[Illustration: Fig. 445. Imbricate pattern of leaves; Begonia.]

=780. Spiral patterns.=—They are very common on stems of the

cylindrical type, which are unbranched, or but little branched. The

sunflower, mullein, chrysanthemum, as it is grown in greenhouses, the

Easter lily, etc., are examples. The spiral arrangement of the leaves

provides that each successive leaf on the stem, as one ascends the

stem, is a little to one side so that it does not cast shade on the leaf

just below. In some stems, according to the leaf arrangement (or

phyllotaxy), one would pass several times around in ascending the stem

before a leaf would be found directly above another, which would be

such a distance below that it would not be shaded to an appreciable

extent. Interesting observations can be made on different plants to

work out the relation of distance of leaves on the stem to length of

the upper and lower leaves; the number of vertical rows on the stem

compared to the width of the leaves; and the relation of these facts to

the problem of light supply. Related to the spiral pattern is that of

erect stems with opposite leaves. Here each pair is set at right angles

to the direction of the pair above or below.

[Illustration: Fig. 446. Palm showing radiate arrangement of leaves and

the petiole of the leaf functions as stem in lifting leaf to the light.]

=781. Radiate pattern.=—This pattern is present in many grasses

and related plants with narrow leaves and short stems. The leaves are

often very crowded at the base, but by radiating in all directions from

the horizontal to the vertical, abundant exposure to light is gained

with little shading. The dragon tree screw-pine, and plants grown in

greenhouses also illustrate this type. It is also shown in cycads,

palms, and many ferns, although these have divided leaves.

[Illustration: Fig. 447. Screw-pine (Pandanus) showing prop roots and

radiate pattern of leaves.]

=782. Compass plants.=—These plants with vertical leaf

arrangement, and exposure of both surfaces to the lateral rays of light

have been mentioned in other sections (Lactuca scariola).

=783. Open patterns.=—Open patterns are presented by divided or

“branched” leaves. Where the leaves are very finely dissected, they may

be clustered in great profusion and yet admit sufficient light for some

depth below. Where the leaflets are broader, the leaves are likely to

be fewer in number and so arranged as to admit light to a great depth

so that successive leaves below on the same or adjacent stems may not

be too much shaded. On such plants, often the leaves lying next the

ground are entire or less divided.

FOOTNOTES:

[44] Some of the different terms used to express the kinds of compound

leaves are as follows:

[45] The most remarkable case is that of the “telegraph” plant

(Desmodium gyrans). Aside from the day and night positions which the

leaves assume, there is a pair of small lateral leaflets to each leaf

which constantly execute a jerky motion, and swing around in a circle

like the second hand of a watch.

[46] Seedlings are usually very sensitive to light and are good objects

to study.

CHAPTER XLI.

THE ROOT.

I. Function of Roots.

=784.= The most obvious function of the roots of ordinary

plants are two: 1st, To furnish anchorage and partial support, and

2d, absorption of liquid nutriment from the soil. The environmental

relation of such roots, then, in broad terms, is with the soil. It is

very clear that in some plants the root serves both functions, while in

other plants the root may fulfil only one of these requirements.

The problems which the plant has to solve in working out these

relations are:

(1) Permeation of the soil or substratum.

(2) Grappling the substratum.

(3) A congenial moisture or water relation.

(4) Distribution of roots for the purpose of reaching

food-laden soil.

(5) Exposure of surface for absorption.

(6) The renewal of the delicate structures for absorption.

(7) Aid in preparation of food from raw material.

(8) The maintenance of the required balance between the

environment as a whole and the increasing or changing

requirements of the plant.

=785.= (1) =Permeation of the soil or substratum.=—The

fundamental divergence of character in the environmental relations of

root and stem are manifest as soon as they emerge from the germinating

seed. Under the influence of the same stimulus (_gravity_) the root

shows its geotropic character by growing downward, while the geotropic

character of the stem is shown in its upward growth.

The medium which the root has to penetrate offers considerable

resistance, and the form of the root as well as its manner of growth is

adapted to overcome this difficulty. The slender, conical, penetrating

root-tip wedges its way between the minute particles of soil or into

the minute crevices of the rock, while the nutation of the root enables

it to search for the points of least resistance. The root-tips having

penetrated the soil, the older portions of the root continue this wedge

action by growth in diameter, though, of course, elongation of the old

parts of the root does not take place. It is the widening growth of the

tapering root that produces the wedge-like action. The crevices of the

rock are sometimes broadened, but the resistance here is so great, the

root is often greatly flattened out.

=786.= (2) =Grappling the substratum.=—The mere penetration

of a single root into the soil gives it some hold on the soil and it

offers some resistance to a “pull” since it has wedged its way in

and the contact of soil particles offers resistance. The root hairs

formed on the first entering root growing laterally in great numbers

and applying themselves very closely to the soil particles, increase

greatly the hold of the plant on the soil, as one can readily see by

pulling up a young seedling. Lateral roots are soon formed, and as

these continue to extend and ramify in all directions, the hold is

increased until in the case of some of the larger plants the resistance

their hold would offer would equal many tons. Even in some of the

smaller shrubs and herbs the resistance is considerable, as one can

easily test by pulling with the hand. To obtain some idea of the amount

of resistance the roots of these smaller plants offer, they can be

tested by pulling with the ordinary spring scales.

=787.= (3) =A congenial moisture, or water relation.=—In

general, the roots seek those portions of the soil provided with a

modicum of moisture. Usually a suitable moisture condition is present

in those portions of the soil containing the plant food. But if

portions of the soil are too dry and very nearby other portions

containing moisture, the roots grow mainly into the moist substratum

(_hydrotropism_). If the soil is too wet, the roots grow away from it

to soil with less water, or in some cases will grow to and upon the

surface of the soil.

The roots need _aeration_, and where the supply of water is too great,

the air is shut out, and we know that corn, wheat, and many other

plants become “sickly” in low and undrained soil in wet seasons. This

can only be said in the case of our ordinary dry land plants, i.e.,

those that occupy an intermediate position between _water-loving_

plants and _dry-conditioned_ plants. This phase of the subject must be

reserved for special treatment. (See Chapter XLVI.)

=788.= (4) =Distribution of roots for the purpose of reaching

food-laden soil.=—This is one of the essential relations of the

root in the case of the land plant, and probably accounts for the very

extensive ramification of the roots. To some extent it also explains

the different root systems in some plants. The pines, spruces, etc.,

usually grow in regions where the soil is very shallow. The root

system does not extend deeply into the soil. It spreads laterally and

extends widely through the shallow surface soil and presents a very

different aspect from the stem system in the air. The root system of

the broad-leaved trees usually extends more deeply into the soil, while

of course, extending laterally to great distances. The hickory, walnut,

etc., especially have strong tap-roots which extend deeply into the

soil, and the root system of such a tree is more comparable in aspect,

if it were entirely uncovered, to the stem system in the air. The

tap-root is more pronounced in some trees than in others. It may be

that in the hickory and walnut the deep tap-root is important in

supplying the tree with water in dry seasons, especially when growing

on dry, gravelly soil which does not retain moisture on the surface

nor hold it within two or three feet of the surface. Experiment has

demonstrated, by pot culture of plants, that where soil rich in plant

food lies adjacent to poor soil, no matter in what part of the pot the

rich soil is, the greatest growth and branching of roots is in the rich

soil.

=789.= (5) =Exposure of root surface for absorption.=—The

principal part of root absorption takes place in the young root and

the root hairs growing near the root-tip. The root-tips and root hairs

in their relation to the root systems on which they are borne are

not to be compared morphologically with the leaves and stem system.

But the root-tips and hairs are absorbing organs of the roots while

the main root system supports them, brings them into relation with

the soil and moisture, and conducts food and other substances to and

from them. One of the important relations of the leaf is that of

light, and since the source of light is restricted, i.e., it is not

equally strong from all sides, an expanded and thin leaf-blade is more

effective than an equal expenditure of plant material in the form of

thread-like outgrowths. It is different, however, with the plant food

dissolved in the soil water. It is equally accessible on all sides. A

greater surface for absorption is exposed with the same expenditure of

material by multiplication of the organs and a reduction in their size.

Numerous delicate root hairs present a greater absorbing surface than

if the same amount of material were massed into leaf-like expansions.

There is another important advantage also. Its slender roots and

thread-like root hairs allow greater freedom of circulation of water,

food solutions, and air than if the absorbing organs of the roots were

broadly expanded.

=790.= (6) =The renewal of the delicate structures for

absorption.=—The delicate root hairs are easily injured. The thin

cell-walls through which food solutions flow become more or less choked

by the gradual deposit of substances in solution in the water, and

continued growth of the root in diameter forms a firmer epidermis and

cortex through which the solutions taken up by the root hairs would

pass with difficulty. For this reason new root hairs are constantly

being formed on the growing root-tip throughout the growing season,

and in the case of perennial plants, through each season of their

growth.

=791.= (7) =Aid in preparation of food from raw

materials.=—For most plants the food obtained from the soil is

already in solution in the soil water. But there are certain substances

(examples, some of the chemical compounds of potash, phosphoric acid,

etc.) which are insoluble in water. Certain acids excreted by the

roots aid in making these substances soluble (see Chapter III). In

a number of plants the roots have become associated with fungus or

bacterial organisms which assist in the manufacture of nitrogenous food

substances, or even in the absorption of ordinary food solution from

the soil, or in making use of the decaying humus of the forest (see

Chapter IX).

=792.= (8) =The maintenance of the required balance between

the environment and the increasing or changing requirements of the

plant.=—In this matter the entire plant participates. Mention is

made here only of the general relation which the root sustains to its

own environment and the increased burden placed upon it by the shoot.

The increase in the root system keeps pace with the increasing size of

the stem system. The roots become stronger, their ramifications wider,

and the number of absorbing rootlets more numerous. The observation

is sometimes offered that the correlation between the root system of

a plant, and the form of the stem system and position of the leaves,

is of such a nature that plants with a tap-root system have their

leaves so arranged as to shed the water to the center of the system,

while plants with a fibrous-root system have their leaves so arranged

as to shed the water outward. In support of this attention is called

to the radiate type of the leaf system of the dandelion, beet, etc.

In the second place the imbricate type as manifested in broad-leaved

trees, and in the overlapping branch systems of many pines, etc. One

should note, however, that in the former class the leaves are often

arranged to shed as much water outward as inward. As to the latter

class, there is need of experiment to determine whether these empirical

observations are correct, for the following reasons: 1st, Root and leaf

distribution are governed by other and more important laws, the root

being influenced by the location of food in the soil which usually

forms a very thin stratum while the shoot and leaf is mainly influenced

by light, and root distribution is much wider in a lateral direction

than that of the branches. 2d, In light rains the leaf surface holds

back practically all the rain which is then evaporated into the air

and lost to the root systems. 3d, In heavy and long-continued rains

the water breaks through the leaf system to such an extent that roots

under the tree would be as well supplied as those outside, and the

ground outside being saturated anyway, the roots do not need the

small additional water which may have been shed outward. 4th, It is

the habit of plants where left undisturbed (except in rare cases), to

grow in more or less dense formations or societies. Here there is no

opportunity for any appreciable centrifugal distribution of rainfall

and yet the root distribution is practically the same, except that the

root systems of adjacent plants are interlaced.

II. Kinds of Roots.

=793. The root system.=—From the foregoing, it will be understood

that the roots of a plant taken together form the _root system_ of that

plant. In soil-roots in general we usually recognize two kinds of root

systems.

=794. The fibrous-root system.=—Roots which are composed of

numerous slender branching roots resembling “fibers,” are termed

_fibrous_, or the plant is said to have a _fibrous-root system_. The

bean, corn, most grasses, and many other plants have fibrous-root

systems.

=795. The tap-root system.=—Plants with a recognizable central

shaft-like root, more or less thickened and considerably stouter

than the lateral roots, are said to have _tap_ roots, or they have a

_tap-root system_. The dandelion, beet, carrot (see crown tuber) are

examples. The hickory, walnut, and some other trees have very prominent

tap-roots when young. The tap-root is maintained in old age, but the

lateral roots often become finally as large as the tap-root. Besides

tap-roots and fibrous-roots, which include the larger number, several

other kinds of roots are to be enumerated.

=796. Aerial roots.=—Aerial roots are most abundantly developed

in certain tropical plants, especially in the orchids and aroids. Many

examples of these plants are grown in conservatories. The amount of

moisture is so great in these tropical regions that the roots are

abundantly supplied without the soil relation. Certain of the roots

hang free in the air and are provided with a special sheath of spongy

tissue called the _velamen_, through which moisture is absorbed from

the air. Other roots attach themselves to the trunk or branches of

the tree on which the orchid is growing, and furnish the support to

the _epiphyte_, as such plants are often called. Among the tangle

of these clinging roots falling leaves are caught. Here they decay

and nourishing roots grow from the clinging roots into this mass

of decaying leaves and supply some of the plant food. Aerial roots

sometimes possess chlorophyll.

There are a number of plants, however, in temperate regions which

have aerial roots. These are chiefly used to give the stem support as

it climbs on trees or on walls. They are sometimes called clinging

roots. A common example is the climbing poison-ivy (Rhus radicans), the

trumpet creeper, etc. Such aerial roots are called _adventitious_ roots.

=797. Bracing roots, or prop roots.=—These are developed in a

great variety of plants and serve to brace or prop the plant where

the fibrous-root system is insufficient to support the heavy shoot

system, or the shoot system branches so widely props are needed to hold

up the branches. In the common Indian corn several whorls of bracing

roots arise from the nodes near the ground and extend outward and

downward to the ground, though the upper whorls do not always succeed

in reaching the ground. The screw-pine so common in greenhouses affords

an excellent example of prop roots. The roots are quite large, and long

before the root reaches the soil the large root cap is evident. The

banyan tree of India is a classic example of prop roots for supporting

the wide-reaching branches. The mangrove in our own subtropical forests

of Florida is a nearer example.

[Illustration: Fig. 448. Bracing roots of Indian corn.]

[Illustration: Fig. 449. Buttresses of silk-cotton tree, Nassau.]

=798. Buttresses= are formed at the junction of the root and

trunk, and therefore are part root and part stem. Splendid examples

of buttresses are formed on the silk-cotton tree. They are sometimes

formed on the elm and other trees in low swampy ground.

=799. Fleshy roots, or root tubers.=—These are enlargements of

the root in the form of tubers, as in the sweet potato, the dahlia,

etc. They are storage reservoirs for food. Portions of the roots become

thick and fleshy and contain large quantities of sugar, as in the sweet

potato, or of _inulin_ (a carbohydrate) in the root tubers of the

dahlia and other composites.

=800. Water-roots and roots of water plants.=—These are roots

which are developed in the water, or in the soil. Water-roots are

sometimes formed on land plants where the root comes in contact with a

body of water, or a stream. Water-roots usually possess no root hairs,

or but a few, as can be seen by comparing water-roots with soil-roots,

or by comparing roots of plants grown in water cultures. The greater

body of water in contact with the root and the more delicate epidermis

of the root render less necessary the root hairs. The duck-meats

(Lemna) are good examples of plants having only water-roots. Other

aquatic plants like the potamogetons, etc., have true roots which grow

into the soil and serve to anchor the plant, but they are not developed

as special organs of absorption, since the stem and leaves largely

perform this function.

=801. Holdfasts.=—These are organs for anchorage which are not

true roots. These are especially well developed in some of the algæ

(Fucus, Laminaria, etc.). They are usually called _holdfasts_. The

holdfasts of the larger algæ are mainly for anchoring the plant. They

do not function as absorbing organs, and the structure is different

from that of true roots.

=802. Haustoria or suckers= is a name applied to another kind of

holdfast employed by parasitic plants. In the dodder the haustorium

penetrates the tissue of the _host_ (the plant on which the parasite

grows), and besides furnishing a means of attachment, it serves as an

absorbing organ by means of which the parasite absorbs food from its

host. The parasitic fungi like the powdery mildews which grow on the

surface of their hosts have simple haustoria which serve both as organs

of attachment and absorption, while in the rusts which grow in the

interior of their hosts the haustoria are merely absorbing organs.

=803. Rootlets, or rhizoids.=—Many of the algæ, liverworts and

mosses have slender, hair-like organs of attachment and absorption.

These plants do not have true roots. Because of the slender form and

small size of these organs, they are called _rhizoids_, or _rootlets_.

In form many of them resemble the root hairs of higher plants.

CHAPTER XLII.

THE FLORAL SHOOT.

I. The Parts of the Flower.

The portion of the stem on which the flowers are borne is the _flower_

shoot or axis, or taken together with the flowers, it is known as the

_Flower Cluster_.

=804. The flower.=—The flower is best understood by an

examination, first of one of the types known as a “complete” flower,

as in the buttercup, the spring-beauty, the blood-root, the apple, the

rose, etc.

There are two sets of organs or members in the complete flower—(1) the

floral envelope; (2) the essential or necessary members or organs.

The floral envelope when complete consists of—1st, an outer envelope,

the _calyx_, made up of several leaf-like structures (_sepals_), very

often possessing chlorophyll, which envelop all the other parts of the

flower when in bud; 2d, an inner envelope, the _corolla_, also made up

of several leaf-like parts (_petals_), usually bright colored and larger

than the sepals. The outer and inner floral envelopes are usually in

whorls (though in close spirals in many of the buttercup family, etc.),

and for reasons discussed elsewhere (Chapter XXXIV) represent leaves.

The essential or necessary members of the flower are also usually in

whorls and likewise represent leaves, but only in rare cases is there

any suggestion, either in their form or color, of a leaf relationship.

These members are in two sets: (1) The outer, or _andrœcium_,

consisting of a few or many parts (_stamens_); (2) the inner set, the

_gynœcium_, consisting of a few or many parts (_carpels_).

=805. Purpose of the flower.=—While the ultimate purpose of

all plants is the production of seed or its equivalent through

which the plant gains distribution and perpetuation, the flower is

the specialized part of the seed plant which utilizes the food and

energies contributed by other members of the plant organization for the

production of seed. In addition to this there are definite functions

performed by the members of the flower, which come under the general

head of plant work, or flower work.

=806. The calyx, or the sepals.=—These are chiefly protective,

affording protection to the young stamens and carpels in the flower

bud. Where the corolla is absent, sepals are usually present and then

assume the function of the petals. In a few instances the calyx may

possibly ultimately join in the formation of the fruit (examples: the

butternut, walnut, hickory).

=807. The corolla, or petals.=—The petals are partly protective

in the bud, but their chief function where well developed seems to be

that of attracting insects, which through their visits to the flower

aid in “_pollination_,” especially “_cross pollination_.”

=808. The stamens.=—The stamens (= microsporophylls) are

flower organs for the production of _pollen_, or _pollen-spores_ (=

microspores). The _stalk_ (not always present) is the _filament_, the

_anther_ is borne on the filament when the latter is present. The

anther consists of the _anther sacs_ or _pollen sacs_ (microsporangium)

containing the pollen-spores, and the _connective_, the sterile tissue

lying between and supporting the anther sac. The stamens are usually

separate, but sometimes they are united by their filaments, or by their

anthers. When the pollen is ripe they open by slits or pores and the

pollen is scattered; or in rarer cases the pollen mass (_pollinium_) is

removed through the agency of insects (see Insect pollination, Chap.

XLIII).

=809. The pistil.=—The pistil consists of the “_ovary_,” the

_style_ (not always present), and the _stigma_. These are well shown in

a _simple pistil_, common examples of which are found in the buttercup,

marsh marigold, the pea, bean, etc. The simple pistil is equivalent to

a _carpel_ (= macrosporophyll), while the _compound pistil_ consists of

two or several carpels joined, as in the toothwort, trillium, lily,

etc. The _ovary_ is the enlarged part which below is attached to

the receptacle of the flower, and contains within the _ovules_. The

_style_, when present, is a slender elongation of the upper end of

the ovary. The _stigma_ is supported on the end of the style when the

latter is present. It is often on a capitate enlargement of the style

or extends down one side, or when the style is absent it is usually

seated directly on the upper end of the ovary. The stigmatic surface is

glutinous or “sticky,” and serves to hold the pollen-spores when they

come in contact with it.

The _ovules_ are within the ovary and are arranged in different ways

in different plants. The pollen grain (or better pollen-spore =

microspore), after it has been transferred to the stigma, “germinates,”

and the pollen tube grows down through the tissue of the stigma and

style, or courses down the stylar canal until it reaches the ovule.

Here it usually enters the ovule (macrosporangium) at the _micropyle_

(in some of the ament-bearing plants it enters at the _chalaza_), and

the sperm cells are emptied into the embryo sac in the interior of the

ovule.

=810. Fertilization.=—One of the sperms unites with the egg

in the embryo sac. This is _fertilization_, and from the fertilized

egg the young embryo is formed still within the ovule. _Double

fertilization_,—the other sperm cell sometimes unites with one or both

of the “polar” nuclei which have united to form the “definitive” or

“endosperm” nucleus. As a result of fertilization, the embryo plant is

formed within the ovule, the coats of which enlarge by growth forming

the seed coats, and altogether forming the seed. (See Chapters XXXIV,

XXXV, XXXVI.)

II. Kinds of Flowers.

=811. Absence of certain flower parts.=—The _complete_ flower

contains all the four series of parts. When any one of the series of

parts is lacking, the flower is said to be _incomplete_. Where only one

series of the floral envelopes is present the flowers are said to be

_apetalous_ (the petals are absent), examples: elm, buckwheat, etc.

Flowers which lack both floral envelopes are _naked_. When pistils are

absent but stamens are present the flowers are _staminate_, whether

floral envelopes are present or not; and so when stamens are absent and

pistils present the flower is _pistillate_. If both stamens and pistils

are absent the flower is said to be _sterile_ or _neutral_ (snowball,

marginal or showy flowers in hydrangea). Flowers with both stamens and

pistils, whether or not they have floral envelopes, are _perfect_ (or

hermaphrodite), so if only one of these sets of _essential organs_

of the flower is present the flower is _imperfect_, or _diclinous_.

Sometimes the imperfect, or diclinous, flowers are on the same plant,

and the plant is said to be _monœcious_ (of one household). When

staminate flowers are on certain individual plants, and the pistillate

flowers of the same species are on other individuals, the plant is

_diœcious_ (or of two households). When some of the flowers of a plant

are diclinous and others are perfect, they are said to be _polygamous_.

Many of these variations relating to the presence or absence of flower

parts in one way or another contribute to the well-being of the plant.

Some indicate a division of labor; thus in the neutral flowers of

certain species of hydrangea or viburnum, the showy petals serve to

attract insects which aid in the pollination of the fertile flowers. It

must not be understood, however, that all variations in plants which

results in new or different forms of flowers is for the good of the

species. For example, under cultivation the flowers of viburnum and

hydrangea sometimes are all neutral and showy. While such variations

sometimes contribute to the happiness of man, the plant has lost the

power of developing seed. In diclinous flowers cross pollination is

necessitated.

=812. Form of the flower.=—The flower as a whole has _form_.

This is so characteristic that in general all flowers of the different

individuals of a species are of the same shape, though they may vary

in size. In general, flowers of closely related plants of different

species are of the same type as to form, so that often in the shape of

the flower alone we can see the relationship of kind, though the form

of the flower is not the most important nor always the sure index of

kinship. Since many flowers resemble certain familiar objects, names

are often used which relate to these objects.

Flowers are said to be _regular_, or _irregular_. In a regular flower

all of the parts of a set or series are of the same shape and size,

while in irregular flowers the parts are of a different shape or size

in some of the sets. The flowers of the pea family (_Papilionaceæ_),

of the mint family (_Labiatæ_), of the morning glory, larkspur,

monkshood, etc., are irregular (fig. 450). The corolla usually gives

the characteristic form to the flower, and the name is usually applied

to the form of the corolla.

[Illustration: Fig. 450. Several forms of flowers. Regular flowers.

_wh_, wheel-shaped corolla; _sa_, salver-shaped; _tub_, tubular-shaped.

Irregular flowers. _pa_, butterfly or papilionaceous; _per_, personate

or masked flower; _lab_, gaping or ringent corolla. The two latter are

called bilabiate flowers.]

Some of the different forms are wheel-shaped or _rotate_ corolla when

the petals spread out at once like the spokes of a wheel, as in the

potato, tomato, or bittersweet; _salver-shaped_ when the petals spread

out at right angles from the end of a corolla tube, as in the phlox;

_bell-shaped_, or _campanulate_, as in the harebell or campanula;

_funnel-shaped_, as in the morning glory; _tubular_, when the ends of

the petals spread but little or none from the end of the corolla tube,

as in the turnip flower or in the disk florets of the composites. The

_butterfly_, or _papilionaceous_ corolla is peculiar as in the pea

or bean. The upper petal is the “banner,” the two lateral ones the

“wings,” and the two lower the “keel.”

The _labiate_ corolla is characteristic of the mint family where the

gamosepalous corolla is unequally divided, so that the two upper lobes

are sharply separated from the three lower forming two “lips.” The

labiate corolla of the toad-flax, or snapdragon is _personate_, or

_masked_, because the lower lip arches upward like a palate and closes

the entrance to the corolla tube; that of the dead nettle (_Lamium_) is

_ringent_ or _gaping_, because the lips are spread wide apart. In some

plants the labiate corolla is not very marked and differs but slightly

from a regular form.

The _ligulate_ or _strap-shaped_ corolla is characteristic of the

flowers of the dandelion or chicory, or of the ray flowers of other

composites (fig. 451). The lower part of the gamosepalous corolla is

tubular, and the upper part is strap-shaped, as if that part of the

tube were split on one side and spread out flat.

These forms of the flower should be studied in appropriate examples.

=813. Union of flower parts.=—In the buttercup flower all the

parts of each series are separate from one another and from other

series of parts. Each one is attached to the _receptacle_ of the

flower, which is a very much shortened portion of the flower axis.

The calyx being composed of separate and distinct parts is said to be

_polysepalous_, and the corolla is likewise _polypetalous_. The stamens

are _distinct_, and the pistils are _simple_. In many flowers, however,

there is a greater or lesser _union_ of parts.

=814. Union of parts of the same series or cycle.=—The parts

_coalesce_, either slightly or to a great extent. Usually they are not

so completely coalesced but what the number of parts of the series can

be determined. Where the sepals are united the calyx is _gamosepalous_,

when the petals are united the corolla is _gamopetalous_.

Union of the sepals or of the corolla is quite common, but union

of the stamens is rare except in a few families where it is quite

characteristic. When the stamens are united by their anthers, they

are _syngenœsious_. This is the case in most flowers of the composite

family. When all the stamens are united into one group by their

filaments, they are _monadelphous_ (one brotherhood), as in

hollyhock, hibiscus, cotton, marsh-mallow, etc. When they are united

by their filaments in two groups, they are _diadelphous_ (two

brotherhoods), as in the pea and most members of the pea family. In

most species of St. John’s wort (Hypericum), the stamens are united in

threes (_triadelphous_).

=815. The carpels are often united.=—The pistil is then said to

be _compound_. Where the pistils are consolidated, usually the adjacent

walls coalesce and thus separate the cavity of each ovary. Each cavity

in the compound pistil is a _locule_. In some cases the adjacent walls

disappear so that there is one common cavity for the compound pistil

(examples: purslane, chickweeds, pinks, etc.). In a few cases there is a

false partition (example, in the toothwort and other crucifers). The

compound pistil is very often lobed slightly, so that the different

pistils can be discerned. More often the styles or stigmas are

distinct, and thus indicate the number of pistils united.

=816. Union of the parts of different series.=—While in the

buttercup and many other flowers, all the different parts are inserted

on the torus or receptacle, in other flowers one series of parts may

be joined to another. This is _adnation_ of parts, or the two or more

series are _adnate_. In the morning glory the stamens are inserted

on the inner face of the corolla tube; the same is true in the mint

family, and there are many other examples. The insertion of parts,

whether free or adnate, is usually spoken of in reference to their

relation to the pistil. Thus, in the buttercup the floral envelopes and

stamens are all free and _hypogynous_, they are _below_ the pistil.

The pistil in this case is _superior_. In the cherry, pear, etc., the

petals and stamens are borne on the edge of the more or less elevated

tube of the calyx, and are said to be _perigynous_, i.e., around the

pistil. In the cranberry, huckleberry, etc., the calyx is for the

most part united with the wall of the ovary with the short calyx

limbs projecting from the upper surface. The petals and stamens are

inserted on the edge of the calyx above the ovary; they are, therefore,

_epigynous_, and the ovary being under the calyx, as it were, is

_inferior_.

III. Arrangement of Flowers, or Mode of Inflorescence.

=817. Flowers are solitary or clustered.=—_Solitary flowers_

are more simple in their arrangement, i.e., it is easier for us to

determine and name their relation to each other and to other parts of

the plant. They are either _axillary_, i.e., on short lateral shoots

in the axils of ordinary foliage leaves, or they are _terminal_, i.e.,

they are borne on the end of the main axis of an ordinary foliage

shoot. In either case they are so far separated, and the foliage

leaves are so prominent, they do not form recognizable groups or

clusters. The manner of arrangement of flowers on the shoot is called

_inflorescence_, while the group of flowers so arranged is the _flower

cluster_.

Two different modes of inflorescence are usually recognized in

the arrangement of flowers on the stem. (1) The _corymbose_, or

_indeterminate inflorescence_ (also indefinite inflorescence), in

which the flowers arise from axillary buds, and the terminal bud may

continue to grow. (2) The _cymose_ or _determinate inflorescence_ (also

_definite inflorescence_) in which the flowers arise from terminal

buds. This arrests the growth of the shoot in length.

There are several advantages to the plant in the different modes of

inflorescence, chief among which is the massing of the flowers, thus

increasing the chances for effective pollination.

A. FLOWER CLUSTERS WITH INDETERMINATE INFLORESCENCE.

=818. The simplest mode of indeterminate inflorescence= is where

the flowers arise in the axils of normal foliage leaves, while the

terminal bud, as in the florist’s smilax, the bellwort, moneywort,

apricot, etc., continues to grow. The flowers are _solitary_ and

_axillary_. In other cases which are far more numerous, the flowers are

associated into more or less definite clusters in which are a number

of recognizable types. The word type used in this sense, it should be

understood, does not refer to an original structure which is the

source of others. It merely refers to a mode of inflorescence which we

attempt to recognize, and about which we group those forms which have

a resemblance to one another. There are many forms of flower clusters

which do not conform to any one of our recognized types, and are very

puzzling. The evolution of the flower clusters has been _natural_, and

we cannot make them all conform to an _artificial_ classification.

These _types_ are named merely as a matter of convenience in the

expression of our ideas. The types usually recognized are as follows:

=819. The raceme.=—The flower-shoot is more or less elongated,

and the leaves are reduced to a minute size termed _bracts_, while the

flowers on lateral axes are solitary in the axils of the bracts. The

reduction in the size of the leaves and the somewhat limited growth

of the shoot in length, makes the flowers more prominent, and brings

them into closer relation than if they were formed in the axils of

the leaves on the ordinary foliage shoot. The choke cherry, currant,

pokeweed, sourwood, etc., are examples of a raceme (fig. 569). In most

plants with the raceme type, while the inflorescence is indeterminate,

and the uppermost flowers (those toward the end of the main shoot)

are younger, still the period of flowering is somewhat restricted

and the raceme stops growing. In a few plants, however, as in the

common “shepherd’s-purse,” the raceme continues to grow throughout the

summer, so that the lower flowers may have ripened their seed while

the terminal portion of the raceme is still growing and producing

new flowers. Compound racemes are formed when by branching of the

flower-shoot there are several racemes in a cluster, as in the false

Solomon’s seal (Smilacina racemosa).

=820. The panicle.=—The panicle is developed from the raceme type

by the branching of the lateral flower-axes forming a loose open flower

cluster, as in the _oat_.

=821. The thyrsus= is a compact panicle of pyramidal form, as in

the lilac, horse-chestnut, etc.

=822. The corymb.=—The corymb shows likewise an easy transition

from the raceme type, by the shortening of the main axis of

inflorescence, and the lengthening of the lower, lateral flower

peduncles so that the flower cluster is more or less flattened on

top. This represents the _simple corymb_. A _compound corymb_ is one

in which some of the flower peduncles branch again forming secondary

corymbs, as in the mountain-ash. It is like a panicle with the lower

flower stalks elongated.

=823. The umbel.=—The umbel is developed from the raceme, or

corymb. The main flower-shoot remains very short or undeveloped with

several flowers on long peduncles arising close together around this

shortened axis, in the form of a whorl or cluster. Examples are found

in the milkweed, water pennywort (Hydrocotyle), the oxheart cherry,

etc. A _compound_ umbel is one in which the peduncles are branched,

forming secondary umbels, as in the caraway, parsnip, carrot, etc.

=824. The spike.=—In the spike the main axis is long, and the

solitary flowers in the axils of the bracts are usually sessile, and

often very much crowded. The plaintain, mullein (fig. 422), etc.,

are examples. The spike is a raceme, only the flowers are sessile

and crowded. In the grasses the flower cluster is branched, and the

branchlets bearing a few flowers are spikelets.

=825. The head.=—When the flower axis is very much shortened

and the flowers crowded and sessile or nearly so, forming a globose

or compressed cluster, it is a _head_ or _capitulum_. The transition

is from a spike by the shortening of the main axis, as in the clover,

button bush (_Cephalanthus_), etc., or in the shortening of the

peduncles in an umbel, as in the daisy, dandelion, and other composite

flowers. In these the head is surrounded by an involucre, which in

the young head often envelopes the mass of flowers, thus affording

them protection. In some other composites (Lactuca, for example) the

involucre affords protection for a longer period, even while the seeds

are ripening.

=826. The spadix.=—When the main axis of the flower cluster is

fleshy, the spike or head forms a _spadix_, as in the Indian turnip,

the skunk-cabbage, the calla, etc. The spadix is usually more or less

enclosed in a _spathe_, a somewhat strap-shaped leaf.

=827. The catkin.=—A spike which is usually caducous, i.e., falls

away after the maturity of the flower or fruit, is called a catkin,

or an _ament_. The flower clusters of the alder, willow, (fig. 555),

poplar, and the staminate flower clusters of the oak, hickory,

hazel, birch, etc., are _aments_. So characteristic is this mode

of inflorescence that the plants are called _amentiferous_, or

_amentaceous_.

[Illustration: Fig. 451. Head of sunflower showing centripetal

inflorescence of tubular flowers. (Photo by the Author.)]

=828. Anthesis of flowers with indeterminate inflorescence.=—In

the anthesis of the raceme as well as in other corymbose forms the

lower (or outer) flowers being older, open first. The opening of the

flowers then takes place from below, upward; or from the outside,

inward toward the center of inflorescence. The _anthesis_, i.e., the

opening of the flowers of corymbose forms is said to be _centripetal_,

i.e., it progresses from outside, inward. The anthesis of the fuller’s

teazel is peculiar, since it shows both types. There are several

distinct advantages to the plant where anthesis extends over a period

of time, as it favors cross pollination, favors the formation of seed

in case conditions should be unfavorable at one period of anthesis,

distributes the drain on the plant for food, etc.

[Illustration: Fig. 452. Heads of fuller’s teazel in different stages

of flowering.]

B. FLOWER CLUSTERS WITH DETERMINATE INFLORESCENCE.

=829. The simplest mode of determinate inflorescence= is a plant

with a solitary terminal flower, as in the hepatica, the tulip,

etc. The leaves in these two plants are clustered in the form of a

rosette, and the aerial shoot is naked and bears the single flower at

its summit. Such a flower-shoot is a _scape_. As in the case of the

indeterminate inflorescence, so here the larger number of flower-shoots

are more complex and specialized, resulting in the evolution of flower

clusters or masses. Accompanying the association of flowers into

clusters there has been a reduction in leaf surface on the flower-shoot

so that the flowers predominate in mass and are more conspicuous. Among

the recognized modes of determinate inflorescence, the following are

the chief ones:

=830. The cyme.=—In the cyme the terminal flower on the main axis

opens first and the remaining flowers are borne on lateral shoots,

which arise from the axils of leaves or bracts, below. These lateral

shoots usually branch and elongate so that the terminal flowers on all

the branches reach nearly the same height as the terminal flower on the

main shoot, forming a somewhat flattened or convex top of the flower

cluster. This is illustrated in the basswood flower. The anthesis of

the cyme is _centrifugal_, i.e., from the inside outward to the margin.

But it is often more or less mixed, since the lateral shoots if they

bear more than one flower are diminutive cymes and the terminal flower

opens before the lateral ones. Where the flower cluster is quite large

and the branching quite extensive, _compound cymes_ are formed, as in

the dogwood, hydrangea, etc.

[Illustration: Fig. 453. Diagrams of cymose inflorescence. _A_,

dichasium; _B_, scorpioid cyme; _C_, helicoid cyme. (After

Strasburger.)]

=831. The helicoid cyme.=—Where successive lateral branching

takes place, and always continues on the same side a curved flower

cluster is formed, as in the forget-me-not and most members of the

borage family. This is known as a _helicoid cyme_ (fig. 453, _C_). Each

new branch becomes in turn the “false” axis bearing a new branch on the

same side.

=832. The scorpioid cyme.=—_A scorpioid cyme_ (fig. 453, _B_) is

formed where each new branch arises on alternate sides of the “false”

axis.

=833. The forking cyme= is where each “false” axis produces two

branches opposite, so that it represents a false dichotomy (example,

the flower cluster of chickweed).

=834.= Some of these flower clusters are peculiar and it is

difficult to see how the helicoid, or scorpioid, cymes are of any

advantage to the plant over a true cyme. The inflorescence of the

plant being determinate, if the flowering is to be extended over a

considerable period a peculiar form would necessarily result. In the

_helicoid cyme_ continued branching takes place on one side, and

the result in the forget-me-not is a continued inflorescence in its

effect like that of a continued raceme (compare shepherd’s-purse).

But we should not expect that all of the complex and specialized

structures from simple and generalized ones are beneficial to the

plant. In many plants we recognize evolution in the direction of

advantageous structures. But since the plant cannot consciously

evolve these structures, we must also recognize that there may be

phases of retrogression in which the structures evolved are not so

beneficial to the plant as the more simple and generalized ones of its

ancestors. Variation and change do not result in advancing the plant

or plant structures merely along the lines which will be beneficial.

The tendency is in all directions. The result in general may be

diagramed by a tree with divergent and wide-reaching branches. Some die

out; others remain subordinate or dormant; while still others droop

downward, showing a retrogression. But in this backward evolution

they do not return to the condition of their ancestors, nor is the

same course retraced. A new downward course is followed just as the

downward-growing branch follows a course of its own, and does not

return in the trunk.

CHAPTER XLIII.

POLLINATION.

Origin of heterospory, and the necessity for pollination.

=835. Both kinds of sexual organs on the same prothallium.=—In

the ferns, as we have seen, the sexual organs are borne on the

prothallium, a small, leaf-like, heart-shaped body growing in moist

situations. In a great many cases both kinds of sexual organs are

borne on the same prothallium. While it is perhaps not uncommon, in

some species, that the egg-cell in an archegonium may be fertilized

by a spermatozoid from an antheridium on the same prothallium, it

happens many times that it is fertilized by a spermatozoid from another

prothallium. This may be accomplished in several ways. In the first

place antheridia are usually found much earlier on the prothallium than

are the archegonia. When these antheridia are ripe, the spermatozoids

escape before the archegonia on the same prothallium are mature.

=836. Cross fertilization in monœcious prothallia.=—By swimming

about in the water or drops of moisture which are at times present in

these moist situations, these spermatozoids may reach and fertilize

an egg which is ripe in an archegonium borne on another and older

prothallium. In this way what is termed cross fertilization is brought

about nearly as effectually as if the prothallia were diœcious, i.e. if

the antheridia and archegonia were all borne on separate prothallia.

=837. Tendency toward diœcious prothallia.=—In other cases

some fern prothallia bear chiefly archegonia, while others bear only

antheridia. In these cases cross fertilization is enforced because

of this separation of the sexual organs on different prothallia.

These different prothallia, the male and female, are largely due to a

difference in food supply, as has been clearly proven by experiment.

=838. The two kinds of sexual organs on different prothallia.=—In

the horsetails (equisetum) the separation of the sexual organs on

different prothallia has become quite constant. Although all the spores

are alike, so far as we can determine, some produce small male plants

exclusively, while others produce large female plants, though in some

cases the latter bear also antheridia. It has been found that when the

spores are given but little nutriment they form male prothallia, and

the spores supplied with abundant nutriment form female prothallia.

=839. Permanent separation of sexes by different amounts of nutriment

supplied the spores.=—This separation of the sexual organs of

different prothallia, which in most of the ferns, and in equisetum, is

dependent on the chance supply of nutriment to the germinating spores,

is made certain when we come to such plants as isoetes and selaginella.

Here certain of the spores receive more nutriment while they are

forming than others. In the large sporangia (macrosporangia) only a few

of the cells of the spore-producing tissue form spores, the remaining

cells being dissolved to nourish the growing macrospores, which are few

in number. In the small sporangia (microsporangia) all the cells of the

spore-producing tissue form spores. Consequently each one has a less

amount of nutriment, and it is very much smaller, a microspore. The

sexual nature of the prothallium in selaginella and isoetes, then, is

predetermined in the spores while they are forming on the sporophyte.

The microspores are to produce male prothallia, while the macrospores

are to produce female prothallia.

=840. Heterospory.=—This production of two kinds of spores

by isoetes, selaginella, and some of the other fern plants is

_heterospory_, or such plants are said to be _heterosporous_.

Heterospory, then, so far as we know from living forms, has originated

in the fern group. In all the higher plants, in the gymnosperms and

angiosperms, it has been perpetuated, the microspores being represented

by the pollen, while the macrospores are represented by the embryo sac;

the male organ of the gymnosperms and angiosperms being the antherid

cell in the pollen or pollen tube, or in some cases perhaps the pollen

grain itself, and the female organ in the angiosperms perhaps reduced

to the egg-cell of the embryo sac.

=841. In the pteridophytes water serves as the medium for conveying

the sperm cell to the female organ.=—In the ferns and their allies,

as well as in the liverworts and mosses, surface water is a necessary

medium through which the generative or sperm cell of the male organ,

the spermatozoid, may reach the germ cell of the female organ. The

sperm cell is here motile. This is true in a large number of cases

in the algæ, which are mostly aquatic plants, while in other cases

currents of water float the sperm cell to the female organ.

=842. In the higher plants a modification of the prothallium is

necessary.=—As we pass to the gymnosperms and angiosperms,

however, where the primitive phase (the gametophyte) of the plants

has become dependent solely on the modern phase (the sporophyte) of

the plant, surface water no longer serves as the medium through which

a motile sperm cell reaches the egg-cell to fertilize it. The female

prothallium, or macrospore, is, in nearly all cases, permanently

enclosed within the sporangium, so that if there were motile sperm

cells on the outside of the ovary, they could never reach the egg to

fertilize it.

=843.= But a modification of the microspore, the pollen tube,

enables the sperm cell to reach the egg-cell. The tube grows through

the nucellus, or first through the tissues of the ovary, deriving

nutriment therefrom.

=844.= But here an important consideration should not escape us.

The pollen grains (microspores) must in nearly all cases first reach

the pistil, in order that in the growth of this tube a channel may be

formed through which the generative cell can make its way to the egg

cell. The pollen passes from the anther locule, then, to the stigma of

the ovary. This process is termed _pollination_.

Pollination.

=845. Self pollination, or close pollination.=—Perhaps very few

of the admirers of the pretty blue violet have ever noticed that there

are other flowers than those which appeal to us through the beautiful

colors of the petals. How many have observed that the brightly colored

flowers of the blue violet rarely “set fruit”? Underneath the soil

or débris at the foot of the plant are smaller flowers on shorter,

curved stalks, which do not open. When the anthers dehisce, they are

lying close upon the stigma of the ovary, and the pollen is deposited

directly upon the stigma of the same flower. This method of pollination

is _self pollination_, or _close pollination_. These small, closed

flowers of the violet have been termed “_cleistogamous_,” because they

are pollinated while the flower is closed, and fertilization takes

place as a result.

But self pollination takes place in the case of some open flowers.

In some cases it takes place by chance, and in other cases by such

movements of the stamens, or of the flower at the time of the

dehiscence of the pollen, that it is quite certainly deposited upon the

stigma of the same flower.

=846. Wind pollination.=—The pine is an example of

wind-pollinated flowers. Since the pollen floats in the air or

is carried by the “wind,” such flowers are _anemophilous_. Other

anemophilous flowers are found in other conifers, in grasses, sedges,

many of the ament-bearing trees, and other dicotyledons. Such plants

produce an abundance of pollen and always in the form of “dust,” so

that the particles readily separate and are borne on the wind.

=847. Pollination by insects.=—A large number of the plants which

we have noted as being anemophilous are monœcious or diœcious, i.e. the

stamens and pistils are borne in separate flowers. The two kinds of

flowers thus formed, the male and the female, are borne either on the

same individual (monœcious) or on different individuals (diœcious). In

such cases cross pollination, i.e. the pollination of the pistil of

one flower by pollen from another, is sure to take place, if it is

pollinated at all. Even in monœcious plants cross pollination often

takes place between flowers of different individuals, so that more

widely different stocks are united in the fertilized egg, and the

strain is kept more vigorous than if very close or identical strains

were united.

[Illustration: Fig. 454. Viola cucullata; blue flowers above,

cleistogamous flowers smaller and curved below. Section of pistil at

right.]

=848.= But there are many flowers in which both stamens and

pistils are present, and yet in which cross pollination is accomplished

through the agency of insects.

=859. Pollination of the bluet.=—In the pretty bluet the stamens

and styles of the flowers are of different length as shown in figures

455, 456. The stamens of the long-styled flower are at about the same

level as the stigma of the short-styled flower, while the stamens of

the latter are on about the same level as the stigma of the former.

What does this interesting relation of the stamens and pistils in the

two different flowers mean? As the butterfly thrusts its “tongue” down

into the tube of the long-styled flower for the nectar, some of the

pollen will be rubbed off and adhere to it. When now the butterfly

visits a short-styled flower this pollen will be in the right position

to be rubbed off onto the stigma of the short style. The positions

of the long stamens and long style are such that a similar cross

pollination will be effected.

[Illustration: Fig. 455. Dichogamous flower of the bluet (Houstonia

cœrulea), the long-styled form.]

[Illustration: Fig. 456. Dichogamous flower of bluet (Houstonia

cœrulea), the short-styled form.]

=850. Pollination of the primrose.=—In the primroses, of which

we have examples growing in conservatories, that blossom during the

winter, we have almost identical examples of the beautiful adaptations

for cross pollination by insects found in the bluet. The general shape

of the corolla is the same, but the parts of the flower are in

fives, instead of in fours as in the bluet. While the pollen of the

short-styled primulas sometimes must fall on the stigma of the same

flower, Darwin has found that such pollen is not so potent on the

stigma of its own flower as on that of another, an additional provision

which tends to necessitate cross pollination.

[Illustration: Fig. 457. Dichogamous flowers of primula.]

In the case of some varieties of pear trees, as the Bartlett, it

has been found that the flowers remain largely sterile not only to

their own pollen, or pollen of the flowers on the same tree, but to

all flowers of that variety. However, they become fertile if cross

pollinated from a different variety of pear.

=851. Pollination of the skunk’s cabbage.=—In many other flowers

cross pollination is brought about through the agency of insects,

where there is a difference in time of the maturing of the stamens and

pistils of the same flower. The skunk’s cabbage (Spathyema fœtida),

though repulsive on account of its fetid odor, is nevertheless a very

interesting plant to study for several reasons. Early in the spring,

before the leaves appear, and in many cases as soon as the frost is out

of the hard ground, the hooked beak of the large fleshy spathe of this

plant pushes its way through the soil.

If we cut away one side of the spathe as shown in fig. 459 we shall

have the flowering spadix brought closely to view. In this spadix the

pistil of each crowded flower has pushed its style through between the

plates of armor formed by the converging ends of the sepals, and stands

out alone with the brush-like stigma ready for pollination, while the

stamens of all the flowers of this spadix are yet hidden beneath. The

insects which pass from the spadix of one plant to another will, in

crawling over the projecting stigmas, rub off some of the pollen which

has been caught while visiting a plant where the stamens are scattering

their pollen. In this way cross pollination is brought about. Such

flowers, in which the stigma is prepared for pollination before the

anthers of the same flower are ripe, are _proterogynous_.

[Illustration: Fig. 458. Skunk’s cabbage.]

[Illustration: Fig. 459. Proterogyny in skunk’s cabbage. (Photograph by

the author.)]

[Illustration: Fig. 460. Skunk’s cabbage; upper flowers proterandrous,

lower ones proterogynous.]

=852.= Now if we observe the spadix of another plant we may see a

condition of things similar to that shown in fig. 460. In the flowers

in the upper part of the spadix here the anthers are wedging their way

through between the armor-like plates formed by the sepals, while the

styles of the same flowers are still beneath, and the stigmas are not

ready for pollination. Such flowers are _proterandrous_, that is, the

anthers are ripe before the stigmas of the same flowers are ready for

pollination. In this spadix the upper flowers are proterandrous, while

the lower ones are proterogynous, so that it might happen here that

the lower flowers would be pollinated by the pollen falling on them

from the stamens of the upper flowers. This would be cross pollination

so far as the flowers are concerned, but not so far as the plants

are concerned. In some individuals, however, we find all the flowers

proterandrous.

=853. Spiders have discovered this curious relation of the flowers

and insects.=—On several different occasions, while studying

the adaptations of the flowers of the skunk’s cabbage for cross

pollination, I was interested to find that the spiders long ago had

discovered something of the kind, for they spread their nets here to

catch the unwary but useful insects. I have not seen the net spread

over the opening in the spathe, but it is spread over the spadix

within, reaching from tip to tip of either the stigmas, or stamens, or

both. Behind the spadix crouches the spider-trapper. The insect crawls

over the edge of the spadix, and plunges unsuspectingly into the dimly

lighted chamber below, where it becomes entangled in the meshes of the

net.

Flowers in which the ripening of the anthers and maturing of the

stigmas occur at different times are also said to be _dichogamous_.

=854. Pollination of jack-in-the-pulpit.=—The jack-in-the-pulpit

(Arisæma triphyllum) has made greater advance in the art of enforcing

cross pollination. The larger number of plants here are, as we have

found, diœcious, the staminate flowers being on the spadix of one

plant, while the pistillate flowers are on the spadix of another. In a

few plants, however, we find both female and male flowers on the same

spadix.

=855.= The pretty bell-flower (Campanula rotundifolia) is

dichogamous and proterandrous (fig. 462). Many of the composites are

also dichogamous.

=856. Pollination of orchids.=—But some of the most marvellous

adaptations for cross pollination by insects are found in the orchids,

or members of the orchis family. The larger number of the members of

this family grow in the tropics. Many of these in the forests are

supported in lofty trees where they are brought near the sunlight,

and such are called “epiphytes.” A number of species of orchids are

distributed in temperate regions.

[Illustration: Fig. 461. A group of jacks.]

=857. Cypripedium, or lady-slipper.=—One species of the

lady-slipper is shown in fig. 468. The labellum in this genus is shaped

like a shoe, as one can see by the section of the flower in fig. 468.

The stigma is situated at _st_, while the anther is situated at _a_,

upon the style. The insect enters about the middle of the boat-shaped

labellum. In going out it passes up and out at the end near the flower

stalk. In doing this it passes the stigma first and the anther last,

rubbing against both. The pollen caught on the head of the insect, will

not touch the stigma of the same flower, but will be in position to

come in contact with the stigma of the next flower visited.

[Illustration: Fig. 462. Proterandry in the bell-flower (campanula).

Left figure shows the syngenœcious stamens surrounding the immature

style and stigma. Middle figure shows the immature stigma being pushed

through the tube and brushing out the pollen; while in the right-hand

figure, after the pollen has disappeared, the lobes of the stigma open

out to receive pollen from another flower.]

=858. Epipactis.=—In epipactis, shown in fig. 469, the action is

similar to that of the blue iris.

[Illustration: Fig. 463. Kalmia latifolia, showing position of anthers

before insect visits, and at the right the scattering of the pollen

when disturbed by insects. Middle figure section of flower.]

=849.= In some of the tropical orchids the pollinia are set free

when the insect touches a certain part of the flower, and are thrown

in such a way that the disk of the pollinium strikes the insect’s head

and stands upright. By the time the insect reaches another flower the

pollinium has bent downward sufficiently to strike against the stigma

when the insect alights on the labellum. In the mountains of North

Carolina I have seen a beautiful little orchid, in which, if one

touches a certain part of the flower with a lead-pencil or other

suitable object, the pollinium is set free suddenly, turns a complete

somersault in the air, and lands with the disk sticking to the pencil.

Many of the orchids grown in conservatories can be used to demonstrate

some of these peculiar mechanisms.

[Illustration: Fig. 464. Spray of leaves and flowers of cytisus.]

[Illustration: Fig. 465. Flower of cytisus grown in conservatory. Same

flower scattering pollen.]

=860. Pollination of the canna.=—In the study of some of the

marvellous adaptations of flowers for cross pollination one is led to

inquire if, after all, plants are not intelligent beings, instead of

mere automatons which respond to various sorts of stimuli. No plant

has puzzled me so much in this respect as the canna, and any one will

be well repaid for a study of recently opened flowers, even though it

may be necessary to rise early in the morning to unravel the mystery,

before bees or the wind have irritated the labellum. The canna flower

is a bewildering maze of petals and petal-like members. The calyx

is green, adherent to the ovary, and the limb divides into three,

lanceolate lobes. The petals are obovate and spreading, while the

stamens have all changed to petal-like members, called _staminodia_.

Only one still shows its stamen origin, since the anther is seen at one

side, while the filament is expanded laterally and upwards to form the

_staminodium_.

[Illustration: Fig. 466. Spartium, showing the dusting of the pollen

through the opening keels on the under side of an insect. (From Kerner

and Oliver.)]

=861.= The ovary has three locules, and the three styles are

usually united into a long, thin, strap-shaped style, as seen in the

figure, though in some cases three, nearly distinct, filamentous styles

are present. The end of this strap-shaped style has a peculiar curve on

one side, the outline being sometimes like a long narrow letter S. It

is on the end of this style, and along the crest of this curve, that

the stigmatic surface lies, so that the pollen must be deposited on the

stigmatic end or margin in order that fertilization may take place.

[Illustration: Fig. 467. Cypripedium.]

[Illustration: Fig. 468. Section of flower of cypripedium. _st_,

stigma; _a_, at the left stamen. The insect enters the labellum at

the center, passes under and against the stigma, and out through the

opening _b_, where it rubs against the pollen. In passing through

another flower this pollen is rubbed off on the stigma.]

=862.= If we open carefully canna flower buds which are nearly

ready to open naturally, by unwrapping the folded petals and

staminodia, we shall see the anther-bearing staminodium is so wrapped

around the flattened style that the anther lies closely pressed against

the face of the style, near the margin _opposite that on which the

stigma lies_.

[Illustration: Fig. 469. Epipactis with portion of perianth removed

to show details. _l_, labellum; _st_, stigma; _r_, rostellum; _p_,

pollinium. When the insect approaches the flower its head strikes the

disk of the pollinium and pulls the pollinium out. At this time the

pollinium stands up out of the way of the stigma. By the time the

insect moves to another flower the pollinia have moved downward so that

they are in position to strike the stigma and leave the pollen. At the

right is the head of a bee, with two pollinia (_a_) attached.]

[Illustration: Fig. 470. Canna flowers with the perianth removed to

show the depositing of the pollen on the style by the stamen.]

=863.= The walls of the anther locules which lie against the style

become changed to a sticky substance for their entire length, so that

they cling firmly to the surface of the style and also to the mass of

pollen within the locules. The result is that when the flower opens,

and this staminodium unwraps itself from the embrace of the style,

the mass of pollen is left there deposited, while the empty anther is

turned around to one side.

=668.= Why does the flower deposit its own pollen on the style?

Some have regarded this as the act of pollination, and have concluded,

therefore, that cannas are necessarily self pollinated, and that cross

pollination does not take place. But why is there such evident care to

deposit the pollen on the side of the style away from the stigmatic

margin? If we visit the cannas some morning, when a number of the

flowers have just opened, and the bumblebees are humming around seeking

for nectar, we may be able to unlock the secret.

=864.= We see that in a recently opened canna flower, the petal

which directly faces the style in front stands upward quite close to

it, so that the flower now is somewhat funnel-shaped. This front petal

is the _labellum_, and is the landing place for the bumblebee as he

alights on the flower. Here he comes humming along and alights on the

labellum with his head so close to the style that it touches it. But

just the instant that the bee attempts to crowd down in the flower the

labellum suddenly bends downward, as shown in fig. 468. In so doing the

head of the bumblebee scrapes against the pollen, bearing some of it

off. Now while the bee is sipping the nectar it is too far below the

stigma to deposit any pollen on the latter. When the bumblebee flies to

another newly opened flower, as it alights, some of the pollen of the

former flower is brushed on the stigma.

=865.= One can easily demonstrate the sensitiveness of the

labellum of recently opened canna flowers, if the labellum has not

already moved down in response to some stimulus. Take a lead-pencil, or

a knife blade, or even the finger, and touch the upper surface of the

labellum by thrusting it between the latter and the style. The labellum

curves quickly downward.

=866.= Sometimes the bumblebees, after sipping the nectar, will

crawl up over the style in a blundering manner. In this way the flower

may be pollinated with its own pollen, which is equivalent to self

pollination. Undoubtedly self pollination does take place often in

flowers which are adapted, to a greater or less degree, for cross

pollination by insects.

[Illustration: Fig. 471. Pollination of the canna flower by bumblebee.]

[Illustration: Canna flower. Pollen on style, stamen at left.]

CHAPTER XLIV.

THE FRUIT.

I. Parts of the Fruit.

=867. After the flower comes the fruit.=—With the perfection of

the fruit the seed is usually formed. This is the end towards which the

energies of the plant have been directed. While the seed consists only

of the ripened ovule and the contained embryo, the fruit consists of

the ripened ovary in addition, and in many cases with other accessory

parts, as calyx, receptacle, etc., combined with it. The wall of the

ripened ovary is called a _pericarp_, and the walls of the ovary form

the walls of the fruit.

=868. Pericarp, endocarp, exocarp, etc.=—This is the part of the

fruit which envelops the seed and may consist of the carpels alone, or

of the carpels and the adherent part of the receptacle, or calyx. In

many fruits the pericarp shows a differentiation into layers, or zones

of tissue, as in the cherry, peach, plum, etc. The outer, which is

here soft and fleshy, is _exocarp_, while the inner, which is hard, is

the _endocarp_. An intermediate layer is sometimes recognized and is

called _mesocarp_. In such cases the skin of the fruit is recognized as

the _epicarp_. Epicarp and mesocarp are more often taken together as

exocarp.

In general fruits are _dry_ or _fleshy_. Dry fruits may be grouped

under two heads. Those which open at maturity and scatter the seed are

_dehiscent_. Those which do not open are _indehiscent_.

II. Indehiscent Fruits.

=869. The akene.=—The thin dry wall of the ovary encloses the

single seed. It usually does not open and free the seed within. Such a

fruit is an _akene_. An _akene_ is a dry, _indehiscent_ fruit. All of

the crowded but separate pistils in the buttercup flower when ripe make

a head of akenes, which form the fruit of the buttercup. Other examples

of akenes are found in other members of the buttercup family, also in

the composites, etc. The sunflower seed is a good example of an akene.

[Illustration: Fig. 472. Seed, or akene, of buttercup.]

[Illustration: Fig. 473. Fruit of red oak. An acorn.]

=870. The samara.=—The winged fruits of the maple, elm, etc., are

indehiscent fruits. They are sometimes called key fruits.

=871. The caryopsis= is a dry fruit in which the seed is

consolidated with the wall of the ovary, as in the wheat, corn, and

other grasses.

=872. The schizocarp= is a dry fruit consisting of several locules

(from a _syncarpous gynœcium_). At maturity the carpels separate from

each other, but do not themselves dehisce and free the seed, as in the

carrot family, mallow family.

=873. The acorn.=—The acorn fruit consists of the acorn and

the “cup” at the base in which the acorn sits. The cup is a curious

structure, and is supposed to be composed of an involucre of numerous

small leaves at the base of the pistillate flower, which become

consolidated into a hard cup-shaped body. When the acorn is ripe it

easily separates from the cup, but the hard pericarp forming the

“shell” of the acorn remains closed. Frost may cause it to crack, but

very often the pericarp is split open at the smaller end by wedge-like

pressure exerted by the emerging radicle during germination.

[Illustration: Fig. 474. Germinating acorn of white oak.]

=874. The hazelnut, chestnut, and beechnut.=—In these fruits a

crown of leaves (involucre) at the base of the flower grows around

the nut and completely envelops it, forming the husk or burr. When

the fruit is ripe the nut is easily shelled out from the husk. In the

beechnut and chestnut the burr dehisces as it dries and allows the nut

to drop out. But the fruit is not dehiscent, since the pericarp is

still intact and encloses the seed.

=875. The hickory-nut, walnut, and butternut.=—In these fruits the

“shuck” of the hickory-nut and the “hull” of the walnut and butternut

are different from the involucre of the acorn or hazelnut, etc. In the

hickory-nut the “shuck” probably consists partly of calyx and partly of

involucral bracts consolidated, probably the calyx part predominating.

This part of the fruit splits open as it dries and frees the “nut,” the

pericarp being very hard and indehiscent. In the walnut and butternut

the “hull” is probably of like origin as the “shuck” of the hickory

nut, but it does not split open as it ripens. It remains fleshy. The

walnut and butternut are often called _drupes_ or _stone-fruits_, but

the fleshy part of the fruit is not of the same origin as the fleshy

part of the true drupes, like the cherry, peach, plum, etc.

III. Dehiscent Fruits.

=876. Of the dehiscent fruits= several prominent types are

recognized, and in general they are sometimes called _pods_. There is a

single carpel (simple pistil), and the pericarp is dry (gynœcium

_apocarpous_); or where there are several carpels united the pistil is

compound (gynœcium _syncarpous_).

[Illustration: Fig. 475. Diagrams illustrating three types (in

cross-section) of the dehiscence of dry fruits. _Loc_, loculicidal;

_Sep_, Septicidal, Septifragal.]

[Illustration: Fig. 476. Fruit of sweet pea; a pod.]

=877. The capsule.=—When the capsule is _syncarpous_ it may

dehisce in three different ways: 1st. When the carpels split along

the line of their union with each other longitudinally (_septicidal

dehiscence_), as in the azalea or rhododendron. 2d. When the carpels

_split down the middle line_ (_loculicidal dehiscence_), as in the

fruit of the iris, lily, etc. 3d. When the carpels open by pores

(_poricidal dehiscence_), as in the poppy. Some syncarpous capsules

have but one locule, the partitions between the different locules when

young having disappeared. The “bouncing-bet” is an example, and the

seeds are attached to a central column in four rows corresponding to

the four locules present in the young stage.

=878. A follicle= is a capsule with a single carpel which splits

open along the ventral or upper suture, as in the larkspur, peony.

=879. The legume, or true pod=, is a capsule with a single carpel

which splits along both sutures, as the pea, bean, etc. As the pod

ripens and dries, a strong twisting tension is often produced, which

splits the pod suddenly, scattering the seeds.

=880. The silique.=—In the toothwort, shepherd’s-purse, and

nearly all of the plants in the mustard family the fruit consists of

two united carpels, which separate at maturity, leaving the partition

wall persistent. Such a fruit is a _silique_; when short it is a

_silicle_, or _pouch_.

=881. A pyxidium, or pyxis=, is a capsule which opens with a lid,

as in the plantain.

IV. Fleshy and Juicy Fruits.

=882. The drupe, or stone-fruit.=—In the plum, cherry, peach,

apricot, etc., the outer portion (exocarp) of the pericarp (ovary)

becomes fleshy, while the inner portion (endocarp) becomes hard and

stony, and encloses the seed, or “pit.” Such a fruit is known as a

drupe, or as a stone-fruit. In the almond the fleshy part of the fruit

is removed.

[Illustration: Fig. 477. Drupe, or stone-fruit, of plum.]

=883. The raspberry and blackberry.=—While these fruits are

known popularly as “berries,” they are not berries in the technical

sense. Each ovary, or pericarp, in the flower forms a single small

fruit, the outer portion being fleshy and the inner stony, just as

in the cherry or plum. It is a _drupelet_ (little drupe). All of the

drupelets together make the “berry,” and as they ripen the separate

drupelets cohere more or less. It is a collection, or aggregation, of

fruits, and consequently they are sometimes called _collective fruits_,

or _aggregate fruits_. In the raspberry the fruit separates from the

receptacle, leaving the latter on the stem, while the drupelets of

the blackberry and dewberry adhere to the receptacle and the latter

separates from the stem.

=884. The berry.=—In the true berry both exocarp (including

mesocarp) and endocarp are fleshy or juicy. Good examples are found

in cranberries, huckleberries, gooseberries, currants, snowberries,

tomatoes, etc. The calyx and wall of the pistil are adnate, and in

fruit become fleshy so that the seeds are imbedded in the pulpy juice.

The seeds themselves are more or less stony. In the case of berries,

as well as in strawberries, raspberries, and blackberries, the fruits

are eagerly sought by birds and other animals for food. The seeds being

hard are not digested, but are passed with the other animal excrement

and thus gain dispersal.

V. Reinforced, or Accessory, Fruits.

When the torus (receptacle) is grown to the pericarp in fruit, the

fruit is said to be _reinforced_. The torus may enclose the pericarps,

or the latter may be seated upon the torus.

[Illustration: Fig. 478. Fruit of raspberry.]

=885. In the strawberry= the receptacle of the flower becomes

larger and fleshy, while the “seeds,” which are akenes, are sunk in the

surface and are hard and stony. The strawberry thus differs from the

raspberry and blackberry, but like them it is not a true berry.

=886. The apple, pear, quince, etc.=—In the flower the calyx,

corolla, and stamens are perigynous, i.e., they are seated on the

margin of the receptacle, or torus, which is elevated around the

pistils. In fruit the receptacle becomes consolidated with the wall

of the ovary (with the pericarp). The torus thus _reinforces_ the

pericarp. The torus and outer portion of the pericarp become fleshy,

while the inner portion of the pericarp becomes papery and forms the

“core.” The calyx persists on the free end of the fruit. Such a fruit

is called a _pome_. The receptacle, or torus, of the rose-flower,

closely related to the apple, is instructive when used in comparison.

The rose-fruit is called a “hip.”

=887. The pepo.=—The fruit of the squash, pumpkin, cucumber,

etc., is called a _pepo_. The outer part of the fruit is the receptacle

(or torus), which is consolidated with the outer part of the

three-loculed ovary. The calyx, which, with the corolla and stamens,

was epigynous, falls off from the young fruit.

VI. Fruits of Gymnosperms.

The fruits of the gymnosperms differ from nearly all of the angiosperms

in that the seed formed from the ripened ovule is naked from the first,

i.e., the ovary, or carpel, does not enclose the seed.

=888. The cone-fruit= is the most prominent fruit of the

gymnosperms, as can be seen in the cones of various species of pine,

spruce, balsam, etc.

=889. Fleshy fruits of the gymnosperms.=—Some of the fleshy

fruits resemble the stone-fruits and berries of the angiosperms. The

_cedar_ “_berries_,” for example, are fleshy and contain several seeds.

But the fleshy part of the fruit is formed, not from pericarp, since

there is no pericarp, but from the outer portion of the ovules, while

the inner walls of the ovules form the hard stone surrounding the

endosperm and embryo. An examination of the pistillate flower of the

cedar (juniper) shows usually three flask-shaped ovules on the end

of a fertile shoot subtended by as many bracts (carpels?). The young

ovules are free, but as they grow they coalesce, and the outer walls

become fleshy, forming a berry-like fruit with a three-rayed crevice

at the apex marking the number of ovules. The red fleshy fruit of the

yew (taxus) resembles a drupe which is open at the apex. The stony

seed is formed from the single ovule on the fertile shoot, while the

red cup-shaped fleshy part is formed from the outer integument of the

ovule. The so-called “aril” of the young ovule is a rudimentary outer

integument.

The fruit of the maidenhair tree (ginkgo) is about the size of a plum

and resembles very closely a stone-fruit. But it is merely a ripened

ovule, the outer layer becoming fleshy while the inner layer becomes

stony and forms the pit which encloses the embryo and endosperm.

The so-called “aril,” or “collar,” at the base of the fruit is the

rudimentary carpel, which sometimes is more or less completely expanded

into a true leaf. The fruit of cycas is similar to that of ginkgo, but

there is no collar at the base. In zamia the fruit is more like a cone,

the seeds being formed, however, on the under sides of the scales.

VII. The “Fruit” of Ferns, Mosses, etc.

=890. The term “fruit”= is often applied in a general or popular

sense to the groups of spore-producing bodies of ferns (_fruit dots_,

or _sori_), the spore-capsules of mosses and liverworts, and also to

the fruit-bodies, or spore-bearing parts, of the fungi and algæ.

CHAPTER XLV.

SEED DISPERSAL.

=891. Means for dissemination of seeds.=—During late summer

or autumn a walk in the woods or afield often convinces us of the

perfection and variety of means with which plants are provided for the

dissemination of their seeds, especially when we discover that several

hundred seeds or fruits of different plants are stealing a ride at

our expense and annoyance. The hooks and barbs on various seed-pods

catch into the hairs of passing animals and the seeds may thus be

transported considerable distances. Among the plants familiar to us,

which have such contrivances for unlawfully gaining transportation,

are the beggar-ticks or stick tights, or sometimes called bur-marigold

(bidens), the tick-treefoil (desmodium), or cockle-bur (xanthium), and

burdock (arctium).

[Illustration: Fig. 479. Bur of bidens or bur-marigold, showing barbed

seeds.]

[Illustration: Fig. 480. Seed pod of tick-treefoil (desmodium); at the

right some of the hooks greatly magnified.]

=892.= Other plants like some of the sedges, etc., living on the

margins of streams and of lakes, have seeds which are provided with

floats. The wind or the flowing of the water transports them often to

distant points.

=893.= Many plants possess attractive devices, and offer a

substantial reward, as a price for the distribution of their seeds.

Fruits and berries are devoured by birds and other animals; the seeds

within, often passing unharmed, may be carried long distances. Starchy

and albuminous seeds and grains are also devoured, and while many

such seeds are destroyed, others are not injured, and finally are

lodged in suitable places for growth, often remote from the original

locality. Thus animals willingly or unwillingly become agents in the

dissemination of plants over the earth. Man in the development of

commerce is often responsible for the wide distribution of harmful as

well as beneficial species.

[Illustration: Fig. 481. Seeds of geum showing the hooklets where the

end of the style is kneed.]

=894.= Other plants are more independent, and mechanisms are

employed for violently ejecting seeds from the pod or fruit. The

unequal tension of the pods of the common vetch (Vicia sativa) when

drying causes the valves to contract unequally, and on a dry summer day

the valves twist and pull in opposite directions until they suddenly

snap apart, and the seeds are thrown forcibly for some distance. In the

impatiens, or touch-me-not as it is better known, when the pods are

ripe, often the least touch, or a pinch, or jar, sets the five valves

free, they coil up suddenly, and the small seeds are thrown for several

yards in all directions. During autumn, on dry days, the pods of the

witch hazel contract unequally, and the valves are suddenly spread

apart, and the seeds are hurled away.

Other plants have seeds provided with tufts of pappus, or hair-like

masses, or wing-like outgrowths which serve to buoy them up as they are

whirled along, often miles away. In late spring or early summer the

pods of the willow burst open, exposing the seeds, each with a tuft of

white hairs making a mass of soft down. As the delicate hairs dry, they

straighten out in a loose spreading tuft, which frees the individual

seeds from the compact mass. Here they are caught by currents of air

and float off singly or in small clouds.

[Illustration: Fig. 482. Touch-me-not (Impatiens fulva); side and front

view of flower below; above unopened pod, and opening to scatter the

seed.]

=895. The prickly lettuce.=—In late summer or early autumn the

seeds of the prickly lettuce (Lactuca scariola) are caught up from the

roadsides by the winds, and carried to fields where they are unbidden

as well as unwelcome guests. This plant is shown in fig. 483.

=896. The wild lettuce.=—A related species, the wild lettuce

(Lactuca canadensis) occurs on roadsides and in the borders of fields,

and is about one meter in height. The heads of small yellow or purple

flowers are arranged in a loose or branching panicle. The flowers are

rather inconspicuous, the rays projecting but little above the apex of

the enveloping involucral bracts, which closely press together, forming

a flowerhead more or less flask-shaped.

[Illustration: Fig. 483. Lactuca scariola.]

At the time of flowering the involucral bracts spread somewhat at the

apex, and the tips of the flowers are a little more prominent. As the

flowers then wither, the bracts press closely together again and the

head is closed. As the seeds ripen the bracts die, and in drying bend

outward and downward, around the flower stem below, or they fall away.

The seeds are thus exposed. The dark brown achenes stand over the

surface of the receptacle, each one tipped with the long slender

beak of the ovary. The “pappus,” which is so abundant in many of the

plants belonging to the composite family, forms here a pencil-like

tuft at the tip of this long beak. As the involucral bracts dry and

curve downward, the pappus also dries, and in doing so bends downward

and stands outward, bristling like the spokes of a small wheel. It is

an interesting coincidence that this takes place simultaneously with

the pappus of all the seeds of a head, so that the ends of the pappus

bristles of adjoining seeds meet, forming a many-sided dome of a

delicate and beautiful texture. This causes the beaks of the achenes to

be crowded apart, and with the leverage thus brought to bear upon the

achenes they are pried off the receptacle. They are thus in a position

to be wafted away by the gentlest zephyr, and they go sailing away on

the wind like a miniature parachute. As they come slowly to the ground

the seed is thus carefully lowered first, so that it touches the ground

in a position for the end which contains the root of the embryo to come

in contact with the soil.

=897. The milkweed, or silkweed.=—The common milkweed, or

silkweed (Asclepias cornuti), so abundant in rich grounds, is

attractive not only because of the peculiar pendent flower clusters,

but also for the beautiful floats with which it sends its seeds

skyward, during a puff of wind, to finally lodge on the earth.

[Illustration: Fig. 484. Milkweed (Asclepias cornuti); dissemination of

seed.]

=898.= The large boat-shaped, tapering pods, in late autumn, are

packed with oval, flattened, brownish seeds, which overlap each other

in rows like shingles on a roof. These make a pretty picture as the pod

in drying splits along the suture on the convex side, and exposes them

to view. The silky tufts of numerous long, delicate white hairs on the

inner end of each seed, in drying, bristle out, and thus lift the seeds

out of their enclosure, where they are caught by the breeze and borne

away often to a great distance, where they will germinate if conditions

become favorable, and take their places as contestants in the battle

for existence.

=899. The virgin’s bower.=—The virgin’s bower (Clematis

virginiana), too, clambering over fence and shrub, makes a show of

having transformed its exquisite white flower clusters into

grayish-white tufts, which scatter in the autumn gusts into hundreds

of arrow-headed, spiral plumes. The achenes have plumose styles, and

the spiral form of the plume gives a curious twist to the falling seed

(fig. 485).

[Illustration: Fig. 485. Seed distribution of virgin’s bower

(clematis).]

CHAPTER XLVI.

VEGETATION IN RELATION TO ENVIRONMENT.[47]

I. Factors Influencing Vegetation Types.

=900.= All plants are subject to the influence of environment

from the time the seed begins to germinate until the seed is formed

again, or until the plant ceases to live. A suitable amount of warmth

and moisture is necessary that the seed may germinate. Moisture may

be present, but if it is too cold, germination will not take place.

So in all the processes of life there are several conditions of the

environment, or the “outside” of plants, which must be favorable for

successful growth and reproduction. Not only is this true, but the

surroundings of plants to a large extent determine the kind of plants

which can grow in particular localities. It is also evident that the

reaction of environment on plants has in a large measure caused them

to take on certain forms and structures which fit them better to exist

under local conditions. In other cases where plants have varied by

mutation (p. 338) some of the new forms may be more suited to the

conditions of environment than others and they are more apt to survive.

These conditions of environment acting on the plant are _factors_ which

have an important determining influence on the existence, habitat,

habit, and form of the plant. These factors are sometimes spoken of

as _ecological factors_, and the study of plants in this relation is

sometimes spoken of as ecology,[48] which means a study of plants in

their home or a study of the household relations of plants. These

factors are of three sorts: 1st, physical factors; 2d, climatic

factors; 3d, biotic factors.

=901. Physical factors.=—Some of these factors are water, light,

heat, wind, chemical or physical condition of the soil, etc. _Water_

is a very important factor for all plants. Even those growing on land

contain a large percentage of water, which we have seen is rapidly lost

by transpiration, and unless water is available for root absorption

the plant soon suffers, and aquatic plants are injured very quickly by

drying when taken from the water. Excess of soil water is injurious to

some plants. _Light_ is important in photosynthesis, in determining

direction of growth as well as in determining the formation of suitable

leaves in most plants, and has an influence in the structure of the

leaf according as the light may be strong, weak, etc. _Heat_ has great

influence on plant growth and on the distribution of plants. The

growth period for most vegetation begins at 6° C. (= 43° F.), or in

the tropics at 10°-12° C., but a much higher temperature is usually

necessary for reproduction. Some arctic algæ, however, fruit at 1.8° C.

The upper limit favorable for plants in general is 45°-50° C., while

the optimum temperature is below this. Very high temperatures are

injurious, and fatal to most plants, but some algæ grow in hot springs

where the temperature reaches 80°-90° C. Some desert plants are able to

endure a temperature of 70° C., while some flowering plants of other

regions are killed at 45° C. Some plants are specifically susceptible

to cold, but most plants which are injured by freezing suffer because

the freezing is a drying process of the protoplasm (see p. 374). _Wind_

may serve useful purposes in pollination and in aeration, but severe

winds injure plants by causing too rapid transpiration, by felling

trees, by breaking plant parts, by deforming trees and shrubs, and by

mechanical injuries from “sand-blast.” _Ground covers_ protect plants

in several ways. Snow during the winter checks radiation of heat from

the ground so that it does not freeze to so great a depth, and this is

very important for many trees and shrubs. It also prevents alternate

freezing and thawing of the ground, which “heaves” some plants from

the soil. Leaves and other plant remains mulch the soil and check

evaporation of water. The influence of the _chemical condition_ of the

soil is very marked in alkaline areas where the concentration of salt

in the soil permits a very limited range of species. So the physical

and mechanical conditions of the soil influence plants because the

moisture content of the ground is so closely dependent on its physical

condition. Rocky and gravelly soil, other things being equal, is dry.

Clay is more retentive of moisture than sand, and moisture also varies

according to the per cent of humus mixed with it, the humus increasing

the percentage of moisture retained.

=902. Climatic factors.=—These factors are operative over very

wide areas. There are two climatic factors: rainfall or atmospheric

moisture, and temperature. A very low annual rainfall in warm or

tropical countries causes a desert; an abundance of rain permits the

growth of forests; extreme cold prevents the growth of forests and

gives us the low vegetation of arctic and alpine regions.

=903. Biotic factors.=—These are animals which act favorably

in pollination, seed distribution, or unfavorably in destroying or

injuring plants, and man himself is one of the great agencies in

checking the growth of some plants while favoring the growth of others.

Plants also react on themselves in a multitude of ways for good or

evil. Some are parasites on others; some in symbiosis (see p. 85) aid

in providing food; shade plants are protected by those which overtop

them; mushrooms and other fungi disintegrate dead plants to make humus

and finally plant food; certain bacteria by nitrification prepare

nitrates for the higher plants (see p. 83).

II. Vegetation Types and Structures.

=904. Responsive type of vegetation.=—In studying vegetation in

relation to environment we are more concerned with the form of the

plants which fits them to exist under the local conditions than we are

with the classification of plants according to natural relationships.

Plants may have the same vegetation type, grow side by side, and still

belong to very different floristic types. For example, the cactus,

yucca, three-leaved sumac, the sage-brush, etc., have all the same

general vegetation type and thrive in desert regions. The red oaks, the

elms, many goldenrods, trillium, etc., have the same general vegetation

type, but represent very different floristic types. The latter plants

grow in regions with abundant rainfall throughout the year, where

the growing season is not very short and temperature conditions are

moderate. Some goldenrods grow in very sandy soil which dries out

quickly. These have fleshy or succulent leaves for storing water,

and while they are of the same floristic type as goldenrods growing

in other places, the vegetation type is very different. The types of

vegetation which fit plants for growing in special regions or under

special conditions, they have taken on in response to the influence

of the conditions of their environment. While we find all gradations

between the different types of vegetation, looking at the vegetation

in a broad way, several types are recognized which were proposed by

Warming as follows:

=905. Mesophytes.=—These are represented by land plants under

temperate or moderate climatic and soil conditions. The normal

land vegetation of our temperate region is composed of mesophytes,

that is, the plants have mesophytic structures during the growing

season. The deciduous forests or thickets of trees and shrubs with

their undergrowth, the meadows, pastures, prairies, weeds, etc., are

examples. In those portions of the tropics where rainfall is great the

vegetation is mesophytic the year around.

=906. Xerophytes.=—These are plants which are provided with

structures which enable them to live under severe conditions of

dryness, where the air and soil are very dry, as in deserts or

semideserts, or where the soil is very dry or not retentive of

moisture, as in very sandy soil which is above ground water, or in

rocky areas. Since the plants cannot obtain much water from the soil

they must be provided with structures which will enable them to

retain the small amount they can absorb from the soil and give it off

slowly. Otherwise they would dry out by evaporation and die. Some

of the structures which enable xerophytic plants to withstand the

conditions of dry climate and soil are lessened leaf surface, increase

in thickness of leaf, increase in thickness of cuticle, deeply sunken

stomates, compact growth, also succulent leaves and stems, and in some

cases loss of the leaf. Evergreens of the north temperate and the

arctic regions are xerophytes.

=907. Hydrophytes.=—These are plants which grow in fresh water

or in very damp situations. The leaves of aerial hydrophytes are very

thin, have a thin cuticle, and lose water easily, so that if the air

becomes quite dry they are in danger of drying up even though the roots

may be supplied with an abundance of water. The aquatic plants which

are entirely submerged have often thin leaves, or very finely divided

or slender leaves, since these are less liable to be torn by currents

of water. The stems are slender and especially lack strengthening

tissue, since the water buoys them up. Removed from the water they

droop of their own weight, and soon dry up. The stems and leaves have

large intercellular spaces filled with air which aids in aeration and

in the diffusion of gases. Some use the term _hygrophytes_.

=908. Halophytes.=—These are salt-loving plants. They grow in

salt water, or in salt marshes where the water is brackish, or in

soil which contains a high per cent of certain salts, for example the

alkaline soils of the West, especially in the so-called “Bad Lands”

of Dakota and Nebraska, and in alkaline soils of the Southwest and

California. These plants are able to withstand a stronger concentration

of salts in the water than other plants. They are also found in soil

about salt springs.

=909. Tropophytes.=[49]—Tropophytes are plants which can live as

mesophytes during the growing season, and then turn to a xerophytic

habit in the resting season. Deciduous trees and shrubs, and perennial

herbs of our temperate regions, are in this sense tropophytes, while

many are at the same time mesophytes if they exist in the portions of

the temperate region where rainfall is abundant. In the spring and

summer they have broad and comparatively thin leaves, transpiration

goes on rapidly, but there is an abundance of moisture in the soil,

so that root absorption quickly replaces the loss and the plant does

not suffer. In the autumn the trees shed their leaves, and in this

condition with the bare twigs they are able to stand the drying effect

of the cold and winds of the winter because transpiration is now at a

minimum, while root absorption is also at a minimum because of the cold

condition of the soil. Perennial herbs like trillium, dentaria, the

goldenrods, etc., turn to xerophytic habit by the death of their aerial

shoots, while the thick underground shoot which is also protected by

its subterranean habit carries the plant through the winter.

=910.= While these different vegetation types are generally

dominant in certain climatic regions or under certain soil conditions,

they are not the exclusive vegetation types of the regions. For

example, in desert or semidesert regions the dominant vegetation

type is made up of xerophytes. But there is a mesophytic flora even

in deserts, which appears during the rainy season where temperature

conditions are favorable for growth. This is sometimes spoken of as the

rainy-season flora. The plants are annuals and by formation of seed can

tide over the dry season. So in the region where mesophytes grow there

are xerophytes, examples being the evergreens like the pines, spruces,

rhododendrons; or succulent plants like the stonecrop, the purslane,

etc. Then among hydrophytes the semiaquatics are really xerophytes. The

roots are in water, and absorption is slow because there are no root

hairs, or but few, and the aerial parts of the plant are xerophytic.

III. Plant Formations.

=911.= The term plant formation is applied to associations of

plants of the same kind, though there is a great difference in the use

of the word by different writers which leads to some confusion.[50] It

is sometimes applied to an association of individuals of a species, or

of several species occupying a rather definite area of ground where the

soil conditions are not greatly different (individual formation); by

others it is applied to the plants of a definite physiographic area, as

a swamp, moor, strand, or beach, bank, rock hill, clay hill, ravine,

bluff, etc. (principal formation); and in a broad sense it is applied

to the plants of climatic regions, of those in bodies of water, etc.

(general formations). Space here is too limited to discuss all these

kinds of formations, but the nature of the general formations will be

pointed out. The general formations may be grouped into four divisions:

1st. Climatic formations.

2d. Edaphic formations.

3d. Aquatic formations.

4th. Culture formations.

=912. Climatic formations.=—Climatic influences extend over

wide regions, so that climate controls the general type of vegetation

of a region. In the sense of control there are two climatic factors,

temperature and moisture, especially soil moisture. Temperature exerts

a controlling influence over the vegetation type only where the total

heat during the period of growth and reproduction is very low. This

occurs in polar lands and at high elevations where the climate is

alpine. In the temperate and tropical regions of the globe moisture,

not heat, controls the general vegetation type. These vegetation types

in general are coincident with rainfall distribution, and Schimper

recognizes here three types, which with the arctic-alpine type would

make four climatic formations as follows:

1st. _The woodland formation._—This formation is characterized by

trees and shrubs, and it is what is called a _close_ formation. By this

it is meant that so far as the climate is concerned the conditions are

favorable for the development of trees and shrubs in such abundance

that they become the dominant vegetation type of the region and grow

close together. Other plants, as herbs, grasses, etc., occur, but

they grow as subordinate elements of the general vegetation type, and

as undergrowth. The land portion of the globe, therefore, outside of

arctic and alpine regions, where the annual precipitation is 40 to 60

or more inches, is the area for woodland formation. In some places,

the eastern part of England, for example, the annual precipitation is

25 to 30 inches, but the cool temperature permits a forest growth. It

is true there are places where forests do not grow,—where man cuts

them down, for example. But if cultivated lands in this region were

allowed to go to waste, they would in time grow up to forest again.

So there are swamps where the soil is too wet for trees, or sandy or

rocky areas where there is not a sufficient amount of soil or water to

support forest trees. But here it is the soil conditions, not climatic

conditions, which prevent the development of the forest. But we know

that swamps are being filled in and the ground gradually becoming

higher and drier, and that soil is slowly accumulating in rocky areas,

so that in time if left to natural forces these places would become

forested. So this area of heavy annual rainfall is a _potential_ forest

area. These areas are determined by warm currents of moisture-laden

air from the ocean moving over cooler land areas where the moisture

is precipitated. In general these areas are along the coasts of great

continents and on mountains. Therefore the interior of a continent is

apt to be dry because most of the moisture has been precipitated before

it reaches the interior. Deserts or steppes are therefore usually near

the interior of continents. Some exceptions to this general rule are

found: central South America, which is a region of exceptional rainfall

because the moisture-laden winds here come from the warmest part of the

ocean; the desert region west of the Andes mountains, where the winds

are not favorable; southern California, where the winds come chiefly

from a cooler portion of the Pacific ocean and move over an area of

high temperature, etc.

[Illustration: Fig. 486. Typical prairie scene, a few miles west of

Lincoln, Nebraska. (Bot. Dept., Univ. Nebraska.)]

2d. _Grassland formation._—Grasses form the dominant vegetation

type where the annual rainfall is approximately 15 to 25 inches. In

true grasslands the formation is a close one since there is still a

sufficient amount of moisture to provide for all the plants which can

stand on the ground. Yet there is not enough moisture to permit the

growth of forest as the dominant type without aid and protection by

man. The so-called prairie regions are examples. Trees and shrubs do

occur, but they cannot compete successfully with the grasses because

the climatic conditions are favorable for the latter and unfavorable

for the former. On the border line between forest and prairie the line

of division is not a clear-cut one because conditions grade from one to

the other. The two formations are somewhat mixed, like the outposts of

contending armies, arms of the forest or prairie extending out here and

there. In the United States the prairies extend from Illinois to about

the 100th meridian, and beyond this to the foothills of the Rockies and

southwest to the Sonora Nevada desert the region is drier, the rainfall

varying from 10 to 20 inches. This is the area of the Great Plains,

and while grasses of the bunch type are dominant, they make a more or

less open formation because the moisture is not sufficient to supply

all the plants which could be crowded on the ground, each individual

tuft needing an area of ground surrounding it on which it can draw

for moisture. Such a formation is an open one, and in this respect is

similar to desert formations.

[Illustration: Fig. 487. Winter range in northwestern Nevada, showing

open formation; white sage (Eurotia lanata) in foreground, salt-bush

(Atriplex confertifolia) and bud-sage (Artemisia spinescens) at base

of hill, red sage (Kochia americana) on the higher slope. (After

Griffiths, Bull. 38, Bureau Plant Ind., U. S. Dept. Agr.)]

3d. _Desert formations._—These occur where the annual rainfall is

still lower, 10 to 4 inches or even less, 2 to 3 inches, while in

one place in Chili it is as low as ½ inch. In the great Sahara desert

it is about 8 inches, while in the Sonora Nevada desert in the

southwestern United States it is 4 to 8 inches. Here the formation is

an open one. In the forest and prairie formations the plants compete

with each other for occupancy of the ground, since climatic conditions

are favorable, so that the struggle against climate is not severe.

But in the desert plants do not compete with each other; since the

climate is so austere, the struggle is against the climate. Hence

plants stand at some distance from each other because the roots need

the moisture from the ground for some distance around them. There is

not enough moisture for all the plants that begin, and those which get

the start take the moisture away from the intervening ones, which then

die. Since the struggle is against the adverse conditions of climate

and not a competition between plants to occupy the ground, no one

floristic type dominates as in the case of the grasses and forests of

the grassland and woodland formations, but grassland and woodland types

grow together. So we find grasses, trees, and shrubs growing without

competition in the desert. The dominant vegetation type is xerophytic.

[Illustration: Fig. 488. Northern limit of tree growth, Alaska.

(Copyright, 1899, by E. H. Harriman.)]

4th. _Arctic-alpine formation._ This formation extends from the limit

of tree growth to the region of perpetual ice and snow. The forest

here comes in competition with climate, with the severe cold of the

long winter night, so that tree growth is limited, and on the border

line with the woodland formation the trees are stunted, bent to one

side by the heavy snows, or the tops are killed by the cold wind. The

arctic zone of plant growth is sometimes spoken of as the “cold waste,”

since conditions here are somewhat similar to those in the desert, the

extreme cold exercising a drying effect on vegetation, and the

vegetation type then is largely xerophytic.

=913. Edaphic[51] formations.=—Edaphic formations may occur

in any of the climatic-formation areas. They are controlled by the

condition of soil or ground. The condition of the soil is unfavorable

for the growth of the general vegetation type of that region, or is

more favorable for another vegetation type, so that soil conditions

overcome the climatic conditions. These areas include swamps, moors,

the strand or beach, rocky areas, etc., as well as oases in the desert,

warm oases in the arctic zone, river bottoms in the prairie and plains

region, alkaline areas, etc. The edaphic formations may be close or

open according to the nature of the soil. The edaphic formations then

are infiltrated in the climatic formations, the different vegetation

types fitting together like pieces of mosaic, which can be seen in some

places from a mountain top, or if one could take a bird’s-eye view of

the landscape or from a balloon.

=914. Aquatic formations.=—These are made up of water plants and

are of two general kinds: fresh-water plant formations in ponds, lakes,

streams; and salt-water plant formations in the ocean and inland salt

seas.

=915. Culture formations.=—Culture formations are largely

controlled by man, who destroys the climatic or edaphic formation and

by cultivation protects cultivated types, or by allowing land to go to

“waste” permits the growth of weeds, though weeds are often abundant

in the culture areas. In general the culture formations may be grouped

into two subdivisions: 1st, the vegetation of cultivated places; and

2d, the vegetation of waste places, as abandoned fields, roadsides, etc.

IV. Plant Societies.

=916. Plant societies= are somewhat definite associations of the

vegetation of an area marked by physiographic conditions. A single

plant society is nearly if not altogether identical with a “_principal

formation_,” but is a more popular expression, and besides includes all

the plants growing on the area, while in the use of the term “principal

formation” we have reference mainly to the dominant plants and the most

conspicuous subordinate species.

=917. Complex character of plant societies.=—In their broadest

analysis all plant societies are complex. Every plant society has one

or several dominant species, the individuals of which, because of

their number and size, give it its peculiar character. The society may

be so nearly pure that it appears to consist of the individuals of a

single species. But even in those cases there are small and conspicuous

plants of other species which occupy spaces between the dominant ones.

Usually there are several or more kinds in the same society. The larger

individuals come into competition for first place in regard to ground

and light, the smaller ones come into competition for the intervening

spaces for shade, and so on down in the scale of size and shade

tolerance. Then climbing plants (lianas) and epiphytes (lichens, algæ,

mosses, ferns, tree orchids, etc.) gain access to light and support by

growing on other larger and stouter members of the society.

Parasites (dodder, mistletoes, rusts, smuts, mildews, bacteria, etc.)

are present, either actually or potentially, in all societies, and in

their methods of obtaining food sap the life and health of their hosts.

Then come the scavenger members, whose work it is to clean house, as it

were, the great army of saprophytic fungi (molds, mushrooms, etc.), and

bacteria ready to lay hold on dead and dying leaves, branches, trunks,

roots, etc., disintegrate them, and reduce them to humus, where other

fungi change them into a form in which the larger members of the plant

society can utilize them as plant food and thus continue the cycle of

matter through life, death, decay, and into life again. Mycorhiza (see

Chapter IX) or other forms of mutualistic symbiosis occur which make

atmospheric nitrogen available for food, or shorten the path from humus

to available food, or the humus plants feed on the humus directly.

Nor should we leave out of account the myriads of nitrate and nitrite

bacteria (see Chapter IX) which make certain substances in the soil

available to the higher members of the society. Most plant societies

are also benefited or profoundly influenced in other ways by animals,

as the flower-visiting insects, birds which feed on injurious insects,

the worms which mellow up the soil and cover dead organic matter so

that it may more thoroughly decay. In short, every plant society is

a great cosmos like the universe itself of which it is a part, where

multitudinous forms, processes, influences, evolutions, degenerations,

and regenerations are at work.

=918. Forest Societies.=[52]—Each different climatic belt or

region has its characteristic forest. For example, the forests of

the Hudsonian zone in North America are different from those of

the Canadian zone, and these in turn different from those in the

transition zone (mainly in northern United States). The forests of

the Rocky mountains and of the Pacific coast differ from those of the

Alleghanian, Carolinian (mainly middle United States) or Austroriparian

(southern United States) areas. Finally, tropical forests are

strikingly different from those of other regions. Similar variations

occur in the forests of other regions of the globe. The character of

these forests depends largely on climatic factors. The character of

the forest varies, however, even in the same climatic area, dependent

on soil conditions, or success in seeding and ground-gaining of the

different species in competition, etc.

=919. General structure of the forest.=—Structurally the forest

possesses three subdivisions: the floor, the canopy, and the interior.

The floor is the surface soil, which holds the rootage of the trees,

with its covering of leaf-mold and carpet of leaves, mosses, or other

low, more or less compact vegetation. The canopy is formed by the

spreading foliage of the tree crowns, which, in a forest of an even

and regular stand, meet and form a continuous mass of foliage through

which some light filters down into the interior. Where the stand is

irregular, i.e., the trees of different heights, the canopy is said to

be “compound” or “storied.” Where it is uneven, there are open places

in the canopy which admit more light, in which case the undergrowth

may be different. The interior of the forest lies between the canopy

and the floor. It provides for aeration of the floor and interior

occupants, and also room for the boles or tree trunks (called by

foresters the wood mass of the forest) which support the canopy and

provide the channels for communication and food exchange between the

floor and canopy. The canopy manufactures the carbohydrate food and

assimilates the mineral and proteid substances absorbed by the roots in

the soil; and also gets rid of the surplus water needed for conveying

food materials from the floor to the place where they are elaborated.

It is the seat where energy is created for work, and also the place for

seed production.

[Illustration: Fig. 489. Mature forest of redwood (Sequoia

sempervirens). (Bureau of Forestry, U. S. Dept. Agr., Bull. 38.)]

=920. Longevity of the forest.=—The forest is capable of

self-perpetuation, and, except in case of unusual disaster or the

action of man, it should live indefinitely. As the old trees die they

are gradually replaced by younger ones. So while trees may come and

trees may go, the forest goes on forever.

=921. Autumn colors.=—One of the striking effects produced by

the deciduous forests is that of the autumn coloring of the leaves.

It is more pronounced in the forests of the United States than in

corresponding life zones in the eastern hemisphere because of the

greater number of species. With the disintegration of the chlorophyll

bodies, other colors, which in some cases were masked by the green,

appear. In other cases decomposition products result in the formation

of other colors, as red, scarlet, yellow, brown, purple, maroon, etc.,

in different species. These coloring substances to some extent are

believed to protect the nitrogenous substances in the leaf from injury.

The colors absorb the sun’s rays, which otherwise might destroy these

nitrogenous substances before they have passed back through the petiole

of the leaf into the stem, where they may be stored for food. The

gorgeous display of color, then, which the leaves of many trees and

shrubs put on is one of the many useful adaptations of the plants.

=922. Importance of the forest in the disposal of rainfall.=—The

importance of the forest in disposing of the rainfall is very great.

The great accumulation of humus on the forest floor holds back the

water both by absorption and by checking its flow, so that it does not

immediately flow quickly off the slopes into the drainage system of the

valley. It percolates into the soil. Much of it is held in the humus

and soil. What is not retained thus filters slowly through the soil

and is doled out more gradually into the valley streams and mountain

tributaries, so that the flood period is extended, and its injury

lessened or entirely prevented, because the body of water moving at any

one time is not dangerously high. The winter snow is shaded and in the

spring melts slowly, and the spring freshets are thus lessened. The

action of the leaves and humus in retarding the flow of the water

prevents the washing away of the soil; the roots of trees bind the soil

also and assist in holding it.

=923. Absence of forest encourages serious floods.=—The great

floods of the Mississippi and its tributaries are due to the rapidity

with which heavy rainfall flows from the rolling prairies of the west,

and from the deforested areas west of the Alleghany system. The serious

floods in recent years in some of the South Atlantic States are in

part due to the increasing area of deforestation in the Blue Ridge and

southern Alleghany system.

=924. The prairie and plains societies.=—These are to be found

in the grassland formation. In the prairies “meadows” are formed in

the lower ground near river courses where there is greater moisture

in soil. The grasses here are principally “sod-formers” which have

creeping underground stems which mat together, forming a dense sod. On

the higher and drier ground the “bunch” grasses, like buffalo-grass,

beard-grass, or broom-sedge, etc., are dominant, and in the drier

regions as one approaches desert conditions the vegetation gradually

takes on more the character of the desert, so that in the plains

sage-brush, the prickly-pear cactus, etc., occur. Besides the dominant

vegetation of the society there are subordinate species, and the

societies are especially marked by a spring and autumn flora of

conspicuous flowering plants which are mixed with the grasses.

=925. Desert societies.=—These are composed of plants which

possess a form or structure which enables them to exist in a very

dry climate where the air is very dry and the soil contains but

little moisture. The true desert plants are perennial. The growth and

flowering period occurs during the rainy season, or those portions

of the rainy season when the temperature is favorable, and they rest

during the very dry season and cold. Characteristic desert plants are

the cacti with thick succulent green stems or massive trunks, the

leaves being absent or reduced to mere spines which no longer function

in photosynthesis; yuccas with thick, narrow and long leaves with a

firm and thick cuticle; small shrubs or herbs with compact rounded

habit and small thick gray leaves. All of these structures conserve

moisture. The mesquite tree is one of the common trees in portions

of the Sonora Nevada desert. Besides the true desert plants, desert

societies have a rainy-season flora consisting of annuals, which can

germinate, vegetate, flower, and seed during the period of rain and

before the ground moisture has largely disappeared, and these pass the

resting period in seed.

[Illustration: Fig. 490. Desert vegetation, Arizona, showing large

succulent trunks of cactus with shrubs and stunted trees. Open

formation. (Photograph by Tuomey.)]

[Illustration: Fig. 491. Polar tundra with scattered flowers, Alaska.

(Copyright by E. H. Harriman.)]

[Illustration: Fig. 492. Perennial rosette plant from alpine flora of

the Andes, showing short stem, rosette of leaves, and large flower.

(After Schimper.)]

=926. Arctic-alpine societies.=—The most striking of the arctic

plant societies are the “polar tundra,” extensive mats of vegetation

largely made up of mosses, lichens, etc., only partially decayed

because of the great cold of the subsoil, and perhaps also because

of humus acid in the partially decayed vegetation. These tundras

are brightened by numerous flowering plants which are characterized

by short stems, a rosette of leaves near the ground, and by large

bright-colored flowers. Heaths, saxifrages, and dwarf willow abound.

Alpine plant societies are similar to the arctic, although some of the

conditions are more severe than in the arctic region. This is

principally due to the fact that during the summer while the plants are

growing they are subject to a high temperature during the day and a

very low temperature at night, whereas during the summer in arctic

regions while the plants are growing there is continuous warmth for

growth and continuous light for photosynthesis. Five types of alpine

plants are recognized by some. 1st. _Elfin tree._ This type has short,

gnarled, often horizontal stems, as seen in pines, birches, and other

trees growing in alpine heights. 2d. _The alpine shrubs._ In the

highest alpine belts they are dwarfed and creeping, richly branched and

spreading close to the ground, while at lower belts they are more like

lowland shrubs. 3d. _The cushion type._ The branching is very profuse

and the branches are short and touch each other on all sides, forming

compact masses (examples saxifrages, androsace, mosses, etc.). 4th.

_Rosette plants._ These are perennial, short stems and very strong

roots, and play an important part in the alpine meadows. 5th. _Alpine

grasses._ These usually have much shorter leaves than grasses of the

lowlands and consequently form a low sward.