MORPHOLOGY OF THE NUCLEUS AND SIGNIFICANCE OF GAMETOPHYTE AND
SPOROPHYTE.
=676.= In the development of the spores of the liverworts,
mosses, ferns, and their allies, as well as in the development of the
microspores of the gymnosperms and angiosperms, we have observed that
four spores are formed from a single mother cell. These mother cells
are formed as a last division of the fertile tissue (archesporium) of
the sporangium. In ordinary cell division the nucleus always divides
prior to the division of the cell. In many cases it is directly
connected with the laying down of the dividing cell wall.
[Illustration: Fig. 402. Forming spores in mother cells (Polypodium
vulgare).]
[Illustration: Fig. 403. Spores just mature and wall of mother cell
broken (Asplenium bulbiferum).]
=677. Direct division of the nucleus.=—The nucleus divides in two
different ways. On the one hand the process is very simple. The nucleus
simply fragments, or cuts itself in two. This is direct division.
=678. Indirect division of the nucleus.=—On the other hand very
complicated phenomena precede and attend the division of the nucleus,
giving rise to a succession of nuclear figures presented by a definite
but variable series of evolutions on the part of the nuclear substance.
This is _indirect division_ of the nucleus, or _karyokinesis_. Indirect
division of the nucleus is the usual method, and it occurs in the
normal growth and division of the cell. The nuclear figures which are
formed in the division of the mother cell into the four spores are
somewhat different from those occurring in vegetative division, but
their study will serve to show the general character of the process.
=679. Chromatin and linin of the nucleus.=—In figure 404 is
represented a pollen mother cell of the May-apple (podophyllum). The
nucleus is in the resting stage. There is a network consisting of very
delicate threads, the _linin_ network. Upon this network are numerous
small granules, and at the junction of the threads are distinct knots.
The nucleolus is quite large and prominent. The numerous small granules
upon the linin stain very deeply when treated with certain dyes used in
differentiating the nuclear structure. This deeply staining substance
is the _chromatin_ of the nucleus.
[Illustration: Fig. 404. Pollen mother cell of podophyllum, resting
nucleus. Chromatin forming a network.]
[Illustration: Fig. 405. Spirem stage of nucleus. _nu_, nuclear cavity;
_n_, nucleolus; _Sp_, spirem.]
[Illustration: Fig. 406. Forming spindle, threads from protoplasm with
several poles, roping the chromosomes up to nuclear plate.
(Figures 404-406 after Mottier.)]
=680. The chromatin skein.=—One of the first nuclear figures in the
preparatory stages of division is the chromatin _skein_ or _spirem_.
The chromatin substance unites to form this. The spirem is in the form
of a narrow continuous ribbon, or band, woven into an irregular skein,
or gnarl, as shown in figure 405. This band splits longitudinally
into two narrow ones, and then each divides into a definite number
of segments, about eight in the case of podophyllum. Sometimes the
longitudinal splitting of the band appears to take place after the
separation into the chromatin segments. The segments remain in pairs
until they separate at the nuclear plate.
[Illustration: Fig. 407.
Karyokinesis in pollen mother cells of podophyllum. At the left the
spindle with the chromosomes separating at the nuclear plate; in the
middle figure the chromosomes have reached the poles of the spindle,
and at the right the chromosomes are forming the daughter nuclei.
(After Mottier.)]
=681. Chromosomes, nuclear plate, and nuclear spindle.=—Each
one of these rod-like chromatin segments is a _chromosome_.
The pairs of chromosomes arrange themselves in a median plane
of the nucleus, radiating somewhat in a stellate fashion, forming
the _nuclear plate_, or _monaster_. At the same time threads of the
protoplasm (kinoplasm) become arranged in the form of a spindle,
the axis of which is perpendicular to the nuclear plate of chromosomes,
as shown in figure 407, at left. Each pair of chromosomes
now separate in the line of the division of the original spirem,
one chromosome of each pair going to one pole of the spindle,
while the other chromosome of each pair goes to the opposite pole.
The chromosomes here unite to form the daughter nuclei. Each of these
nuclei now divide as shown in figure 409 (whether the chromosomes in
this second division in the mother cell split longitudinally or divide
transversely has not been definitely settled), and four nuclei are
formed in the pollen mother cell. The protoplasm about each one of
these four nuclei now surrounds itself with a wall and the spores are
formed.
[Illustration: Fig. 408. Different stages in the separation of divided
U-shaped chromosomes at the nuclear plate. (After Mottier.) In
podophyllum.]
[Illustration: Fig. 409. Second division of nuclei in pollen mother
cell of podophyllum, chromosomes at poles.]
[Illustration: Fig. 410. Chromosomes uniting at poles to form the
nuclei of the four spores. (After Mottier.)]
=The number of chromosomes usually the same in a given species
throughout one phase of the plant.=—In those plants which have been
carefully studied, the number of chromosomes in the dividing nucleus
has been found to be fairly constant in a given species, through all
the divisions in that stage or phase of the plant, especially in the
embryonic, or young growing parts. For example, in the prothallium, or
gametophyte, of certain ferns, as osmunda, the number of chromosomes
in the dividing nucleus is always twelve. So in the development of the
pollen of lilium from the mother cells, and in the divisions of the
antherid cell to form the generative cells or sperm cells, there are
always twelve chromosomes so far as has been found. In the development
of the egg of lilium from the macrospore there are also twelve
chromosomes.
=When fertilization takes place the number of chromosomes is doubled
in the embryo.=—In the spermatozoid of osmunda then, as well as in
the egg, since these are developed on the gametophyte, there are twelve
chromosomes each. The same is true in the sperm cell (generative cell)
of lilium, and also in the egg-cell. When these nuclei unite, as they
do in fertilization, the paternal nucleus with the maternal nucleus,
the number of chromosomes in the fertilized egg, if we take lilium as
an example, is twenty-four instead of twelve; the number is doubled.
The fertilized egg is the beginning of the sporophyte, as we have seen.
Curiously throughout all the divisions of the nucleus in the embryonic
tissues of the sporophyte, so far as has been determined, up to the
formation of the mother cells of the spores, the number of chromosomes
is usually the same.
[Illustration: Fig. 411. Karyokinesis in sporophyte cells of
podophyllum (twice the number of chromosomes here that are found in the
dividing spore mother cells).]
=682. Reduction of the number of chromosomes in the nucleus.=—If
there were no reduction in the number of chromosomes at any point in
the life cycle of plants, the number would thus become infinitely
large. A reduction, however, does take place. This usually occurs,
either in the mother cell of the spores or in the divisions of its
nucleus, at the time the spores are formed. In the mother cells a sort
of pseudo-reduction is effected by the chromatin band separating into
one half the usual number of nuclear segments. So that in lilium during
the first division of the nucleus of the mother cell the chromatin band
divides into twelve segments, instead of twenty-four as it has done
throughout the sporophyte stage. So in podophyllum during the first
division in the mother cell it separates into eight instead of into
sixteen. Whether a qualitative reduction by transverse division of the
spirem band, unaccompanied by a longitudinal splitting, takes place
during the first or second karyokinesis is still in doubt. Qualitative
reduction does take place in some plants according to Beliaieff and
others. Recently the author has found that it takes place in Trillium
grandiflorum during the second karyokinesis, and in Arisæma triphyllum
the chromosomes divide both transversely and longitudinally during
the first karyokinesis forming four chromosomes, and a qualitative
reduction takes place here.
=683. Significance of karyokinesis and reduction.=—The precision
with which the chromatin substance of the nucleus is divided, when
in the spirem stage, and later the halves of the chromosomes are
distributed to the daughter nuclei, has led to the belief that this
substance bears the hereditary qualities of the organism, and that
these qualities are thus transmitted with certainty to the offspring.
In reduction not only is the original number of chromosomes restored,
it is believed by some that there is also a qualitative reduction of
the chromatin, i.e. that each of the four spores possesses different
qualitative elements of the chromatin as a result of the reducing
division of the nucleus during their formation.
The increase in number of chromosomes in the nucleus occurs with the
beginning of the sporophyte, and the numerical reduction occurs at the
beginning of the gametophyte stage. The full import of karyokinesis and
reduction is perhaps not yet known, but there is little doubt that a
profound significance is to be attached to these interesting phenomena
in plant life.
=684. The gametophyte may develop directly from the tissue of the
sporophyte.=—If portions of the sporophyte of certain of the
mosses, as sections of a growing seta, or of the growing capsule, be
placed on a moist substratum, under favorable conditions some of the
external cells will grow directly into protonemal threads. In some
of the ferns, as in the sensitive fern (onoclea), when the fertile
leaves are expanding into the sterile ones, protonemal outgrowths occur
among the aborted sporangia on the leaves of the sporophyte. Similar
rudimentary protonemal growths sometimes occur on the leaves of the
common brake (pteris) among the sporangia, and some of the rudimentary
sporangia become changed into the protonema. In some other ferns, as
in asplenium (A. filix-fœmina, var. clarissima), prothallia are borne
among the aborted sporangia, which bear antheridia and archegonia. In
these cases the gametophyte develops from the tissue of the sporophyte
without the intervention or necessity of the spores. This is _apospory_.
[Illustration: Fig. 412. Apogamy in Pteris cretica.]
=685. The sporophyte may develop directly from the tissue of the
gametophyte.=—In some of the ferns, Pteris cretica for example,
the embryo fern sporophyte arises directly from the tissue of the
prothallium, without the intervention of sexual organs, and in some
cases no sexual organs are developed on such prothallia. Sexual organs,
then, and the fusion of the spermatozoid and egg nucleus are not here
necessary for the development of the sporophyte. This is _apogamy_.
Apogamy occurs in some other species of ferns, and in other groups of
plants as well, though it is in general a rare occurrence except in
certain species, where it may be the general rule.
=686. Types of nuclear division.=—The nuclear figures in the
vegetative cells are usually different from those in the spore
mother cells. In the spore mother cells there are two types of
nuclear division. (1) The first division in the mother cell is called
_heterotypic_. The early stages of this division usually extend over a
longer period than the second, and the figures are more complex. Before
the chromosomes arrive at the nuclear plate they are often in the form
of rings, or tetrads, or in the form of X, V, or Y, and the number is
usually one half the number in the preceding cells of the sporophyte.
(2) The _homotypic_ division immediately follows the heterotypic and
the figures are simpler, often the chromosomes being of a hook form,
or sometimes much stouter than in the heterotypic division. In the
vegetative cells (sometimes called somatic cells, or body cells in
contrast with reproductive cells) there is another type, called by some
the _vegetative type_. The chromosomes here are often in the form of
the letter U, and the figures are much simpler than in the heterotypic
division. In the somatic cells of the sporophyte, as stated above,
the number of chromosomes is double that found in the heterotypic
and homotypic divisions of the mother cells and in the somatic cells
of the gametophyte, Fig. 411 represents a late stage in the division
of somatic cells in the sporophyte of podophyllum. The root-tips of
various plants as the onion, lily, etc., are excellent places in which
to study nuclear division in the somatic cells of the sporophyte.
=687. Comparison with animals.=—In animals there does not seem
to be anything which corresponds with the gametophyte of plants unless
the sperm cells and eggs themselves represent it. Heterotypic and
homotypic division with the accompanying reduction of the number of the
chromosomes takes place in animals usually in the mother cells of the
sperms and eggs. At the time of fertilization the number of chromosomes
is doubled, so that all the somatic cells (except in rare instances)
from the fertilized egg to the mother cells of sperms and eggs have the
doubled number of chromosomes. Reduction, therefore, takes place in
animals just prior to the formation of the gametes, while in plants it
takes place just prior to the formation of the gametophytes.
=688. Perhaps there is not a fundamental difference between
gametophyte and sporophyte.=—This development of sporophyte, or
leafy-stemmed plant of the fern (parag. 685), from the tissue of the
gametophyte is taken by some to indicate that there is not such a great
difference between the gametophyte and sporophyte of plants as others
contend. In accordance with this view it has been suggested that the
leafy-stemmed moss plant, as well as the leafy stem of the liverworts,
is homologous with the sporophyte or leafy stem of the fern plant; that
it arises by budding from the protonema; and that the sexual organs are
borne then on the sporophyte.
PART III.
PLANT MEMBERS IN RELATION TO ENVIRONMENT.
CHAPTER XXXVIII.
THE ORGANIZATION OF THE PLANT.
I. Organization of Plant Members.[37]
=689.= It is now generally conceded that the earliest plants to
appear in the world were very simple in form and structure. Perhaps the
earliest were mere bits of naked protoplasm, not essentially different
from early animal life. The simplest ones which are clearly recognized
as plants are found among the lower algæ and fungi. These are single
cells of very minute size, roundish, oval, or oblong, existing
during their growing period in water or in a very moist substratum
or atmosphere. Examples are found in the red snow plant (_Sphærella
nivalis_), the Pleurococcus, the bacteria; and among small colonies of
these simple organisms (Pandorina) or the thread-like forms (Spirogyra,
Œdogonium, etc.). It is evident that some of the life relations of such
very simple organisms are very easily obtained—that is, the adjustment
to environment is not difficult. All of the living substance is very
closely surrounded by food material in solution. These food solutions
are easily absorbed. Because of the minute size of the protoplasts and
of the plant body, they do not have to solve problems of transport of
food to distant parts of the body. When we pass to more bulky organisms
consisting of large numbers of protoplasts closely compacted together,
the problem of relation to environment and of food transport become
felt; the larger the organism usually the greater are these problems.
A point is soon reached at which there is a gain by a differentiation
in the work of different protoplasts, some for absorption, some for
conduction, some for the light relation, some for reproduction, and
so on. There is also a gain in splitting the form of the plant body
up into parts so that a larger surface is exposed to environment
with an economy in the amount of building material required. In this
differentiation of the plant body into parts, there are two general
problems to be solved, and the plant to be successful in its struggle
for existence must control its development in such a way as to preserve
the balance between them. (1) A ready display of a large surface to
environment for the purpose of acquiring food and the disposition of
waste. (2) The protection of the plant from injuries incident to an
austere environment.
It is evident with the great variety of conditions met with in
different parts of the same locality or region, and in different parts
of the globe, that the plant has had very complex problems to meet and
in the solution of them it has developed into a great variety of forms.
It is also likely that different plants would in many cases meet these
difficulties in different ways, sometimes with equal success, at other
times with varied success. Just as different persons, given some one
piece of work to do, are likely to employ different methods and reach
results that are varied as to their value. While we cannot attribute
consciousness or choice to plants in the sense in which we understand
these qualities in higher animals, still there is something in their
“constitution” or “character” whereby they respond in a different
manner to the same influences of environment. This is, perhaps,
imperceptible to us in the different individuals of the same species,
but it is more marked in different species. Because of our ignorance of
this occult power in the plant, we often speak of it as an “inherent”
quality.
Perhaps the most striking examples one might use to
illustrate the different line of organization among
plants in two regions where the environment is very
different are to be found in the adaptation of the
cactus or the yucca to desert regions, and the oak or
the cucurbits to the land conditions of our climate.
The cactus with stem and leaf function combined in a
massive trunk, or the yucca with bulky leaves expose
little surface in comparison to the mass of substance,
to the dry air. They have tissue for water storage and
through their thick epidermis dole it out slowly since
there is but little water to obtain from dry soil.
The cucurbits and the oak in their foliage leaves
expose a very large surface in proportion to the mass
of their substance, to an atmosphere not so severely
dry as that of the desert, while the roots are able
to obtain an abundant supply of water from the moist
soil. The cactus and the yucca have differentiated
their parts in a very different way from the oak or the
cucurbits, in order to adapt themselves to the peculiar
conditions of the environment.
When we say that certain plants have the power to
adapt themselves to certain conditions of environment,
we do not mean to say that if the cucurbits were
transferred to the desert they would take on the form
of the cactus or the yucca. They could do neither.
They would perish, since the change would be too great
for their organization. Nor do we mean, that, if the
cactus or yucca were transferred from the desert to our
climate, they would change into forms with thin foliage
leaves. They could not. The fact is that they are
enabled to live in our climate when we give them some
care, but they show no signs of assuming characters
like those of our vegetation. What we do mean is, that
where the change is not too great nor too sudden, some
of the plants become slightly modified. This would
indicate that the process of organization and change of
form is a very slow one, and is therefore a question of
time—ages it may be—in which change in environment
and adaptation in form and structure have gone on
slowly hand in hand.
=690. Members of the plant body.=—The different parts into which
the plant body has become differentiated are from one point of view,
spoken of as members. It is evident that the simplest forms of life
spoken of above do not have members. It is only when differentiation
has reached the stage in which certain more or less prominent parts
perform certain functions for the plant that members are recognized.
In the algæ and fungi there is no differentiation into stem and leaf,
though there is an approach to it in some of the higher forms. Where
this simple plant body is flattened, as in the sea-wrack, or ulva, it
is a _frond_. The Latin word for frond is _thallus_, and this name is
applied to the plant body of all the lower plants, the algæ and fungi.
The algæ and fungi together are sometimes called _thallophytes_, or
_thallus plants_. The word thallus is also sometimes applied to the
flattened body of the liverworts. In the foliose liverworts and mosses
there is an axis with leaf-like expansions. These are believed by some
to represent true stems and leaves; by others to represent a flattened
thallus in which the margins are deeply and regularly divided, or in
which the expansion has only taken place at regular intervals.
In the higher plants there is usually great differentiation of the
plant body, though in many forms, as in the duckweeds, it is in the
form of a frond. While there is a great variety in the form and
function of the members of the plant body, they are all reducible to a
few fundamental members. Some reduce these forms to three, the _root_,
_stem_, _leaf_; while others to two, the _root_, and _shoot_, which is
perhaps the best primary subdivision, and the shoot is then divided
into stem and leaf, the leaf being a lateral outgrowth of the stem, and
can be indicated by the following diagram:
{Stem.
{Shoot····{
Plant body····{ {Leaf.
{Root.
KINDS OF SHOOTS.
=691.= Since it is desirable to consider the shoot in its relation
to environment, for convenience in discussion we may group shoots into
four prominent kinds: (1) _Foliage shoots_; (2) _Shoots without foliage
leaves_; (3) _Floral shoots_; (4) _Winter conditions of shoots and
buds._ Topic (4) will be treated in Chapter XXXIX, section IV.
[Illustration: Fig. 413. Lupinus perennis. Foliage shoot and floral
shoot.]
=692. (1st) Foliage shoots.=—Foliage shoots are either aerial,
when their relation is to both light and air; or they are aquatic,
when their relation is to both light and water. They bear green
leaves, and whether in the air or water we see that light is one of
the necessary relations for all. Naturally there are several ways in
which a shoot may display its leaves to the light and air or water.
Because of the great variety of conditions on the face of the earth
and the multitudinous kinds of plants, there is the greatest diversity
presented in the method of meeting these conditions. There is to be
considered the problem of support to the shoot in the air, or in the
water. The methods for solving this problem are fundamentally different
in each case, because of the difference in the density of air and
water, the latter being able to buoy up the plant to a great degree,
particularly when the shoot is provided with air in its intercellular
spaces or air cavities. In the solution of the problem in the relation
of the shoot to aerial environment, stem and leaf have in most cases
coöperated;[38] but in view of the great variety of stems and their
modifications, as well as of leaves, it will be convenient to discuss
them in separate chapters.
[Illustration: Fig. 413_a_. Burrowing type, the mandrake, a “rhizome.”]
=693. (2d) Shoots without foliage leaves.=—These are subterranean
or aerial. Nearly all subterranean shoots have also aerial shoots,
the latter being for the display of foliage leaves (foliage-shoots),
and also for the display of flowers (flower-shoots). The subterranean
kinds bear scale leaves, i.e., the leaves not having a light relation
are reduced in size, being small, and they lack chlorophyll. Examples
are found in Solomon’s seal, mandrake (fig. 413_a_), etc. Here the
scale leaves are on the bud at the end of the underground stem from
which the foliage shoot arises. Aerial shoots which lack foliage
leaves are the dodder, Indian-pipe-plant, beech drops, etc. These
plants are saprophytes or parasites (see Chapter IX). Deriving their
carbohydrate food from other living plants, or from humus, they do not
need green leaves. The leaves have, therefore, probably been reduced
in size to mere scales, and accompanying this there has been a loss of
the chlorophyll. Other interesting examples of aerial shoots without
foliage leaves are the cacti where the stem has assumed the leaf
function and the leaves have become reduced to mere spines. The various
modifications which shoots have undergone accompanying a change in
their leaf relation will be discussed under stems in Chapter XXXIX.
=694. (3d) Floral shoots.=—The floral shoot is the part of the
plant bearing the flower. As interpreted here it may consist of but a
single flower with its stalk, as in Trillium, mandrake, etc., or of
the clusters of flowers on special parts of the stem, termed flower
clusters, as the _catkin_, _raceme_, _spike_, _umbel_, _head_, etc. In
the floral shoot as thus interpreted there are several peculiarities to
observe which distinguish it from the foliage shoot and adapt it to its
life relations.
The floral shoot in many respects is comparable to the foliage shoot,
as seen from the following peculiarities:
(1) It usually possesses, beside the flowers, small
green leaves which are in fact foliage though they are
very much reduced in size, because the function of
the shoot as a foliage shoot is subordinated to the
function of the floral shoot. These small leaves on the
floral shoot are termed _bracts_.
(2) It may be (_a_) unbranched, when it would consist
of a single flower, or (_b_) branched, when there would
be several to many flowers in the flower cluster.
(3) The flower bud has the same origin on the shoot
as the leaf bud; it is either terminal or axillary, or
both.
(4) The members of the flower belong to the leaf
series, i.e., they are leaves, but usually different
in color from foliage leaves, because of the different
life relation which they have to perform. Evidence
of this is seen in the transition of sepals, petals,
stamens, or pistils, to foliage leaves in many flowers,
as in the pond lily, the abnormal forms of trillium,
and many monstrosities in other flowers (see Chapter
XXXIV).
(5) The position of the members of the flower on
its axis, though usually more crowded, in many cases
follows the same plan as the leaves on the stem.
The various kinds of floral shoots or flower clusters will be discussed
in Chapter XLII, on the Floral Shoot.
II. Organization of Plant Tissues.
=695.= A tissue is a group of cells of the same kind having a
similar position and function. In large and bulky plants different
kinds of tissue are necessary, not only because the work of the plant
can be more economically performed by a division of labor, but also
cells in the interior of the mass or at a distance from the source
of the food could not be supplied with food and air unless there
were specialized channels for conducting food and specialized tissue
for support of the large plant body. In these two ways most of the
higher plants differ from the simple ones. The tissues for conduction
are sometimes called collectively the _mestome_, while tissues for
mechanical support are called _stereome_. Division of labor has
gone further also so that there are special tissues for absorption,
assimilation, perception, reproduction, and the like. The tissues of
plants are usually grouped into three systems: (1) The Fundamental
System, (2) The Fibrovascular System, (3) The Epidermal System. Some of
the principal tissues are as follows:
1. THE FUNDAMENTAL SYSTEM.
=696. Parenchyma.=—Tissue composed of thin-walled cells which
in the normal state are living. Parenchyma forms the loose and spongy
tissue in leaves, as well as the palisade tissue (see Chapter IV); the
soft tissue in the cortex of root and stem (Fig. 414); as well as that
of the pith, of the pith-rays or medullary rays of the stem; and is
mixed in with the other elements of the vascular bundle where it is
spoken of as wood parenchyma and bast parenchyma; and it also includes
the undifferentiated tissue (meristem) in the growing tips of roots and
shoots; also the “intrafascicular” cambium (i.e., between the bundles,
some also include the cambium within the bundle).
=697. Collenchyma.=—This is a strengthening tissue often found
in the cortex of certain shoots. It also is composed of living cells.
The cells are thickened at the angles, as in the tomato and many other
herbs (fig. 414).
=698. Sclerenchyma, or stone-tissue.=—This is also a
strengthening tissue and consists of cells which do not taper at the
ends and the walls are evenly thickened, sometimes so thick that the
inside (lumen) of the cell has nearly disappeared. Usually such cells
contain no living contents at maturity. Sclerenchyma is very common in
the hard parts of nuts, and underneath the epidermis of stems and
leaves of many plants, as in the underground stems of the bracken fern,
the leaves of pines (fig. 415), etc.
[Illustration: Fig. 414. Transverse section of portion of tomato stem.
_ep_, epidermis; _ch_ chlorophyll-bearing cells; _co_, collenchyma;
_cp_, parenchyma.]
[Illustration: Fig. 415. Margin of leaf of Pinus pinaster, transverse
section, _c_, cuticularized layer of outer wall of epidermis;
_i_, inner non-cuticularized layer; _c´_, thickened outer wall of
marginal cell; _g_, _i´_, hypoderma of elongated sclerenchyma; _p_,
chlorophyll-bearing parenchyma; _pr_, contracted protoplasmic contents.
×800. (After Sachs.)]
[Illustration: Fig. 416. Section through a lenticel of Betula alba
showing stoma at top, phellogen below producing rows of flattened
cells, the cork. (After De Bary.)]
=699. Cork.=—In many cases there is a development of “cork”
tissue underneath the epidermis. Cork tissue is developed by repeated
division of parenchyma cells in such a way that rows of parallel cells
are formed toward the outside. These are in distinct layers, soon lose
their protoplasm and die; there are no intercellular spaces and the
cells are usually of regular shape and fit close to each other. In
some plants the cell walls are thin (cork oak), while in others they
are thickened (beech). The tissue giving rise to cork is called “cork
cambium,” or phellogen, and may occur in other parts of the plant. For
example, where plants are wounded the living exposed parenchyma cells
often change to cork cambium and develop a protective layer of cork.
The walls of cork cells contain a substance termed _suberin_, which
renders them nearly waterproof.
=700. Lenticels.=—These are developed quite abundantly underneath
stomates on the twigs of birch, cherry, beech, elder, etc. The
phellogen underneath the stoma develops a cushion of cork which presses
outward in the form of an elevation at the summit of which is the stoma
(fig. 416). The lenticels can easily be seen.
2. THE FIBROVASCULAR SYSTEM.
=701. Fibrous tissue.=[39]—This consists of thick-walled cells,
usually without living contents which are elongated and taper at the
ends so that the cells, or fibers, overlap. It is common as one of the
elements of the vascular bundles, as wood fibers and bast fibers.
=702. Vascular tissue, or tracheary tissue.=—This consists of the
vessels or ducts, and tracheides, which are so characteristic of the
vascular bundle (see Chapter V) and forms a conducting tissue for the
flow of water. The vascular tissue contains spiral, annular, pitted,
and scalariform vessels and tracheides according to the marking on the
walls (figs. 58, 59). These are all without protoplasmic contents when
mature. There are also thin-walled living cells intermingled called
wood parenchyma. In the conifers (pines, etc.) the tracheary tissue is
devoid of true vessels except a few spiral vessels in the young stage,
while it is characterized by tracheides with peculiar markings. These
marks on the tracheides are due to the “bordered” pits appearing as two
concentric rings one within the other. These can be easily seen in a
longitudinal section of wood of conifers.
=703. Sieve tissue.=—This consists of elongated tubular cells
connected at the ends, the cross walls being perforated at the ends.
These are in the phloem part of the bundle, and serve to conduct
downwards the dissolved substances elaborated in the leaves.
=704. Fascicular cambium.=—This is the living, cell-producing
tissue in the vascular bundle, which in the open bundle adds to the
phloem on one side and the xylem on the other.
3. THE EPIDERMAL SYSTEM.
=705.= To the epidermal system belong the epidermis and the
various outgrowths of its cells in the form of hairs, or _trichomes_,
as well as the guard cells of the stomates, and probably some of the
reproductive organs.
=706. The epidermis.=—The epidermis proper consists of a
single layer of external cells originating from the outer layer of
parenchyma cells at the growing apex of the stem or root. These cells
undergo various modifications of form. In many cases they lose their
protoplasmic contents. In many cases the outer wall becomes thickened,
especially in plants growing in dry situations or where they are
exposed to drying conditions. The epidermal cells generally become
considerably flattened, and are usually covered with a more or less
well developed waterproof cuticle, a continuous layer over the
epidermis. In many plants the cuticle is covered with a waxy exudation
in the form of a thin layer, or of rounded grains, or slender rods,
or grains and needles in several layers. These waxy coverings are
sometimes spoken of as “bloom” on leaves and fruit.
=707. Trichomes.=—Trichome is a general term including various
hair-like outgrowths from the epidermis, as well as scales, prickles,
etc. These include root hairs, rhizoids, simple or branched hairs,
glandular hairs, glandular scales, etc. Glandular hairs are found on
many plants, as tomato, verbena, primula, etc.; glandular scales on
the hop; simple-celled hairs on the evening primrose, cabbage, etc.;
many-celled hairs on the primrose, pumpkin; branched hairs on the
shepherd’s-purse, mullein, etc., stellate hairs on some oak leaves.
For stomates see Chapter IV.
4. ORIGIN OF THE TISSUES.
=708. Meristem tissue.=—The various tissues consisting of cells
of dissimilar form are derived from young growing tissue known as
_meristem_. Meristem tissue consists of cells nearly alike in form,
with thin cell walls and rich in protoplasm. It is situated at the
growing regions of the plants. In the higher plants these regions in
general are three in number, the stem and root apex, and the cambium
cylinder beneath the cortex. Tissues produced from the stem and root
apex are called _primary_, those from the cambium _secondary_. In most
cases the main bulk of the plant is secondary tissue, while in the corn
plant it is all primary.
[Illustration: Fig. 417. Section through growing point of stem, _d_,
dermatogen; _p_, plerome; periblem between. (After De Bary.)]
=709. Origin of stem tissues.=—Just back of the apical meristem
in a longitudinal section of a growing point it can be seen that the
cells are undergoing a change in form, and here are organized three
formative regions. The outer layer of cells is called _dermatogen_
(skin producer), because later it becomes the epidermis. The central
group of elongating cells is the _plerome_ (to fill). This later
develops the _central cylinder_, or _stele_, as it is called
(fig. 417). Surrounding the plerome and filling the space between it
and the dermatogen is the third formative tissue called the _periblem_,
which later forms the cortex (bark or rind), and consists of
parenchyma, collenchyma, sclerenchyma, or cork, etc., as the case may
be. It should be understood that all these different forms and kinds
of cells have been derived from meristem by gradual change. In the
mature stems, therefore, there are three distinct regions, the central
cylinder or stele, the cortex, and the epidermis.
[Illustration: Fig. 418. Concentric bundle from stem of Polypodium
vulgare. Xylem in the center, surrounded by phloem, and this by the
endodermis. (From the author’s Biology of Ferns.)]
=710. Central cylinder or stele.=—As the central cylinder is
organized from the plerome it becomes differentiated into the vascular
bundles, the pith, the pith-rays (medullary rays) which radiate from
the pith in the center between the bundles out to the cortex, and
the pericycle, a layer of cells lying between the central cylinder
and the cortex. The bundles then are farther organized into the
xylem and phloem portions with their different elements, and the
fascicular cambium (meristem) separating the xylem and phloem, as
described in Chapter V. Such a bundle, where the xylem and phloem
portions are separated by the cambium is called an open bundle (as
in fig. 58). Where the phloem and xylem lie side by side in the same
radius the bundle is a _collateral_ one. Dicotyledons and conifers are
characterized by open collateral bundles. This is why trees and many
other perennial plants continue to grow in diameter each year.
The cambium in the open bundle forms new tissue each spring and
summer, thus adding to the phloem on the outside and the xylem on
the inside. In the spring and early summer the large vessels in the
xylem predominate, while in late summer wood fibers and small vessels
predominate and this part of the wood is firmer. Since the vascular
bundles in the stem form a circle in the cylinder, this difference
in the size of the spring and late summer wood produces the “annual”
rings, so evident in the cross-section of a tree trunk. Branches
originate at the surface involving epidermis, cortex, and the bundles.
In monocotyledonous plants (corn, palm, etc.) the bundles are not
regularly arranged to form a hollow cylinder, but are irregularly
situated through the stele. There is no meristem, or cambium, left
between the xylem and phloem portions of the bundle and the bundle is
thus _closed_ (as in fig. 60), since it all passes over into permanent
tissue. In most monocotyledons there is, therefore, practically no
annual increase in diameter of the stem.
[Illustration: Fig. 419. Section of stem (rhizome) of Pteris aquilina.
_sc_, thick-walled sclerenchyma; _a_, thin-walled sclerenchyma; _par_,
parenchyma.]
=711. Ferns.=—In the ferns and most of the Pteridophytes an
apical meristem tissue is wanting, its place being taken by a single
apical cell from the several sides of which cells are successively
cut off, though Isoetes and many species of Lycopodium have an apical
meristem group. In most of the Pteridophytes also the bundles are
_concentric_ instead of collateral. Fig. 418 represents one of the
bundles from the stem of the polypody fern. The xylem is in the center,
this surrounded by the phloem, the phloem by the phloem sheath, and
this in turn by the endodermis, giving a concentric arrangement of the
component tissues. A cross-section of the stem (fig. 419) shows two
large areas of sclerenchyma, which gives the chief mechanical support,
the bundles being comparatively weak.
=712. Origin of root tissues.=—A similar apical meristem exists
in roots, but there is in addition a fourth region of formative
tissue in front of the meristem called _calyptrogen_ (fig. 420). This
gives rise to the “root cap” which serves to protect the meristem.
The vascular cylinder in roots is very different from that of the
stem. There is a solid central cylinder in which the groups of xylem
radiate from the center and groups of phloem alternate with them but
do not extend so near the center (fig. 421). As the root ages, changes
take place which obscure this arrangement more or less. Branches of
the roots arise from the central cylinder. In fern roots the apical
meristem is replaced by a single four-sided (tetrahedral) apical cell,
the root cap being cut off by successive divisions of the outer face,
while the primary root tissues are derived from the three lateral faces.
[Illustration: Fig. 420. Median longitudinal section of the apex of a
root of the barley, Hordeum vulgare. _k_, calyptrogen; _d_, dermatogen;
_c_, its thickened wall; _pr_, periblem; _pl_, plerome; _en_,
endodermis; _i_, intercellular air-space in process of formation; _a_,
cell row destined to form a vessel; _r_, exfoliated cells of the root
cap. (After Strasburger.)]
[Illustration: Fig. 421. Cross-section of fibrovascular bundle in
adventitious root of Ranunculus repens. _w_, pericycle; _g_, four
radial plates of xylem; alternating with them are groups of phloem.
This is a radial bundle. (After De Bary.)]
=Function of the root cap.=—The root cap serves an important
function in protecting the delicate meristem or cambium at the tip of
the root. These cells are, of course, very thin-walled, and while there
is not so much danger that they would be injured from dryness, since
the soil is usually moist enough to prevent evaporation, they would be
liable to injury from friction with the rough particles of soil. No
similar cap is developed on the end of the stem, but the meristem here
is protected by the overlapping bud-scales. One of the most striking
illustrations of a root cap may be seen in the case of the Pandanus, or
screw-pine, often grown in conservatories (see fig. 447). On the prop
roots which have not yet reached the ground the root caps can readily
be seen, since they are so large that they fit over the end of the root
like a thimble on the finger.
=713. Descriptive Classification of Tissues.=
{ Epidermis.
{
{ { Simple hairs.
{ { Many-celled hairs.
{ { Branched hairs, often stellate.
Epidermal { Trichomes. { Clustered, tufted hairs.
System. ····{ { Glandular hairs.
{ { Root hairs.
{ { Prickles.
{
{ Guard cells of stomates.
{ Spiral vessels.
{ Pitted vessels.
{ Scalariform vessels.
{ Xylem (wood). { Annular vessels.
{ { Tracheides.
{ { Wood fibers.
{ { Wood parenchyma.
Fibrovascular {
System. ····{ Cambium (fascicular).
{
{ { Sieve tubes.
{ Phloem (bast). { Bast fibers.
{ Companion cells.
{ Bast parenchyma.
{ Cork.
{ Collenchyma.
{ Cortex.····{ Parenchyma.
{ { Fibers.
{ { Milk tissue.
{
{ Pith-ray.··{ Parenchyma.
{ { Intrafascicular
{ Stem and root. { cambium.
{ { Pith.······{ Parenchyma.
{ { { Sclerenchyma.
{ {
Fundamental { { Bundle-sheath.
System. ····{ {
{ { Endodermis.
{
{ Leaves. { Palisade tissue.
{ { Spongy parenchyma.
{
{ Reproductive Organs (mainly fundamental).
=714. Physiological Classification of Tissues.=
_Formative Tissue._
Thin-walled cells composing the meristem, capable of division and from
which other tissues are formed.
_Protective Tissue._
_Tegumentary System._—Epidermis, periderm, bark protecting the plant
from external contact.
_Mechanical System._—Bast tissue, bast-like tissue, collenchyma,
sclerenchyma, afford protection against harmful bending, pulling, etc.
_Nutritive Tissues._
_Absorptive System._—Root hairs and cells, rhizoids, aerial root
tissue, absorptive leaf glands, absorptive organs in seeds, haustoria
of parasites, etc.
_Assimilatory System._—Assimilating cells in leaf and stem.
_Conductive System._—Sieve tissue, tracheary tissue, milk tissue,
conducting parenchyma, etc.
_Food-storing System._—Water reservoir, water tissue, slime tissue,
fleshy roots and stems, endosperm and cotyledons, etc.
_Aerating System._—Air spaces and tubes, special air tissue,
air-seeking roots, stomates, lenticels, etc.
_Secretory and Excretory System._—Water glands, digestive glands,
resin glands, nectaries, tannin, pitch and oil receptacles, etc.
_Apparatus and Tissues for Special Duties._
Holdfasts.
Tissues of movement, parachute hairs, floating tissue, hygroscopic
tissue, living tissue.
For perceiving stimuli.
For conducting stimuli, etc.
FOOTNOTES:
[37] =Suggestions to the teacher.=—In the study of the flowering plants
in the secondary school and in elementary courses three general topics
are suggested. 1st, the study of the form and members of the plant and
their arrangement, as in Chapters XXXVIII-XLV. 2d, the study of a few
plants representative of the more important families, in order that the
members of the plant, as studied under the first topic, may be seen in
correlation with the plant as a whole in a number of different types.
3d, the study of plants in their relation to environment, as in Chapter
XLVI. The first and second topics can be conducted consecutively in the
classroom and laboratory. The third topic can be studied at opportune
times during the progress of topics 1 and 2. For example, while
studying topic 1 excursions can be made to study winter conditions of
buds, shoots, etc., if in winter period, or the relations of leaves,
etc., to environment, if in the growing period. While studying topic 2
excursions can be made to study flower relations, and also vegetation
relations to environment (see Chapters XLVI-LVII of the author’s
“College Text-book of Botany”). It is believed that a study of these
three general topics is of much more value to the beginning student
than the ordinary plant analysis and determination of species.
[38] It is interesting to note that in some foliage shoots the stem is
entirely subterranean. See discussion of the bracken fern and sensitive
fern in Chapter XXXIX.
[39] Some fibers occur also very frequently in the Fundamental System,
forming bundle-sheaths, or strands of mechanical tissue in the cortex.
CHAPTER XXXIX.
THE DIFFERENT TYPES OF STEMS. WINTER SHOOTS AND BUDS.
I. Erect Stems.
=715. Columnar type.=—The columnar type of stem may be simple or
branched. When branching occurs the branches are usually small and in
general subordinate to the main axis. The sunflower (Helianthus annuus)
is an example. The foliage part is mainly simple. The main axis remains
unbranched during the larger part of the growth-period. The principal
flowerhead terminates the stem. Short branches bearing small heads
then arise in the axils of a few of the upper leaves. In dry, poor
soil, or where other conditions are unfavorable, there may be only the
single terminal flowerhead, when the stem is unbranched. The mullein
is another columnar stem. The foliage part is rarely branched, though
branches sometimes occur where the main axis has become injured or
broken. The flower stem is terminal. The corn plant and the Easter lily
are good illustrations also of the columnar stem.
Among trees the Lombardy poplar (Populus fastigiata) is an excellent
example of the columnar type. Though this is profusely branched, the
branches are quite slender and small in contrast with the main axis,
unless by some injury or other cause two large axes may be developed.
As the technical name indicates, the branching is fastigiate, i.e., the
branches are crowded close together and closely surround the central
axis. The royal palm and some of the tree ferns have columnar, simple
stems, but the large, wide-spreading leaves at the top of the stem give
the plant anything but a cylindrical habit. Some cedars and arbor-vitæ
are also columnar.
The advantages of the columnar habit of stem are three: (1) That the
plant stands above other neighboring ones of equal foliage area and
thus is enabled to obtain a more favorable light relation; (2) where
large numbers of plants of the same species are growing close together,
they can maintain practically the same habit as where growing alone;
(3) the advantage gained by other types in their neighborhood in less
shading than if the type were spreading. The cylindrical type can,
therefore, grow between other types with less competition for existence.
[Illustration: Fig. 422. Cylindrical stem of mullein.]
[Illustration: Fig. 423. Conical type of larch.]
=716. The cone type.=—This is well exampled in the larches,
spruces, the gingko tree, some of the pines, cedars, and other
gymnosperms. In the cone type, the main axis extends through the system
of branches like a tall shaft, i.e., the trunk is _excurrent_. The
lower branches are wide-spreading, and the branches become successively
shorter, usually uniformly, as one ascends the stem. The branching is
of two types: (1) the branches are in false whorls; (2) the branches are
distributed along the stem. To the first type belong the pines, Norway
spruce, Douglas spruce, etc. _The white pine_ is an exquisite example,
and in young and middle-aged trees shows the style of branching to
very good advantage. The branches are nearly horizontal, with a
slight sigmoid graceful curve, while towards the top the branches are
ascending. This direction of the branches is due to the light relation.
The few whorls at the top are ascending because of the strong light
from above. They soon become extended in a horizontal direction as the
main source of light is shifting to the side by the shading of the top.
The ascending direction first taken by the upper branches and their
subsequent turning downward, while the ends often still have a slight
ascending direction gives to the older branches their sigmoid curve.
The young vernal shoots of the pines show some very interesting
growth movements. There are two growth periods: (1) the elongation of
the shoot, and (2) the elongation of the leaves. The elongation of the
shoot takes place first and is completed in about six weeks or two
months’ time. The direction of the shoot in the first period seems to
be entirely influenced by geotropism. It grows directly upward and
stands up as a very conspicuous object in strong contrast with the dark
green foliage of the more or less horizontal shoots. When the second
period of growth takes place, and the leaves elongate, the shoot bends
downward and outward in a lateral direction.
The rate of growth of the pines can be very easily observed since each
whorl of branches (between the whorls of long shoots there are short
shoots bearing the needle leaves), whether on the main axis or on the
lateral branches, marks a year, the new branches arising each year
at the end of the shoot of the previous year. The rate of growth is
sometimes as high as twelve to twenty-four inches or more per year.
The _spruces_ form a more perfect cone than the pines. The long
branches are mostly in whorls, but often there are intermediate ones,
though the rate of growth per year can usually be easily determined. In
the _hemlock-spruce_, the branching is distributed. The _larch_ has a
similar mode of branching, but it is deciduous, shedding its leaves in
the autumn, and it has a tall, conical form.
It would seem that trees of the cone type possessed certain advantages
in some latitudes or elevations over other trees. (1) A conical tree,
like the spruces and larches and the pines, and hemlocks also, before
they get very old, meets with less injury during high winds than trees
of an oval or spreading type. The slender top of the tree where the
force of the wind is greatest presents a small area to the wind, while
the trunk and short slender branches yield without breaking. Perhaps
this is one reason why trees of this type exist in more northern
latitudes and at higher elevations in mountainous regions, and why
the spruce type reaches a higher latitude and altitude even than the
pines. (2) The form of the tree is such as to admit light to a large
foliage area, even where the trees are growing near each other. The
evergreen foliage, persistent for several years, on the wide-spreading
lower branches, probably affords some protection to the trees since
this cover would aid in maintaining a more equable temperature in the
forest cover than if the trees were bare during the winter. (3) There
is less danger of injury from the weight of snow since the greater
load of snow would lie on the lower branches. The form of the branches
also, especially in the spruces, permits them to bend downward without
injury, and if necessary unload the snow if the load becomes too heavy.
=717. The oval type.=—This type is illustrated by the oak,
chestnut, apple, etc. The trees are usually deciduous, i.e., cast
their leaves with the approach of winter. The main axis is sometimes
maintained, but more often disappears (trunk is _deliquescent_),
because of the large branches which maintain an ascending direction,
and thus lessen the importance of the central axis which is so marked
in the cone type. Trees of this type, and in fact all deciduous trees,
exhibit their character or habit to better advantage during the winter
season when they are bare. Trees of this type are not so well adapted
to conditions in the higher altitudes and latitudes as the cone type,
for the reason given in the discussion of that type. The deciduous
habit of the oaks, etc., enables them to withstand heavy winds far
better than if they retained their foliage through the winter, even
were the foliage of the needle kind and adapted to endure cold.
=718. The deliquescent type.=—The elm is a good illustration of
this type. The main axes and the branches fork by a false dichotomy, so
that a trunk is not developed except in the forest. The branches rise
at a narrow angle, and high above diverge in the form of an arch. The
chief foliage development is lofty and spreading.
Trees possess several advantages over vegetation less lofty. They may
start their growth later, but in the end they outgrow the other kinds,
shade the ground and drive out the sun-loving kinds.
II. Creeping, Climbing, and Floating Stems.
=719. Prostrate type.=—This type is illustrated by creeping or
procumbent stems, as the strawberry, certain roses, of which a good
type is one of the Japanese roses (Rosa wichuriana), which creeps very
close to the ground, some of the raspberries, the cucurbits like the
squash, pumpkin, melons, etc. These often cover extensive areas by
branching and reaching out radially on the ground or climbing over low
objects. The cucurbits should perhaps be classed with the climbers,
since they are capable of climbing where there are objects for support,
but they are prostrate when grown in the field or where there are no
objects high enough to climb upon. In the prostrate type, there is
economy in stem building. The plants depend on the ground for support,
and it is not necessary to build strong, woody trunks for the display
of the foliage which would be necessary in the case of an erect plant
with a foliage area as great as some of the prostrate stems. This
gain is offset, at least to a great extent, by the loss in ability to
display a great amount of foliage, which can be done only on the upper
side of the stem.
[Illustration: Fig. 424. Prostrate type of the water fern (_marsilia_).]
Other advantages gained by the prostrate stems are protection from
wind, from cold in the more rigorous climates, and some propagate
themselves by taking root here and there, as in certain roses, the
strawberry plant, etc. Some plants have erect stems, and then send
out runners below which take root and aid the plant in spreading and
multiplying its numbers.
=720. The decumbent type.=—In this type the stem is first erect,
but later bends down in the form of an arch, and strikes root where the
tip touches the ground. Some of the raspberries and blackberries are of
this type.
=721. The climbing type.=—The grapes, clematis, some roses, the
ivies, trumpet-creeper, the climbing bittersweet, etc., are climbing
stems. Like the prostrate type, the climbers economize in the material
for stem building. They climb over shrubs, up the trunks of trees and
often reach to a great height and acquire the power of displaying a
great amount of foliage by sending branches out on the limbs of the
trees, sometimes developing an amount of foliage sufficient to cover
and nearly smother the foliage of large trees; while the main stem of
the vine may be not over two inches in diameter and the trunk of the
supporting tree may be three feet in diameter.
=722. Floating stems.=—These are necessarily found in aquatic
plants. The stems may be ascending or horizontal. The stems are
usually not very large, nor very strong, since the water bears them
up. The plants may grow in shallow water, or in water 10-12 feet or
more deep, but the leaves are usually formed at or near the surface of
the water in order to bring them near the light. Various species of
Potamogeton, Myriophyllum, and other plants common along the shores of
lakes, in ponds, sluggish streams, etc., are examples. Among the algæ
are examples like Chara, Nitella, etc., in fresh water; Sargassum,
Macrocystis, etc., in the ocean. In these plants, however, the plant
body is a thallus, which is divided into stem-like (_caulidium_) and
leaf-like (_phyllidium_) structures.
=723. The burrowing type, or rhizomes.=—These are horizontal,
subterranean stems. The bracken fern, sensitive fern, the mandrake
(see fig. 413_a_), Solomon’s seal, Trillium, Dentaria, and the like,
are examples. The subterranean habit affords them protection from the
cold, the wind, and from injury by certain animals. Many of these stems
act as reservoirs for the storage of food material to be used in the
rapid growth of the short-lived aerial shoot. In the ferns mentioned,
the subterranean is the only shoot, and this bears scale leaves which
are devoid of chlorophyll, and foliage leaves which are larger, and the
only member of the plant body which is aerial. The foliage leaf has
assumed the function of the aerial shoot. The latter is not necessary
since flowers are not formed. The mandrake, Solomon’s seal, Trillium,
etc., have scale leaves on the fleshy underground stems, while foliage
leaves are formed on the aerial stems, the latter also bearing the
flowers. Some of the advantages of the rhizomes are protection from
injury, food storage for the rapid development of the aerial shoot, and
propagation.
Many of the grasses have subterranean stems which ramify for great
distances and form a dense turf. For the display of foliage and for
flower and seed production, aerial shoots are developed from these
lateral upright branches.
III. Specialized Shoots and Shoots for Storage of Food.[40]
=724. The bulb.=—The bulb is in the form of a bud, but the scale
leaves are large, thick, and fleshy, and contain stored in them food
products manufactured in the green aerial leaves and transported to
the underground bases of the leaves. Or when the bulb is aerial in its
formation, it is developed as a short branch of the aerial stem from
which the reserve food material is transported. Examples are found
in many lilies, as Easter lily, Chinese lilies, onion, tulip, etc.
The thick scale leaves are closely overlapped and surround the short
stem within (also called a _tunicated_ stem). In many lilies there
is a sufficient amount of food to supply the aerial stem for the
development of flower and seed. There are roots, however, from the bulb
and these acquire water for the aerial shoot, and when planted in soil
additional food is obtained by them.
[Illustration: Fig. 425. Bulb of hyacinth.]
[Illustration: Fig. 426. Corm of Jack-in-the-pulpit.]
=725. Corm.=—A corm is a thick, short, fleshy, underground stem.
A good example is found in the jack-in-the-pulpit (Arisæma).
=726. Tubers.=—These are thickened portions of the subterranean
stems. The most generally known example is the potato tuber (“Irish”
potato, not the sweet potato, which is a root). The “eyes” of the
potato are buds on the stem from which the aerial shoots arise when the
potato sprouts. The potato tuber is largely composed of starch which is
used for food by the young sprouts.
=726=_a_. =Phylloclades.=—These are trees, shrubs, or
herbs in which the leaves are reduced to mere bracts and stems,
are not only green and function as leaves, but some or all of the
branches are flattened and resemble leaves in form as in Phyllanthus,
Ruscus, Semele, Asparagus, etc. The flowers are borne directly on
these flattened axes. The prickly-pear cactus (Opuntia) is also
a phylloclade. Examples of phylloclades are often to be found in
greenhouses.
=727. Undifferentiated stems= are found in such plants as the
duckweed, or duckmeat (Lemna, Wolffia, etc. See Chapter III).
IV. Annual Growth and Winter Protection of Shoots and Buds.[41]
=728. Winter conditions.=[42]—While herbs are subjected only to
the damp warm atmosphere of summer, woody plants are also exposed
during the cold dry winter, and must protect themselves against such
conditions. The air is dryer in winter than in summer; while at the
same time root absorption is much retarded by the cold soil. Then, too,
the osmotic activity of the dormant twig-cells being much reduced, the
water-raising forces are at a minimum. It is easy to see, therefore,
that a tree in winter is practically under desert conditions. Moreover,
it has been found by various investigators, contrary to the general
belief, that cold in freezing is only indirectly the cause of death.
The real cause is the abstraction of water from the cell by the ice
crystals forming in the intercellular spaces. Death ensues because the
water content is reduced below the danger-point for that particular
cell. It was formerly thought that on freezing, the cells in the tissue
were ruptured. This is not so. Ice almost never forms within the cell,
but in the spaces between. Freezing then is really a drying process,
and dryness, not cold, causes death in winter. To protect themselves
in winter, trees provide various waterproof coverings for the exposed
surfaces and reduce the activity of the protoplasm so that it will be
less easily harmed by the loss of water abstracted by the freezing
process.
[Illustration: Fig. 427. Two-year-old twig of horse-chestnut, showing
buds and leaf-scars. (A twig with a terminal bud should have been
selected for this figure.)]
=729. Protection of the twig.=—Woody plants protect the living
cells within the twigs by the production of a dull or rough corky bark,
or by a thick glossy epidermis over the entire surface. At intervals
occur small whitish specks called lenticels, which here perform nearly
the same function as do stomates in the leaf.
=730. Bark of trunk.=—A similar service is performed by the
bark for the main trunk and branches of the tree. To admit of growth
in diameter the old bark is constantly being thrown off in strips,
flakes, etc., and replaced by a new but larger cylinder of young bark.
The external appearance thus produced enables experienced persons to
recognize many kinds of trees by the trunk alone.
=731. Leaf-scars and bundle-scars.=—The presence of foliage
leaves during the winter would greatly increase the transpiring surface
without being of use to the plant; hence they are usually thrown off on
the approach of winter. The scars left by the fallen leaves are termed
leaf-scars. The small dots on the leaf-scars left by the vascular
bundles which extended through the petiole into the twig are termed
bundle-scars. Sometimes stipule-scars are left on each side of the
leaf-scar by the fallen stipules.
=732. Nodes and internodes.=—The region upon a stem where a leaf
is borne is termed a node. The space between two nodes is an internode.
[Illustration: Fig. 428.—Shoot of butternut showing leaf-scars,
axillary buds, and adventitious buds (buds coming from above the
axils).]
[Illustration: Fig. 429.—Shoot and bud of white oak.]
=733. Phyllotaxy.=—Investigation of a horse-chestnut or willow
twig will show that the leaf-scars occupy definite positions which
are constant for each plant but different for the two species.
The arrangement of the leaves on the stem in any plant is termed
phyllotaxy. In the horse-chestnut we find two scars placed at the same
node, but on opposite sides of the stem. Somewhat higher up we find two
more similarly placed, but in a position perpendicular to that of the
first pair. Such phyllotaxy is termed opposite. If in any plant several
leaves occur at a node, the phyllotaxy is whorled. If but one at each
node, as in the willow, the phyllotaxy is alternate. The opposite and
alternate types are very commonly met with. Closer observation will
show that in the willow, if a line be drawn connecting the successive
leaf-scars, it will pass spirally up the twig until at length a scar is
reached directly over the one taken as a starting-point. Such spiral
arrangement always accompanies alternate phyllotaxy. The section of the
spiral thus delineated is termed a cycle. We express the nature of the
cycle by the fractions ½, ⅓, ⅖, ⅜, ⁵/₁₃, etc., in which the numerator
denotes the number of turns around the stem in each cycle, and the
denominator the number of leaf-scars in the same distance. In a general
way we find in plants only such arrangements as are represented by
the fractions given above. These fractions show the curious condition
that the numerator and denominator of each is equal to the sum of
the numerator or denominator of the two preceding fractions. Much
speculation has been indulged in regarding the significance of these
definite laws of leaf arrangement. In part they may be due to the
desire that each leaf receive the maximum amount of light. Only certain
definite geometrical conditions will insure this. More likely it is due
to the economy of space allotted to the leaf-fundaments in the bud.
Here, again, geometrical laws govern this economy. The phyllotaxy is
nearly constant for a given species.
=734. Buds.=—The growing point of the stem or branch together
with its leaf or flower fundaments and protective structures is termed
a bud. Winter buds on woody plants are terminal when inclosing the
growing point of the main axis of the twig; lateral when the growing
point is that of a branch of the main axis. Lateral buds are always
axillary, i.e., situated on the upper angle between a leaf and the main
axis.
=735. Buds occupying special positions.=—Several species of trees
and shrubs produce more than one bud in each leaf-axil. The additional
ones are termed accessory or supernumerary buds. These may be lateral
to one another or they may be superposed as in the walnut or butternut.
In such cases some of the buds usually contain simply floral shoots and
are termed flower buds. In some species buds are frequently produced
on the side of the branches and trunk at some distance from the
leaf-axils, and entirely without regard for the latter; or more rarely
may occur upon the root. Such buds are termed adventitious, and are the
source of the feathery branchlets upon the trunks of the American elm.
=736. Branching follows the phyllotaxy.=—Since the lateral or
branch-producing buds are always located in the axil of a leaf, the
branches necessarily follow the same arrangement upon the main axis
as do the leaves. Since, however, many of the axillary buds fail to
develop, this arrangement may be more or less obscured.
[Illustration: Fig. 430. Bud of European elm in section, showing
overlapping of scales.]
=737. Coverings of winter buds.=—These are of two sorts, hair
and cork, or scales. Buds protected simply by dense hair or sunk in
the cork of the twig are termed naked buds, and are comparatively
rare. Most species protect their buds by the addition of an imbricated
covering of closely appressed scales, the whole frequently being
rendered still more waterproof by the excretion of resin between the
scales or over the whole surface. The scales when studied carefully
are found to be much reduced leaves or parts of leaves. In some cases
they represent a modified whole leaf, when they are said to be laminar,
or a leaf-petiole, when they are petiolar, or stipular, when they are
much-specialized stipules of a leaf which itself is usually absent. The
latter type is much the less common. The form of the bud, the nature
and form of the scales, when combined with characters furnished by the
leaf- and bundle-scars, enable one to recognize and classify the winter
twigs of the various woody species.
=738. Phyllotaxy of the bud-scales.=—Since the bud-scales are
leaves, they follow a definite phyllotaxy. This may or may not be
the same as that of the foliage leaves. Twigs with opposite leaves
have opposite bud-scales, or if with alternate leaves, then alternate
bud-scales, but the fractions vary. If the scales are stipular, then
there are of course two at each node.
=739. Function of the bud coverings.=—It is popularly believed
that the scales and hairy coverings serve to keep the bud warm.
Research, however, shows this to be almost entirely erroneous, and that
the thin bud coverings are entirely inadequate to keep out the cold of
winter. They cannot keep the bud even a degree or two warmer than the
outside air, except when the changes are very rapid. Experiment also
shows that the modifying effect of the covering when the bud thaws
out is so slight as to be almost negligible. Indeed, a thermometer
bulb covered with scales taken from a horse-chestnut bud warmed up
more rapidly than a naked one when exposed to sunshine. The wool in
the horse-chestnut bud is not for the purpose of keeping it warm, but
to protect the young shoot from too great transpiration after the bud
opens the following spring. Research has also shown that such tempering
of the heat conditions is not especially beneficial to the plant,
as was once thought. Neither can we find the main function in the
prevention of water from entering the bud. This might be accomplished
in much simpler ways, even if we could demonstrate the desirability of
keeping the water out at all.
The true functions of the bud-scales are two in number: Firstly, the
prevention of too great loss of water from the young and delicate
parts within; and secondly, the protection of these same parts from
mechanical injury. Without some such protection the delicate young
structures would be beaten off by the wind, or become the food for
hungry birds during the long winter months.
[Illustration: Fig. 431. Opening buds of hickory.]
=740. Opening of the buds.=—When the young shoot begins to
grow in the spring, the bud-scales are forced apart or open of their
own accord. During the young condition the shoot is very soft and
brittle, and also possesses a very thin, little cutinized epidermis.
In this condition it is especially liable to mechanical injury and to
injury from drying out. We find, therefore, a tendency for the inner
bud-scales to elongate during vernation, thus forming a tube around the
delicate tissue much like the opening out of a telescope. The young
leaves and internodes themselves are often provided with a woody or
hairy covering to retard transpiration. When the epidermis becomes more
efficient the hairy covering often falls away.
In the case of naked buds protection is afforded in other ways: by
the protection of hairy covering, by physiological adaptation of the
tissue, or in many cases by the late appearance of the shoot in spring
after the very dry April and May winds have ceased.
=741. Bud-scars, and how to tell the age of the plant.=—In
general the bud-scales when they fall away in the spring leave scars
termed scale-scars, and the whole aggregate of scale-scars makes up the
bud-scar. The position of the buds of previous winters is, therefore,
marked. It becomes an easy matter to determine the age of a branch,
since all that is necessary is to follow back from one bud-scar to
another, the portion of the stem between representing, except in rare
cases, one year’s growth.
A woody plant grows in height only by the formation of new sections
of stem added to the apex or side of similar sections produced the
previous season, never, as is commonly supposed, by the further
elongation of the previous year’s growth. Hence a branch once formed
upon a tree is fixed as regards its distance from the ground. The
apparent rise of the branches away from the ground in forest trees is
an illusion caused by the dying away of the lower branches.
=742. Definite and indefinite growth.=—With the opening of the
buds in spring, growth begins. In some cases, when all the members
for the season were formed, but still minute, within the bud, such
growth consists solely in the expansion of parts already formed; in
others only a few members are thus present to expand, while new ones
are produced by the growing point as the season progresses. In most
cases growth is completed by the middle of July, soon after which buds
are formed for next year’s growth. Such a method of growth is termed
definite.
In a few woody plants, as, for example, sumach, locust, and raspberry,
growth continues until late in the autumn. In such cases the most
recently formed nodes and internodes are unable to become sufficiently
“hardened” before winter sets in, and are killed back more or less.
Next season’s shoot is a branch from some axillary bud. Such growth is
termed indefinite.
[Illustration: Fig. 432. Three-year-old twig of the American ash, with
sections of each year’s growth showing annual rings.]
=743. Structure of woody stems.=—If we make a cross-section of a
woody twig three general regions are presented to view. On the outside
is the rather soft, often greenish “bark,” so called, made up of
sieve tubes, ordinary parenchyma cells, and in many cases long fibrous
cells composing the “fibrous bark.” From a growing layer in this
region, termed the phellogen, the true corky bark of the older trunk is
formed.
Next within the bark we find the so-called “woody” portion of the
twig. This is strong and resistant to both breaking and cutting. The
microscope shows it to be composed of the ordinary already known woody
elements,[43] wood fibers, for strengthening purposes, pitted and spiral
vessels as conducting tissue; and intermixed with these some living
parenchyma cells. A cross-section of the stem also shows narrow radial
lines through the wood. These are pith-rays, composed of vertical
plates of living parenchyma cells. These cells, unlike the others in
the wood, are elongated radially, not vertically. The height of the
pith-rays as well as their thickness varies with the species studied.
In the older trunk only the outer portion, a few inches in thickness,
remains light-colored and fresh, and is called sap-wood. The inner
wood is usually darker and harder, and is termed heart-wood. Living
parenchyma cells, in general, are present only in the sap-wood, and
in this almost solely the ascent of sap occurs. Dyestuffs and other
substances are frequently deposited in the walls of the heart-wood.
The third region occupying the center of the twig is the pith. This is
composed ordinarily of angular, little elongated, parenchyma cells,
when mature mostly without cell-contents and filled with air. The pith
region in different trees is quite diversified. It may be hollow,
chambered, contain scattered thick-walled cells, have woody partitions,
or rarely be entirely thick-walled.
The nature of the woody ring is rather perplexing at first; but
its origin is simple. We may conceive that it has developed from a
stem-type like the sunflower, in which the bundles, though separate,
are connected by a continuous cambium ring. In the woody twigs the
numerous bundles are closely packed together, and only separated by
the primary pith-rays extending from the pith to the cortex. Other
secondary pith-rays are produced within each bundle, but they usually
extend only part way from the cortex to the pith. The wood represents
the xylem of the bundle, and the sieve tubes of the bark, the phloem.
=744. Growth in thickness.=—Although the year’s growth does not
increase in length after the first season has passed, it does increase
in diameter very much. From the size of an ordinary little twig it may
at length become a large tree trunk several feet in thickness. Only a
portion of the first year’s growth is produced by the growing point.
All the rest is a product of the cambium, a cylinder of wood being
added to the exterior of the old wood each season. The cambium, here,
as in the sunflower, lies between the phloem and the xylem, forming a
cylinder entirely around the stem. In spring, when active, it becomes
soft and delicate, thus enabling one to easily strip off the bark from
some trees, such as willow, etc., at that season.
=745. Annual rings in woody stems.=—The wood produced by the
cambium each season is not homogeneous throughout, but is usually much
denser toward the outer part of the yearly cylinder, wood fibers here
predominating. In the inner portion vessels predominate, giving a much
more porous effect. The transition from one year’s growth to another is
very abrupt, giving rise to the appearance of rings in cross-section.
Since ordinarily in temperate climates but one cylinder of wood is
added each year, the number of rings will indicate the age of the trunk
or branch. This is not absolutely accurate, since in some trees under
certain conditions more than one ring may be produced in a summer. The
porous part of the ring is often termed “spring wood,” and the denser
portion “fall wood,” but since growth from the cambium ceases in most
trees by the middle of July, “summer wood” would be more appropriate
for the latter. It is mainly the alternation of the cylinders of the
spring and summer wood that gives the characteristic grain to lumber.
Pith-rays play an important part in wood graining only in a few woods,
as, for instance, in quartered oak. The reason for the production of
porous spring wood and dense summer wood is still one of the unsolved
problems of botany.
FOOTNOTES:
[40] Besides these specialized shoots for the storage of food,
food substances are stored in ordinary shoots. For example, in the
trunks of many trees starch is stored. With the approach of cold
weather the starch is converted into oil, in the spring it is converted
into starch again, and later as the buds begin to grow the starch is
converted into glucose to be used for food. In many other trees, on the
other hand, the starch changes to sugar on the approach of winter.
[41] This topic was prepared by Dr. K. M. Wiegand.
[42] See discussion of Tropophytes in Chapter XLVI.
[43] Chapter V, and Organization of Tissues in Chapter XXXVIII.
CHAPTER XL.
FOLIAGE LEAVES.
I. General Form and Arrangement of Leaves.
=746. Influence of foliage leaves on the form of the stem.=—The
marked effect which foliage has upon the aspect of the plant or upon
the landscape is evident to all observers. Perhaps it is usual to
look upon the stem as having been developed for the display of the
foliage without taking into account the possibility that the foliage
may have a great influence upon the form or habit of the stem. It is
very evident, however, that the foliage exercises a great influence
on the form of the stem. For example, as trees increase in age and
size, the development of branches on the interior ceases and some of
those already formed die, since the dense foliage on the periphery
of the trees cuts off the necessary light stimulus. The tree,
therefore, possesses fewer branches and a more open interior. In the
forest also, the dense foliage above makes possible the shapely,
clean timber trunks. Note certain trees where by accident, or by
design, the terminal foliage-bearing branches have been removed that
foliage-bearing branches may arise in the interior of the tree system.
Without foliage leaves the stems of green plants would develop a very
different habit from what they do. This development could take place in
three different directions under the influence of light: (1) The light
stimulus would induce profuse branching, so that there would be many
small branches. (2) The stem would develop fewer branches, but they
would be flattened. (3) Massive trunks with but few or no branches. In
fact, all these forms are found in certain green stems which do not
bear leaves. An example of the first is found in asparagus with its
numerous crowded slender branches. But such forms in our climate are
rare, since foliage leaves are more efficient. The second and third
forms are found among cacti, which usually grow in dry regions under
conditions which would be fatal to ordinary thin foliage leaves.
=747. Relation of foliage leaves to the stem.=—In the study
of the position of the leaves on the stem we observe two important
modes of distribution: (1) the distribution along the individual
stem or branch which bears them, usually classed under the head of
_Phyllotaxy_; (2) the distribution of the leaves with reference to the
plant as a whole.
=748. Phyllotaxy, or arrangement of leaves.=—In examining buds on
the winter shoots of woody plants, we cannot fail to be impressed with
some peculiarities in the arrangement of these members on the stem of
the plant.
In the horse-chestnut, as we have already observed, the leaves are in
pairs, each one of the pair standing opposite its partner, while the
pair just below or above stand across the stem at right angles to the
position of the former pair. In other cases (the common bed-straw) the
leaves are in whorls, that is, several stand at the same level on the
axis, distributed around the stem. By far the larger number of plants
have their leaves arranged alternately. A simple example of alternate
leaves is presented by the elm, where the leaves stand successively on
alternate sides of the stem, so that the distance from one leaf to the
next, as one would measure around the stem, is exactly one half the
distance around the stem. This arrangement is one half, or the angle of
divergence of one leaf from the next is one half. In the case of the
sedges the angle of divergence is less, that is one-third.
By far the larger number of those plants which have the alternate
arrangement have the leaves set at an angle of divergence represented
by the fraction two fifths. Other angles of divergence have been
discovered, and much stress has been laid on what is termed a law in
the growth of the stem with reference to the position which the leaves
occupy. Singularly by adding together the numerators and denominators
of the last two fractions gives the next higher angle of divergence.
Example:
1 + 2 3 2 + 3 5
—- —- = —-; —- —- = —-;
3 + 5 8 5 + 8 13
and so on. There are, however, numerous exceptions to this regular
arrangement, which have caused some to question the importance of any
theory like that of the “spiral theory” of growth propounded by Goethe
and others of his time.
=749. Adaptation in leaf arrangement.=—As a result, however, of
one arrangement or another we see a beautiful adaptation of the plant
parts to environment, or the influence which environment, especially
light, has had on the arrangement of the leaves and branches of the
plant. Access to light and air are of the greatest importance to
green plants, and one cannot fail to be profoundly impressed with the
workings of the natural laws in obedience to which the great variety of
plants have worked out this adaptation in manifold ways.
=750. Distribution of leaves with reference to the entire
plant.=—In this case, as in the former, we recognize that it is
primarily a light relation. As the plant becomes larger and more
branched the lower and inner leaves disappear. The trees and shrubs
have by far the larger number of leaves on the periphery of the branch
system. A comparison of different kinds of trees in this respect shows,
however, that there is great variation. Trees with dense foliage (elm,
Norway maple, etc.) present numerous leaves on the periphery which
admit but little light to the interior where leaves are very few or
wanting. The sugar maple and red maple do not cast such a dense shade
and there are more leaves in the interior. This is more marked in the
silver maple, and still more so in the locust (Gleditschia tricanthos).
=751. Color of foliage leaves.=—The great majority of foliage
leaves are green in color. This we have learned (Chapter VII) is due
to the presence of a green pigment, chlorophyll, in the chloroplastids
thickly scattered in the cells of the leaf. We have also learned that
in the great majority of cases, the light stimulus is necessary for
the production of chlorophyll green. There are many foliage leaves
which possess other colors, as red (Rosa rubrifolia), purple (the
purple barberry, hazel, beech, birch, etc.), yellow (the golden oak,
elder, etc.); while many others have more or less deep tints of pink,
red, purple, yellow, when young. All of these leaves, however, possess
chlorophyll in addition to red, yellow, purple or other pigment.
These other pigments are sometimes developed in great quantity in the
cell-sap. They obscure the chlorophyll from view, but do not interfere
seriously with the action of light and the function of chlorophyll, and
perhaps in some cases serve as a screen to protect the protoplast.
=752. Autumn colors.=—Foliage leaves of many trees display in
the autumn gorgeous colors. These colors are principally shades of
red or yellow, and sometimes purple. The autumn color is more marked
in some trees than in others. In the red maple, the red and scarlet
oak, the sourwood, etc., red predominates, though sometimes yellow
may be present with the red in a single leaf. Sugar maples, poplars,
hickories, etc., are principally yellow in autumn. The sweet gum has a
rich variety of color-red, purple, maroon, yellow; sometimes all these
colors are present on the same tree.
The red and purple colors are found suffused in the cell-sap of
certain cells in the leaf much as we have found it in the cells of
the red beet. The yellow color is chiefly due to the disappearance
and degeneration of the chlorophyll while the leaf is in a moribund
state. A similar phenomenon is seen in the yellowing of crops when
the soil becomes too wet, or in the blanching of grass when covered
with a board, or of celery as the earth is ridged up over the leaves
in late summer and autumn. A number of different theories have been
advanced to explain autumn coloring, i.e., the appearance of the
red coloring matter. It has been attributed to the approach of cold
weather, and this has likely led to the erroneous belief on the part
of some that it is caused by frost. It very often precedes frost. Some
have attributed it to the action of the more oblique light rays during
autumn, and still others to the diminishing water-supply with the
approach of cool weather. The question is one which has not met as yet
with a satisfactory solution, and is certainly a very obscure one. It
is likely that the low temperature or the declining activities of the
leaf affect certain organic substances in the leaf and give rise to
the red color, and it is quite certain that in some years the display
is more brilliant than in others. The color is more striking in some
regions than in others and the different soil, as well as climate, has
been supposed to have some influence. The North American forests are
noted for the brilliant display of autumnal color. This is perhaps due
to some extent to the great variety or number of species which display
color. It would seem that there is some specific as well as individual
peculiarities in certain trees. Some individuals, for example, exhibit
brilliant colors every autumn, while others near of the same species
are more subdued.
It has been shown by experiment that when sunlight passes through
red colors the temperature is slightly increased, and it has been
suggested that this may be of protection to the living substance which
has ceased working and is in danger of injury from cold. There does
not seem to be much ground for this suggestion, however. It certainly
could not protect the protoplasm of the leaf at night when the cold
is more intense, and during the day would only aggravate matters
by supplying an increased amount of heat, since extremes of heat
and cold in alternation are more harmful to plant life than uniform
cold. Especially would this be the case in alpine climates where the
alternation of heat and cold between day and night is extreme, and
brilliancy of the colors of alpine plants is well known. It seems
more reasonable to suppose that the red color acts as a screen, as
the chlorophyll is disappearing, to protect from the injurious action
of light, certain organic substances which are to be transferred back
from the leaf to the stem for winter storage. So in the case of many
stems in the spring or early summer when the young leaves often have
a reddish color, it is likely that it acts as a screen to protect the
living substance from the strong light at that season of the year until
the chlorophyll screen, which is weak in young leaves, becomes darker
in color and more effective, when the red color often disappears.
=753. Function of foliage leaves.=—In general the function of
the foliage leaf as an organ of the plant is fivefold (see Chapters
IV, VII, VIII, XI), (1) that of carbon dioxide assimilation or
_photosynthesis_, (2) that of transpiration, (3) that of the synthesis
of other organic compounds, (4) that of respiration, and (5) that of
assimilation proper, or the making of new living substance. While none
of these functions are solely carried on in the leaf, it is the chief
seat of the first three of these processes, its form, position, and
structure being especially adapted to the purpose. Assimilation proper,
as well as respiration, probably take place equally in all growing or
active parts.
=754. Parts of the leaf.=—All foliage leaves possess a _blade_
or _lamina_, so called because of its _expanded_ and _thin_ character.
The blade is the essential part. Many leaves, however, are provided
with a stalk or _petiole_ by which the blade is held out at a greater
or lesser distance from the stem. Leaves with no petiole are _sessile_,
the blade is attached by one end directly on the stem. In some cases
the base of the blade is wrapped partly around the stem, or in others
it extends entirely around the stem and is _perfoliate_. Besides, many
leaves have short appendages, termed _stipules_, attached usually on
opposite sides of the petiole at its junction with the stem. In some
species of magnolia the stipules are so large that each one envelops
the entire portion of the bud which has not yet opened. Many leaves
possess outgrowths in the form of hairs, scales, etc. (See leaf
protection.)
=755. Simple leaves.=—Simple leaves are those in which the
blade is plane along the edge, not divided. The edge may be entire or
indented (serrate) to a slight extent as in the elm. The form of the
simple leaf varies greatly but is usually constant for a given species,
or it may vary in shape in the same species on different parts of the
plant. Some of the terms applied to the outline of the leaf are ovate,
oval, elliptical, lanceolate, linear, needle-like, etc., but it is idle
for one to waste time on matters of minute detail in form until it
becomes necessary for those in the future who pursue taxonomic work. It
is evident that a simple leaf, except those of minute size, possesses
advantages over a divided leaf in the amount of surface it exposes to
the light. But in other respects it is at a disadvantage, especially
as it increases in size, since it casts a deeper shade and does not
admit of such a free circulation of air. It will be found, however, in
our study of the relation of leaves to light and air that the balance
between the leaf and its environment is obtained in the relation of the
leaves to each other.
=756. Venation of leaves.=—A very prominent character of the
leaf is its “venation.” This is indicated by the presence of numerous
“veins,” indicated usually by narrow depressed lines on the upper
surface, and by more or less distinct elevated lines on the under
surface. There are two general types: (1) In the corn, Smilacina,
Solomon’s seal, etc., the veins extend lengthwise of the leaf and
are nearly parallel. Such leaves are said to be _parallel-veined_.
It is generally, though not always, a character of monocotyledenous
plants. (2) In the elm, rose, hawthorn, maple, oak, etc., the veins
are not all parallel. The larger ones either diverge from the base
of the blade (palmate leaf, maple), or the midvein extends through
the middle line of the leaf, while other prominent ones branch off
from this and extend, nearly parallel, toward the edge of the leaf
(pinnate venation). The smaller intermediate veins which are also
very distinct extend irregularly and branch and anastomose in such a
fashion as to give the figure of a net with very fine meshes. These
are _netted-veined_ leaves. These are characteristic of most of the
dicotyledenous plants. It is evident from what has been said of the
examples cited that there are two types of netted-veined leaves, the
_palmate_ and _pinnate_.
NOTE. As we have already learned in Chapter
V the veins contain the vascular bundles of the
leaf. Through them the water and food solutions are
distributed to all parts of the leaf, and the return
current of food material elaborated in the leaf moves
back through the bast portion into the shoot. The veins
also possess a small amount of mechanical tissue. This
forms the framework of the leaf and aids in giving
rigidity to the leaf and in holding it in the expanded
position. The mechanical tissue in the framework
alone could not support the leaf. Turgescence of the
mesophyll is needed in addition.
=757. Cut or lobed leaves.=—In many leaves, the indentations
on the margin are few and deep. Such leaves present several lobes
the proportionate size of which is dependent upon the depth of the
indentation or “incision.” Several of the maples, oaks, birches, the
poison ivy, thistles, the dandelion, etc., have lobed leaves. Where
the indentation reaches to or very near the midrib the leaf is said
to be cut. A study of various leaves will show all gradations from
simple leaves with plane edges to those which are cut or divided, as in
compound leaves, and the lobes are often variously indented.
[Illustration: Fig. 433. Lobed leaves of oak forming a mosaic.]
[Illustration: Fig. 434. Twice compound leaf. Leaflets arranged in one
plane, but open spaces permit free circulation of air through the large
leaf.]
=758. Divided, or compound leaves.=—The rose, sumac, elder,
hickory, walnut, locust, pea, clover, American creeper, etc., are
examples of divided or compound leaves. The former are pinnately
compound, and the latter are palmately compound. The leaf of the
honey-locust is twice pinnately compound or bipinnate, and some are
three times pinnately compound.[44] It is evident that compound leaves
are only extreme forms of lobed or cut leaves and that the form of all
bears a definite relation to the primary venation. There has been a
reduction of mesophyll and of the area of smaller venation.
_Unifoliate_ (for a single leaflet, as in orange and lemon where
the compound leaf is greatly reduced and consists of one pinna
attached to the petiole by a joint). _Bifoliate_ for one with two
leaflets; _trifoliate_ for one with three leaflets, as in the clover;
_plurifoliate_ for many leaflets. _Odd pinnate_ for a pinnate leaf with
one or more pairs of leaflets and one odd leaflet at the end.
So leaves are _palmately bifoliate_, etc., _pinnately bifoliate_, etc.
_Decompound_ leaves are those where they are more than twice compound,
as _ternately decompound_ in the common meadow rue (Thalictrum).
_Perfoliate_ leaves are seen in the bellwort (Uvularia), _connate
perfoliate_, as in some of the honeysuckles where the bases of opposite
leaves are joined together around the stem. _Equitant_ leaves are found
in the iris, where the leaves fit over one another at the base like a
saddle.
=759.= These forms of leaves probably have some definite
significance. It is not quite clear why they should have developed as
they have; though it is possible to explain several important relations
of these forms to their environment. (1) The reduction of the surface
of the leaf, with the retention of the firmer portions, allows freer
movement of the air and affords the leaf greater protection from injury
during violent winds, just as the finely dissected leaves of some
water plants are less liable to injury from movement of the more dense
medium in which they live. It is possible that here we may have an
explanation of one of the factors involved in this reduction of leaf
surface. (2) In trees with compound leaves, like the hickory, walnut,
locust, ailanthus, etc., the midvein, and in the case of the Kentucky
coffee-tree (Gymnocladus) the primary lateral veins also, serve in
place of terminal branches of the stem. By the increase in the outline
of the leaf and the reduction of its surface between the larger veins,
the tree has attained the same leaf development that it would were the
larger veins replaced by stems bearing simple leaves. The tree as
it is, however, has the advantage of being able to cast off for the
winter period a layer of what otherwise would have been a portion of
the stem system, to retain which through the winter would use more
energy than with the present reduced stem system, and the stouter
stem is less liable to dry out. In the case of herbaceous plants, in
the case of plants like most of the ferns where the stem is on the
underground rootstock (Pteris), or a very short erect stem, as in the
Christmas fern, the leaf replaces the aerial stem, and the division (or
branching, as it is sometimes styled) of the leaf corresponds to the
branching of the stem. This is more marked in the gigantic exotics like
Cibotium regale, and in the tree ferns which have quite tall trunks,
the massive compound leaves replace branches. In the palms and cycads
are similar examples. Those who choose to observe can doubtless find
many examples close at hand. (3) While divided leaves have probably not
been evolved in response to the light relation, still their relation in
this respect is an important one, since if the leaf with its present
size were entire, it would cast too dense a shade on other leaves below.
=760. General structure of the leaf.=—The general structure of
the leaf has been already studied (see Chapters IV, V, VII). It is
only necessary to recall the main points. The upper and lower surfaces
of the leaf are provided with a layer of cells usually devoid of
chlorophyll. The mesophyll of the leaf consists usually of a layer of
palisade cells beneath the epidermis, and the remainder consists of
loose parenchyma with large intercellular spaces. Through the mesophyll
course the “veins,” or fibrovascular strands, consisting of the xylem
and phloem portions and serving as conduits for water, salts, and
foodstuffs. In the epidermis are the stomata, each one protected by
the two guard cells. The guard cells as well as the mesophyll contain
chlorophyll. The stomata and the communicating intercellular spaces
furnish the avenues for the ingress and egress of gases, and for the
escape of water vapor.
=761. Protection of leaves.=—There are many modifications of
the general plan of structure in different leaves, many of them being
adaptations for the protection of the leaf under adverse or trying
conditions. Many leaves are also capable of assuming certain positions
which afford them protection. The discussion of this subject may be
presented under two general heads: Protective modifications; protective
positions.
II. Protective Modification of Leaves.
=762. General directions in which these modifications have taken
place.=—The usual type of foliage leaf selected is that of
deciduous trees or shrubs or of our common herbs. Such a leaf is
usually greatly expanded and thin in order to present as great a
surface as possible in comparison with its mass, since the kind of
work which the leaf has to do can be more effectually carried on when
it possesses this form. This form of leaf is best adapted for work in
regions where there is a medium amount of moisture such as exists in
the temperate zones. But since there are very great variations in the
climatic and soil conditions of these regions, and even greater changes
in desert and arctic regions, the type of leaf described is unsuited
for all. Its own life would be endangered, and it would also endanger
the life of the plant. Modifications have therefore taken place to
meet these conditions, or at least those plants whose leaves have
become modified in those directions which are suited to the surrounding
conditions have been able to persist. Excessive cold or heat, drought,
winds, intense light, rain, etc., are some of the conditions which
endanger leaves. The protective modifications of leaves may be grouped
under four general heads: (1) Structural adaptations; (2) Protective
covering; (3) Reduction of surface; (4) Elimination of the leaf through
the complete assumption of the leaf function by the stem.
[Illustration: Fig. 435. Structure of leaf of Lactuca scariola. Upper
one grown in sunlight, palisade cells on both sides. Lower one grown in
shade, no palisade tissue.]
=763.= (1) =Structural adaptations.=—The general structure
of the leaf presents certain features which are protective. The
palisade layer of cells found usually beneath the upper epidermis forms
a compact layer of long cells which not only acts as a light screen
cutting off a certain amount of the light, since too intense light
would be harmful; it also aids in lessening the loss of water from the
upper surface, where radiation is greater. The stomata are usually on
the under side of aerial leaves, and the mechanism which closes them
when the leaf is losing too much water is protective. As a protection
against intense light the number of palisade layers is sometimes
increased or the cells of this layer are narrow and long. This is often
beautifully shown when comparing leaves of the same plant grown in
strong light with those grown in the shade. The compass plant (Lactuca
scariola) affords an interesting example. The leaves grown in the light
are usually vertical, so that the light reaches both sides. Such leaves
often have all of the mesophyll organized into palisade cells (fig.
435), while leaves grown in the deep shade may have no palisade cells.
=764.= (2) =Protective covering.=—_Epidermis and
cuticle._—The walls of the epidermal cells are much thickened in some
plants. Sometimes this thickening occurs in the outer wall, or both
walls may be thickened. Variation in this respect as well as the extent
of the thickening occur in different plants and are often correlated
with the extremes of conditions which they serve to meet. The cuticle,
a waxy exudation from the thick wall of the epidermis of many leaves,
also serves as a protection against too great loss of water, or against
the leaf becoming saturated with water during rains. The cabbage,
carnation, etc., have a well-developed cuticle. The effect of the
cuticle in shedding water can be nicely shown by spraying water on a
cabbage leaf or by immersing it in water. Sunken stomata also retard
the loss of water vapor.
_Covers of hair or scales._—In many leaves certain of the cells of the
epidermis grow out into the form of hairs or scales of various forms,
and they serve a variety of purposes. When the hairs form a felt-like
covering as in the common mullein, some antennarias, etc., they lessen
the loss of water vapor because the air-currents close to the surface
of the leaf are retarded. Spines (see the thistles, etc.) also afford a
protection against certain animals.
=765.= (3) =Reduction of surface.=—Reduction of leaf surface
is brought about in a variety of ways. There are two general modes:
(1st) Reduction of surface along with reduction of mass; (2d) Reduction
of surface inversely as the mass. Examples of the first mode are
seen in the dissected leaves of many aquatic plants. In this finely
dissected condition the mass of the leaf substance is much reduced as
well as the leaf surface, but the leaf is less liable to be injured
by movement of the water. In addition it has already been pointed out
that lobed and divided aerial leaves are much less liable to injury
from violent movements of the air, than if a leaf with the same general
outline were entire. The needle leaves of the conifers are also
examples, and they show as well structural provisions for protection
in the thick, hard cell-walls of the epidermis. To offset the reduced
surface there are numerous crowded leaves. Reduction of surface
inversely as the mass, i.e., the mass of the leaf may not be reduced
at all, or it may be more or less increased. In other words, there is
less leaf surface in proportion to the mass of leaf substance. It is
probable in many cases, example: the crowded, overlapping small scale
leaves of the juniper, arbor-vitæ, cypress, cassiope, pyxidanthera,
etc., that there has been a reduction in the size of the leaf, and at
the same time an increase in thickness. This with the crowding together
of the leaves and their thick cell-walls greatly lessens the radiation
of moisture and heat, thus protecting the leaves both in dry and cold
weather. The succulents, like “live-forever,” have a small amount of
surface in proportion to the mass of the leaf. In the yucca, though the
leaves are often large, they are very thick and expose a comparatively
small amount of surface to the dry air and intense sunlight of the
desert regions. The epidermal covering is also hard and thick. In
addition, such leaves, as well as those of many succulents, are so
thick they provide water storage sufficient for the plants, which
radiate so slowly from their surface.
[Illustration: Fig. 436. A “Phylloclade,” leaves absent, stems
broadened to function as leaves, on the edges numerous flowers are
borne.]
=766.= (4) =Elimination of the leaf.=—Perhaps the most
striking illustration of the reduction of leaf surface is in those
cases where the leaf is either completely eliminated as in certain
euphorbias, or in certain of the cacti where the leaves are thought
to be reduced to spines. Whether the cactus spine belongs to the
leaf series or not, the leaf as an organ for assimilation and
transpiration has been completely eliminated and the same is true in
the phylloclades. The leaf function has been assumed by the stem. The
stem in this case contains all the chlorophyll; is bulky, and provides
water storage.
III. Protective Positions.
=767.= In many cases the leaves are arranged either in relation
to the stem, or to each other, or to the ground, in such a way as to
give protection from too great radiation of heat or moisture. In the
examples already cited the imbricated leaves of cassiope, pyxidanthera,
juniper, etc., come also under this head. In the junipers the leaves
spread out in the summer, while in the winter they are closely
overlapped. An interesting example of protective position is to be seen
in the case of the leaves of the white pine. During quite cold winter
weather the needles are appressed to the stem, and sometimes the trees
present a striking appearance in contrast with the spreading position
of the needles in summer. On windy days in winter, the needles turn
with the wind and become rigid in that position so that they remain in
a horizontal position for some time, often until the wind dies down,
or until milder weather. The following day, should there be a cold
strong wind from the opposite direction, the needles again assume a
leeward direction. In quiet weather appressed to the stem and in the
form of a brush there is less radiation of heat than if they diverged.
In strong winds by turning in the leeward direction the wind is not
driven between the needle bases and scales. Some plants, especially
many of those in arctic and alpine regions, have very short stems and
the leaves are developed near the ground, or the rock. Lying close on
the ground they do not feel the full force of the drying winds, there
is less radiation from them, and the radiation of heat from the ground
protects them. Many plants exhibit movement in response to certain
stimuli which place them in a position for protection. Some of these
examples have been discussed under the head of irritability (see
Chapter XIII). The night position of leaves and cotyledons presented
by many plants, but especially by many of the Leguminosæ, is brought
about by the removal of the light stimulus at evening. In many leaves,
when the light influence is removed, the influence of growth turns
the leaves downward, or the cotyledons of some plants upward. In this
vertical position of the leaf-blade there is less radiation of heat
during the cool night. The most striking cases of protection movements
are seen in the sensitive plant. As we have seen, the leaves of mimosa
close in a vertical position at midday if the light and heat are too
strong. Excessive transpiration is thus prevented. At night the vertical
position prevents excessive radiation of heat. The vertical or profile
position of the leaves of the compass plant already referred to not
only lessens transpiration, but the intense heat and light of the
midday sun is avoided. This profile position is characteristic of
certain plants in the dry regions of Australia, and the topmost leaves
of tropical forests.
IV. Relation of Leaves to Light.
=768.= It is very obvious from our study of the function of the
foliage leaf that its most important relation to environment is that
which brings it in touch with light and air. It is necessary that light
penetrate the leaf tissue that the gases of the air and plant may
readily diffuse and that water vapor may pass out of the leaf. The thin
expanded leaf-blade is the most economical and efficient organ for leaf
work. We have seen that leaves respond to fight stimulus in such a way
as to bring their upper sides usually to face the source of fight, at
right angles to it or nearly so (_heliotropism_, see Chapter XIII). How
fully this is brought about depends on the kind of plant, as well as on
other elements of the environment, for as we have seen in our study of
leaf protection there is danger to some plants in any region, and to
other plants in certain regions that the intense light and heat may
harm the protoplast, or the chlorophyll, or both.
[Illustration: Fig. 437. Mosaic form by trailing shoots of Panicum
variegatum, “ribbon-grass.”]
The statement that leaves usually face the light at right angles is to
be taken as a generalized one. The source of the strongest illumination
varies on different days and again at different times of the day.
On cloudy days the zenith is the source of strongest illumination.
The horizontal position of a leaf, where there are no intercepting
lateral or superior objects would receive its strongest light rays
perpendicular to its surface. The fact is, however, that leaves on the
same stem, because of taller or shorter adjacent stems, are so situated
that the rays of greatest illuminating power are directed at some angle
between the zenith and horizon. Many leaves, then, which may have
their upper sides facing the general source of strongest illumination,
do not necessarily face the sun, and they are thus protected from
possible injury from intense light and heat because the direct rays of
sunlight are for the most part oblique. This does not apply, of course,
to those leaves which “follow the sun” during the day. Their specific
constitution is such that intense illumination is beneficial.
The leaf is adjusted as well as may be in different species of varying
constitution, and under different conditions, to a certain balance in
its relation to the factors concerned. The problem then is to interpret
from this point of view the positions and grouping of leaves. Because
of the specific constitution of different plants, and because of a
great variety of conditions in the environment, we see that it is a
more or less complex question.
[Illustration: Fig. 438. Sunflower with young head turned toward morning
sun.]
=769. Day and night positions contrasted.=—In many plants the
day and night positions of the leaves are different. At night the
leaves assume a position more or less vertical, known as the _profile_
position. This is generally regarded as a protective position, since
during the cool of the night the radiation of heat is less than if the
leaf were in a vertical position. In many of these plants, however, the
leaves in assuming the night position become closely appressed which
would also lessen the radiation. This peculiarity of leaves is largely
possessed by the members of the family Leguminoseæ (clovers, peas,
beans, etc.), and by the sensitive plants.[45] But it is also shared by
some other plants as well (oxalis, for example). The leaves of these
plants are usually provided with a mechanism which enables them to
execute these movements with ease. There is a cushion (_pulvinus_) of
tissue at the base of the petiole, and in the case of compound leaves,
at the base of the pinnæ and pinnules which undergoes changes in turgor
in its cells. The collapsing of the cells by loss of water into the
intercellular spaces causes the leaf to droop. When the cells regain
their turgor by the absorption of the water from the intercellular
spaces the leaf is raised to the horizontal, or day position. The light
stimulus induces turgor of the pulvinus, the disappearance of the
stimulus is accompanied by a loss of turgor. It is a remarkable fact
that in some sensitive plants, intense light stimuli are alarm signals
which result in the same movement as if the light stimulus were
entirely removed. As we know also contact or pressure stimulus, or
jarring produces the same result in “sensitive” plants like mimosa,
some species of rubus, etc. In many plants there is no well-developed
pulvinus, and yet the leaves show similar movements in assuming the
day and night positions. Examples are seen in the sunflower, and in
the cotyledons of many plants. A little observation will enable any
one interested to discover some of these plants.[46] In these cases the
night position is due to epinastic growth, and while this influence is
not removed during the day the light stimulus overcomes it and the leaf
is raised to the day position.
[Illustration: Fig. 439. Same sunflower plant photographed just at
sundown.]
=770. Leaves which rotate with the sun.=—During the growth period
the leaves of the sunflower as well as the growing end of the stem
respond readily to the direct sunlight. The response is so complete
that during sunny days the leaves toward the growing end of the stem
are drawn close together in the form of a rosette and the entire
rosette as well as the end of the stem are turned so that they face the
sun directly. In the morning under the stimulus of the rising sun the
rosette is formed and faces the east. All through the day, if the sun
continues to shine, the leaves follow it, and at sundown the rosette
faces squarely the western horizon. For a week or more the young
sunflower head will also face the sun directly and follow it all day as
surely as the rosette of leaves. At length, a little while before the
flowers in the head blossom, the head ceases to turn, but the rosette
of leaves and the stem also, to some extent, continue to turn with the
sun. When the leaves become mature they also cease to turn. This is
well shown in all three photographs (figs. 438-439). The lower leaves
on the stem being older have assumed the fixed horizontal position
usually characteristic of the plant with cylindrical habit.
[Illustration: Fig. 440. Same plant a little older when the head does
not turn, but the stem and leaves do.]
It is not true, as is commonly supposed, that the fully opened
sunflower head turns with the sun. But I have observed young heads
four or five inches in diameter rotate with the sun all day. This is
because the growing end of the stem as well as the young head responds
to the light stimulus. So there is some truth as well as a great deal
of fiction in the popular belief that the sunflower head follows the
sun. The young head will follow the sun all day even if all the leaves
are cut off, and the growing stem will also if all the leaves as well
as the flower head are cut away. Young seedlings will also turn even if
the cotyledons and plumule are cut off.
This phenomenon of the rotation of leaves with the sun is much more
general than one would infer, as may be seen from a little careful
observation of rapidly growing plants on bright sunny days. In Alabama
I have observed beautiful rosettes of _Cassia marilandica_ rotate with
the sun all day. The peculiarity is very striking in the cotton plant,
especially when the rows extend north and south. In the forenoon or
afternoon it is most striking as the entire row shows the leaves tilted
up facing the sun. There are many of our weeds and common flowers of
field and garden which show this rotation of the leaves. Some of these
form rotating rosettes; while in others the leaves rotate independently
as in the sweet clover.
=771. Fixed position of old leaves.=—In many of the cases cited
in the preceding paragraph, the rotation of the leaf only occurs
on sunny days. During cloudy days the leaves of the sunflower, for
example, are in a nearly horizontal position, or the lower ones may
be somewhat oblique, since the stronger illumination on such a plant
would be the oblique rays rather than the zenith rays. As the leaves
reach maturity also the epinastic growth is equalized by hyponastic
growth so that the growth movements bring the leaf to stand in a nearly
horizontal position, or that position in which it receives the best
illumination. In age, then, many leaves have a fixed position and this
corresponds with the position assumed on cloudy days.
=772. Position on horizontal stems.=—On horizontal stems the
leaves have a horizontal position, and if such a stem is stood in an
erect position the appearance is very odd. If the leaf arises directly
from the horizontal stem, its petiole will be twisted part way around
in order to bring the face of the leaf uppermost. It is interesting
to observe the different relation of stem, petiole and blade and the
amount of twisting as the horizontal stem or vine trails over
irregularities in the surface, or climbs over and through other
vegetation.
=773. Position of leaflets on divided leaves.=—An interesting
comparison can be made with entire, lobed, divided and dissected
leaves. The entire leaf usually lies in one plane, since usually the
problem of adjustment is the same for the entire surface. So the lobes
of a leaf usually lie all in the same plane as they would if the leaf
were entire. We find the same is true usually of the compound leaf.
It forms an incomplete mosaic. Some of the pieces having been removed
allow much of the light to pass through to leaves beneath. Leaves,
especially those of some size rarely lie in a flat plane. Some are more
or less depressed. Some curve downward. Compound leaves often curve
more or less and the leaflets often droop more or less in a graceful
fashion. It is interesting, however, that these far separated leaflets
all lie in the same general plane. This is because the area of the
leaf, if not too large, makes the problem of position with reference
to light much the same as if the leaf were entire. The leaflets or
divisions, though separated, are laminate, and they can work more
efficiently facing the light. But suppose we extend our observation to
the finely dissected capillary leaves of some of the parsley family
(Umbelliferæ), or to the upper leaves of the fennel-leaved thoroughwort
(Eupatorium fœniculaceum) among the aerial plants, and to Myriophyllum
among the aquatic plants. The divisions are thread-like or cylindrical.
One side of the leaflet is just as efficient when presented to the
light as another. As a result the leaflets are not arranged in the same
plane, but stand out in many directions.
Occasionally one finds a divided or compound leaf in such a position
that one portion, because of being shaded above, receives the stronger
light stimulus from the side, while the other portion is lighted from
above. If this relation continues throughout the growth-period of the
leaf the leaflets of one portion may lie in a different plane from
those of the other portion. In such cases, some of the leaflets are
permanently twisted to bring them into their proper light relation.
V. Leaf Patterns.
MOSAICS, OR CLOSE PATTERNS.
[Illustration: Fig. 441. Fittonia showing leaves arranged to form
compact mosaic. The netted venation of the leaf is very distinctly
shown in this plant. (Photo by the Author.)]
=774.= Where the leaves of a plant, or a portion of a plant, are
approximate and arranged in the form of a pattern, the leaves fitting
together to form a more or less even and continuous surface, such
patterns are sometimes termed “mosaics,” since the relation of leaves
to one another is roughly like the relation of the pieces of a mosaic.
A good illustration of a mosaic is presented by a greenhouse plant
Fittonia (fig. 441). The stems are prostrate and the erect branches
quite short, but it may have quite a wide system by the spreading of
the runners; the branches of such a length that the leaves borne near
the tips all fit together forming a broad surface of leaves so closely
fitted together often that the stems cannot be seen. The advantage of
a mosaic over a separate disposition of leaves at somewhat different
levels is that the leaves do not shade one another. Were all the light
rays coming down at right angles to the leaves, there would not be
any shading of the lower ones, but the oblique rays of light would be
cut off from many of the leaves. In the case of a mosaic all the rays
of light play upon all the leaves. Some of the mosaics which can be
observed are as follows:
[Illustration: Fig. 442. Rosette pattern of leaves.]
=775. Rosette pattern.=—The rosette pattern is presented by
many plants with “radial” leaves, or leaves which arise in a cluster
near the surface of the ground, and are thus more or less crowded in
their arrangement on the stem. The pretty gloxinia often presents fine
examples of a loose rosette. In the rosette pattern the petioles of
the lower leaves are longer than the upper ones, and the blade is thus
carried out beyond the inner leaves. The leaves being so crowded in
their attachment to the stem lie very nearly in the same plane.
=776. Vines and climbers.=—Some of the most extensive mosaic
patterns are shown in creeping and climbing vines. A very common
example is that of the ivies trained on the walls of buildings,
covering in some instances many square yards of surface. Where the
vines trail over the ground or clamber over other vegetation, it is
interesting to observe the various patterns, and the distortion of
petioles brought about by turning of the leaves. Of examples found in
greenhouses, the Pellonia is excellent, and the trailing ribbon-grass
often forms loose mosaics.
=777. Branch patterns.=—These patterns are very common. They
are often formed in the woods on the ends of branches by the leaves
adjusting themselves so as to largely avoid shading each other. Figure
443 illustrates one of them from a maple branch. It is interesting to
note the way in which the leaves fit themselves in the pattern, how in
some the petioles have elongated, while others have remained short. Of
course, it should be understood that the pattern is made during the
growth of the leaves.
[Illustration: Fig. 443. Spray of leaves of striped maple, showing
different lengths of leafstalks.]
[Illustration: Fig. 444. Cedar of Lebanon, strong light only from one
side of tree (Syria).]
=778. The tree pattern.=—Mosaics are often formed by the exterior
foliage on a tree, though they are rarely so regular as some of those
mentioned above. Still it is common to see in some trees with drooping
limbs like the elm, beautiful and large mosaics. The weeping elm
sometimes forms a very close and quite even pattern over the entire
outer surface. In most trees the leaf arrangement is not such as to
form large patterns, but is more or less open. While the conifers do
not form mosaics there are many interesting examples of grouping of
foliage on branch systems into broadly expanded areas, as seen in the
branches of white pine trees, especially in the edge of a wood, or as
seen in the arbor-vitæ.
OTHER PATTERNS.
=779. Imbricate pattern of short stems.=—This pattern is quite
common, and differs from the rosette in that the leaves are distributed
further apart on the stem so that the central ones are considerably
higher up than in the mosaic. The lower petioles are longer, as in
the rosette, so that the outer lower leaves extend further out. Some
begonias show fine imbricate patterns.