I. MODES OF BRANCHING
MATERIAL.—For determinate growth, have twigs of an alternate and
an opposite-leaved plant showing well-developed terminal buds:
hickory, sweet gum, cottonwood, poplar, chestnut, are good examples
of the first; maple, ash, horse-chestnut, viburnum, of the second;
for the two-forked kind, mistletoe, buckeye, horse-chestnut, jimson
weed, lilac. For showing indefinite growth: rose, willow, sumach,
and ailanthus are good examples. Gummy buds, like horse-chestnut
and poplar, should be soaked in warm water before dissecting, to
soften the gum; the same treatment may be applied when the scales
are too brittle to be handled without breaking. Buds with heavy fur
on the scales cannot very well be studied in section; the parts
must be taken out and examined separately.
[Illustration: FIG. 147.—Diagram of excurrent growth.]
[Illustration: FIG. 148.—Diagram of deliquescent growth.]
=144. Modes of branching.=—Compare the arrangement of the boughs
on a pine, cedar, magnolia, etc., with those of the elm, maple,
apple, or any of our common deciduous trees. Draw a diagram of each,
showing the two modes of growth. The first represents the _excurrent_
kind, from the Latin _excurrere_, to run out; the second, in which
the trunk seems to divide at a certain point and flow away, losing
itself in the branches, is called _deliquescent_, from the Latin
_deliquescere_, to melt or flow away. The great majority of stems,
as a little observation will show, present a combination of the two
modes.
[Illustration: FIG. 149.—Winter twig of sugar maple: _t_, terminal
bud; _ax_, axillary buds; _ls_, leaf scars; _tr_, leaf traces; _l_,
lenticels; _rs_, ring of scars left by bud scales of preceding
season.]
=145. Terminal and axillary buds.=—Notice the large bud at the end of
a twig of hickory, sweet gum, beech, cottonwood, etc. This is called
the _terminal_ bud because it terminates its branch. Notice the
scars left by the leaves of the season as they fell away, and look
for small buds just above them. These are _lateral_, or _axillary_,
buds, so called because they spring from the axils of the leaves. How
many leaves did your twig bear? What difference in size do you notice
between the terminal and lateral buds?
=146. The leaf scars.=—Examine the leaf scars with a hand lens,
and observe the number and position of the little dots in them.
Ailanthus, varnish tree, sumach, and China tree show these very
distinctly. They are called _leaf traces_, and mark the points where
the fibrovascular bundles from the leaf veins passed into the stem.
Look on the bark, or epidermis, for lenticels.
=147. Bud scales and scars.=—Notice the stout, hard scales by which
the winter buds are covered in most of our hardy trees and shrubs.
Remove these from the terminal one of your specimen, and notice the
ring of scars left around the base. Look lower down on your twig
for a ring of similar scars left from last year’s bud. Is there any
difference in the appearance of the bark above and below this ring?
If so, what is it, and how do you account for it? Is there more than
one of these rings of scars on your twig, and if so, how many? How
old is the twig and how much did it grow each year? Has its growth
been uniform, or did it grow more in some years than in others?
[Illustration: FIG. 150.—Diagram of opposite bud scales.]
=148. Arrangement and use of the scales.=—Notice the manner in
which the scales overlap so as to “break joints,” like shingles
on the roof of a house. Where the leaves are opposite, the manner
of superposition is very simple. Remove the scales one by one,
representing the number and position of the pairs by a diagram
after the model given in Fig. 150. In the bud of an alternately
branched twig the order will be different, and the diagram must be
varied accordingly. Do you observe any difference as to size and
texture between the outer and inner scales? Notice how the former
inclose the tenderer parts within like a protecting wall. In cold
climates the outer scales are frequently coated with gum, as in the
horse-chestnut, for greater security against the weather. The hickory
and various other trees have the inner scales covered with fur or
down that envelops the tender bud like a warm blanket.
=149. Nature of the scales.=—The position of the scales shows
that they occupy the place of leaves or of some part of a leaf.
In expanding buds of the lilac and many other plants, they can be
found in all stages of transition, from scales to true leaves. In
the buckeye and horse-chestnut, they will easily be recognized as
modified leaf stalks (Fig. 151). In the tulip tree, magnolia, India
rubber tree, fig, elm, and many others, they represent appendages
called _stipules_, often found at the bases of leaves. (See 165,
166.) In this case a pair of scales is attached with each separate
leaflet, and as the growing axis lengthens in spring, they are
carried apart by the elongation of the internodes so that the scars
are separated, a pair at each node, making rings all along the stem,
as shown in Fig. 152, instead of having them compacted into bands at
the base of the bud. These scars are sometimes very persistent, and
in the common fig and magnolia may often be traced on stems six to
eight years old. Do they furnish any indication as to the relative
age of the different parts of the stem, like the bands of scars on
twigs of horse-chestnut and hickory? Give a reason for your answer.
(Fig. 152.)
[Illustration: FIG. 151.—Development of the parts of the bud in the
buckeye. (_After_ GRAY.)]
[Illustration: FIG. 152.—Stem of tulip tree: _s_, _s_, scars left by
stipular scales; _l_, _l_, leaf scars.]
=150. Different rates of growth.=—Notice the very great difference
between branches in this respect. Sometimes the main stem will have
lengthened from twenty to fifty centimeters or more in a single
season, while some of the lateral ones will have grown but an inch
or two in four or five seasons. One reason for this is because the
terminal bud, being on the great trunk line of sap movement, gets
a larger share of nourishment than the others, and being stronger
and better developed to begin with, starts out in life with better
chances of success.
Make a drawing of your specimen, showing all the points brought out
in the examination just made. Cut sections above and below a set of
bud scars and count the rings of annual growth in each section. What
is the age of each? How does this agree with your calculation from
the number of scar clusters left by the bud scales?
=151. Irregularities.=—Take a larger bough of the same kind that you
have been studying, and observe whether the arrangement of branches
on it corresponds with the arrangement of buds on the twig. Did
all the buds develop into branches? Do those that did develop all
correspond in size and vigor? If all the buds developed, how many
branches would a tree produce every year?
In the elm, linden, beech, hornbeam, hazelnut, willow, and various
other plants, the terminal bud always dies and the one next in order
takes its place, giving rise to the more or less zigzag axis that
generally characterizes trees of these species. (Fig. 153.)
[Illustration: FIG. 153.—Bud development of beech: _a_, as it is,
many buds failing to develop; _b_, as it would be if all the buds
were to live.]
=152. Forked stems.=—Take a twig of buckeye, horse-chestnut, or
lilac, and make a careful sketch of it, showing all the points that
were brought out in the examination of your previous specimen. Which
is the larger, the lateral or the terminal bud? Is their arrangement
alternate or opposite? What was the leaf arrangement? Count the leaf
traces in the scars; are they the same in all? If all the buds had
developed into branches, how many would spring from a node? Look for
the rings of scars left by the last season’s bud scales. Do you find
any twig of more than one year’s growth, as measured by the scar
rings?
[Illustration: FIG. 154.—Two-forked twig of horse-chestnut.]
Look down between the forks of a branched stem for a round scar.
This is not a leaf scar, as we can see by its shape, but one left by
the last season’s flower cluster. The flower, as we know, dies after
perfecting its fruit, and so a flower bud cannot continue the growth
of its axis as other buds do, but has just the opposite effect and
stops all further growth in that direction. Hence, stems and branches
that end in a flower bud cannot continue to develop their main axis,
but their growth is usually carried on, in alternate-leaved stems, by
the nearest lateral bud, or in opposite-leaved ones, by the nearest
pair of buds. In the first case there results the zigzag spray
characteristic of such trees as the beech and elm (Fig. 155, _B_); in
the second, the two-forked, or _dichotomous_ branching, exemplified
by the buckeye, horse-chestnut, jimson weed, mistletoe, and dogwood
(Fig. 155, _A_).
Draw a diagram of the buckeye, or other dichotomous stem, as it would
be if all the buds developed into branches, and compare it with
your diagrams of excurrent and deliquescent growth. Draw diagrams
to illustrate the branching of the elm, beech, lilac, linden, rose,
maple, or their equivalents.
[Illustration: FIG. 155.—Diagrams of two-forked branching. The
pointed bodies in the forks shows where terminal flower buds or
flower clusters have changed the direction of growth.]
=153. Definite and indefinite annual growth.=—The presence or absence
of terminal buds gives rise to another important distinction in
plant development—that of _definite_ and _indefinite_ annual growth.
Compare with any of the twigs just examined, a branch of rose, honey
locust, sumac, mulberry, etc., and note the difference in their
modes of termination. The first kind, where the bough completes its
season’s increase in a definite time and then devotes its energies to
developing a strong terminal bud to begin the next year’s work with,
are said to make a _definite or determinate annual growth_. Those
plants, on the other hand, which make no provision for the future,
but continue to grow till the cold comes and literally nips them in
the bud, are _indefinite_, or _indeterminate_ annual growers. Notice
the effect of this habit upon their mode of branching. The buds
toward the end of each shoot, being the youngest and tenderest, are
most readily killed off by frost or other accident, and hence new
branches spring mostly from the older and stronger buds near the base
of the stem. It is their mode of branching that gives to plants of
this class their peculiar bushy aspect. Such shrubs generally make
good hedges on account of their thick undergrowth. The same effect
can be produced artificially by pruning.
[Illustration: FIG. 156.—A mixed wood in winter, showing the trend of
the branches.]
=154. Differences in the branching of trees.=—We are now prepared
to understand something about the causes of that endless variety in
the spread of bough and sweep of woody spray that makes the winter
woods so beautiful. Where the terminal bud is undisputed monarch
of the bough, as in the pine and fir, or where it is so strong and
vigorous as to overpower its weaker brethren and keep the lead, as
in the magnolia, tulip tree, and holly, we have excurrent growth. In
plants like the oak and apple, where all the buds have a more nearly
equal chance, the lateral branches show more vigor, and the result is
either deliquescent growth, or a mixture of the two kinds. In the elm
and beech, where the usurping pseudo-terminal bud keeps the mastery,
but does not completely overpower its fellows, we find the long,
sweeping, delicate spray characteristic of those species. Examine a
sprig of elm, and notice further that the flower buds are all down
near the base of the stem, while the leaf buds are near the tip. The
chief development of the season’s growth is thus thrown toward the
end of the branch, giving rise to that fine, feathery spray which
makes the elm an even more beautiful object in winter than in summer
(Fig. 158).
An examination of the twigs of other trees will bring out the various
peculiarities that affect their mode of branching. The angle, for
instance, which a twig makes with its bough has a great effect in
shaping the contour of the tree. Compare in this respect the elm and
hackberry; the tulip tree and willow; ash and hickory. As a general
thing, acute angles produce slender, flowing effects; right or obtuse
angles, more bold and rugged outlines.
[Illustration: FIG. 157.—Winter spray of ash, an opposite-leaved
tree.]
[Illustration: FIG. 158.—Winter spray of elm.]
Practical Questions
1. Has the arrangement of leaves on a twig anything to do with the
way a tree is branched? (145, 151, 152.)
2. Why do most large trees tend to assume the excurrent, or axial,
mode of growth if let alone? (150, 154.)
3. If you wished to alter the mode of growth, or to produce what
nurserymen call a low-headed tree, how would you prune it? (152,
153.)
4. Would you top a timber tree? (152, 153.)
5. Are low-headed or tall trees best for an orchard? Why?
6. Why is the growth of annuals generally indefinite?
7. Name some trees of your neighborhood that are conspicuous for
their graceful winter spray.
8. Name some that are characterized by sharpness and boldness of
outline.
9. Account for the peculiarities in each case.
II. BUDS
MATERIAL.—Expanding leaf and flower buds in different stages of
development; large ones show the parts best and should be used
where attainable. Some good examples for the opposite arrangement
are horse-chestnut, maple, lilac, ash; for the alternate: hickory,
sweet gum, balsam poplar, beech, elm. Where material is scarce, the
twigs used in the last section may be placed in water and kept till
the buds begin to expand.
=155. Folding of the leaves.=—Remove the scales from a bud of
horse-chestnut nearly ready to open, and notice the manner in
which the young leaves are folded. This is called _vernation_, or
_prefoliation_, words meaning respectively “spring condition” and
“condition preceding the leaf.” Leaves are packed in the bud so as to
occupy the least space possible, and in different plants they will be
found folded in a great many different ways, according to the shape
and texture of the leaf and the space available for it in the bud.
When doubled back and forth like a fan, or crumpled and folded as in
the buckeye, horse-chestnut, and maple, the vernation is _plicate_
(Figs. 160, 162).
[Illustration: FIG. 159.—Expanding bud of English walnut, showing
twice conduplicate vernation.]
[Illustration: FIG. 160.—A partly expanded leaf of beech, showing
plicate-conduplicate vernation.]
=156. Position of the flower cluster.=—What do you find within the
circle of leaves? Examine one of the smaller axillary buds, and
see if you find the same object within it. If you are in any doubt
as to what this object is, examine a bud that is more expanded,
and you will have no difficulty in recognizing it as a rudimentary
flower cluster. Notice its position with reference to the scales and
leaves. If at the center of the bud, it will, of course, terminate
its axis when the bud expands, and the growth of the branch will
culminate in the flower. The branching of any kind of stem that
bears a central flower cluster must, then, be of what order? Compare
your drawings with the section of a hyacinth bulb or jonquil, and
note the similarity in position of the flower clusters. In a bud
of the hickory, walnut, oak, etc., the position of the flower
clusters is different from that of flowers in the buds of lilac
and horse-chestnut. Look for a bud containing them, and find out
where they occur. Can the axis continue to grow after flowering,
in this kind of stem? Give a reason for your answer. Make sketches
in transverse and longitudinal section (see Figs. 162, 163) of two
different kinds of buds, illustrating the terminal and axillary
position of the flower cluster.
[Illustration: FIGS. 161, 162.—Buds of maple: 161, vertical section
of a twig; 162, cross section through bud, showing folded leaves in
center and scales surrounding them.]
[Illustration: FIG. 163.—Vertical section of hickory bud: _a_, furry
inner scales; _b_, outer scales; _l_, folded leaf; _r_, receptacle.]
=157. Dormant buds.=—A bud may often lie dormant for months or even
years, and then, through the injury or destruction of its stronger
rivals, or some other favoring cause, develop into a branch. Such
buds are said to be _latent_ or _dormant_. The sprouts that often put
up from the stumps of felled trees originate from this source.
[Illustration: FIG. 164.—Twig of red maple, showing supernumerary
bud, _b_; _rs_, ring of scars left by last year’s bud scales.
(_After_ GRAY.)]
=158. Supernumerary buds.=—Where more than one bud develops at a
node, as is so often the case in the oak, maple, honey locust,
etc., all except the normal one in the axil are _supernumerary_
or _accessory_. These must not be confounded with _adventitious_
buds—those that occur elsewhere than at a node.
Practical Questions
1. Would protected buds be of any use to annuals? Why, or why not?
2. Of what use is the gummy coating found on the buds of the
horse-chestnut and balm of Gilead? (148.)
3. Can you name any plants the buds of which serve as food for man?
4. How do flower buds differ in shape from leaf buds?
5. At what season can the leaf bud and the flower bud first be
distinguished? Is it the same for all flowering plants?
6. Watch the different trees about your home, and see when the buds
that are to develop into leaves and flowers the next season are
formed in each species.
III. THE BRANCHING OF FLOWER STEMS
MATERIAL.—Typical flower clusters illustrating the definite and
indefinite modes of inflorescence. Some of those mentioned in the
text are:—
Indefinite: hyacinth, shepherd’s purse, wallflower, carrot, lilac,
blue grass, smartweed (_Polygonum_), wheat, oak, willow, clover.
Definite: chickweed, spurge (_Euphorbia_), comfrey, dead nettle,
etc. Any examples illustrating the principal kinds of cluster will
answer.
=159. Inflorescence= is a term used to denote the position and
arrangement of flowers on the stem. It is merely a mode of branching,
and follows the same laws that govern the branching of ordinary stems.
The stalk that bears a flower is called the _peduncle_. In a cluster
the main axis is the common peduncle, and the separate flower stalks
are _pedicels_. A simple leafless flower stalk that rises directly
from the ground, like those of the dandelion and daffodil, is called
a _scape_ (Fig. 165).
[Illustration: FIG. 165.—Solitary terminal flower of a lily.]
[Illustration: FIG. 166.—Indeterminate inflorescence of moneywort.
(_After_ GRAY.)]
=160. Two kinds of inflorescence.=—The growth of flower stems,
like that of leaf stems, is of two principal kinds, definite and
indefinite, or, as it is frequently expressed, determinate and
indeterminate. The simplest kind of each is the solitary, a single
flower either terminating the main axis, as the tulip, daffodil,
trillium, magnolia, etc., or springing singly from the axils, as the
running periwinkle, moneywort, and cotton.
[Illustration: FIG. 167.—Raceme of milk vetch (_Astragalus_).]
[Illustration: FIG, 168.—Catkins of aspen.]
=161. Indeterminate inflorescence= is always axillary, since the
production of a terminal flower would stop further growth in that
direction and thus terminate the development of the axis. The
_raceme_ is the typical flower cluster of the indefinite sort. In
such an arrangement the oldest flowers are at the lower nodes,
new ones appearing only as the axis lengthens and produces new
internodes. The little scale or _bract_ usually found at the base of
the pedicel in flower clusters of this sort is a reduced leaf, and
the fact that the flower stalk springs from the axil shows it to be
of the essential nature of a branch. When the flowers are sessile
and crowded on the axis in various degrees, the cluster produced may
be a _spike_, as seen in the plantain, knotweed, etc., or a _head_,
like that of the clover, buttonwood, and sycamore. The _catkins_ that
form the characteristic inflorescence of most of our forest trees
are merely pendant spikes. The _corymb_ is a modification of the
raceme in which the lower pedicels are elongated so as to place their
flowers on a level with those of the upper nodes, making a convex, or
more or less flat-topped cluster, as in the wallflower and hawthorn.
The _umbel_ differs from the corymb in having the pedicels with their
bracts all gathered at the top of the peduncle, from which they
spread in every direction like the rays of an umbrella, as the name
implies. This is the prevalent type of flower cluster in the parsley
family, which takes its botanical name, _Umbelliferæ_, from its
characteristic form of inflorescence. The pedicels of an umbel are
called _rays_, and the circle of bracts at the base of the cluster is
an _involucre_.
[Illustration: FIG. 169.—Corymb of plum blossoms.]
[Illustration: FIG. 170.—Umbel of milkweed.]
[Illustration: FIG. 171.—Panicle of grass, a compound cluster of the
racemose type.]
[Illustration: FIG. 172.—Flat-topped cyme of sneezeweed.]
=162. Determinate, or cymose, inflorescence.=—In the _cyme_, the
typical cluster of the determinate kind, the older blossoms in the
center, being terminal, stop the axis of growth in that direction and
force the stem, in continuing its growth, to send out side branches
from the axils of the topmost leaves, in a manner precisely similar
to the two-forked branching of stems like the horse-chestnut and
jimson weed. When the older peduncles are lengthened as described
in 161, a flat-topped cyme is produced, which is distinguished
from the corymb by its order of flowering, the oldest blossoms
being at the center, while in the corymb they appear in the reverse
order. A peculiar form of cyme is found in the scorpioid or coiled
inflorescence of the pink-root (_Spigelia_), heliotrope, comfrey,
etc. Its structure will be made clear by an inspection of Figs.
174-176.
[Illustration: FIG. 173.—Scorpioid cyme.]
=163. The nature of flower stems.=—A comparison of the types of
inflorescence with the modes of branching in ordinary stems (144,
152, 153) will show a strict correspondence between them. Both bear
leaves and buds, and the individual flowers of a cluster usually
spring from the axils of leaves or from bracts, which are merely
reduced leaves. What, then, is the essential nature of flower stems?
[Illustration: FIGS. 174-176.—Diagrams of cymose inflorescence, with
flowers numbered in the order of their development: 174, cyme half
developed (scorpioid); 175, a flat-topped or corymbose cyme; 176,
development of a typical cyme.]
=164. Significance of the clustered arrangement.=—As a general thing
the clustered arrangement marks a higher stage of development than
the solitary, just as in human life the rudest social state is a
distinct advance upon the isolated condition of the savage. In plant
life it is the beginning of a system of coöperation and division of
labor among the associated members of the flower cluster, as will be
seen later when we take up the study of the flower.
Practical Questions
1. Name as many solitary flowers as you can think of.
2. Do you, as a rule, find very small flowers solitary, or in
clusters?
3. Would the separate flowers of the clover, parsley, or grape be
readily distinguished by the eye among a mass of foliage?
4. Should you judge from these facts that it is, in general,
advantageous to plants for their flowers to be conspicuous?
Field Work
(1) In connection with 144-154, the characteristic modes of
branching among the common trees and shrubs of each neighborhood
should be observed and accounted for. The naked branches of the
winter woods afford exceptional opportunities for studies of this
kind, which cannot well be carried on except out of doors. Note
the effect of the mode of branching upon the general outline of
the tree; compare the direction and mode of growth of the larger
boughs with that of small twigs in the same species, and see if
there is any general correspondence between them; note the absence
of fine spray on the boughs of large-leaved trees, and account
for it. Account for the flat sprays of trees like the elm, beech,
hackberry, etc.; the irregular stumpy branches of the oak and
walnut; the stiff straight twigs of the ash; the zigzag switches of
the black locust, Osage orange, elm, and linden. Measure the twigs
on various species, and see if there is any relation between the
length and thickness of branches. Notice the different trend of
the upper, middle, and lower boughs in most trees, and account for
it. Observe the mode of branching of as many different species as
possible of some of the great botanical groups of trees; the oaks,
hickories, hawthorns, and pines, for instance, and notice whether
it is, as a general thing, uniform among the species of the same
group, and how it differs from that of other groups.
(2) In connection with 155-158, buds of as many different kinds
as possible should be examined with reference to their means of
protection, their vernation and leaf arrangement, and the resulting
modes of growth. Compare the folding of the cotyledons in the seed
with the vernation of the same plants, and observe whether the
folding is the same throughout a whole group of related plants,
or only for the same species. Notice which modes seem to be most
prevalent. Select a twig on some tree near your home or your
schoolhouse, and keep a record of its daily growth from the first
sign of the unfolding of its principal bud to the full development
of its leaves. Any study of buds should include an observation of
them in all stages of development.
(3) With 160-165, study the inflorescence of the common plants and
weeds that happen to be in season, until you have no difficulty
in distinguishing between the definite and indefinite sorts, and
can refer any ordinary cluster to its proper form. Notice whether
there is any tendency to uniformity in the mode of inflorescence
among flowers of the same family. Consider how each kind is adapted
to the shape and habit of the flowers composing it, and what
particular advantage each of the specimens examined derives from
the way its flowers are clustered. In cases of mixed inflorescence,
see if you can discover any reason for the change from one form to
the other.