45. Most plants increase the amount of their vegetation by branching,
that is, by producing lateral shoots.
46. Roots branch from any part and usually without definite order. Stems
normally give rise to branches only at definite points, namely, at the
nodes, and there only from the axils of leaves.
47. =Buds= (Fig. 72, 73). Every incipient shoot is a _Bud_ (12). A stem
continues its growth by its _terminal bud_; it branches by the formation
and development of _lateral buds_. As normal lateral buds occupy the
axils of leaves, they are called _axillary buds_. As leaves are
symmetrically arranged on the stem, the buds in their axils and the
branches into which axillary buds grow partake of this symmetry. The
most conspicuous buds are the scaly winter-buds of most shrubs and trees
of temperate and cold climates; but the name belongs as well to the
forming shoot or branch of any herb.
[Illustration: Fig. 72. Shoot of Horse-chestnut, of one year's growth,
taken in autumn after the leaves have fallen; showing the large terminal
bud and smaller axillary buds.]
[Illustration: Fig. 73. Similar shoot of Shagbark Hickory, Carya alba.]
48. =The Terminal Bud=, in the most general sense, may be said to exist
in the embryo,--as cotyledons, or the cotyledons and plumule,--and to
crown each successive growth of the simple stem so long as the summit is
capable of growth. The whole ascending growth of the Palm, Cycas, and
the like (such as in Fig. 71) is from a terminal bud. Branches, being
repetitions of the main stem and growing in the same way, are also
lengthened by terminal buds. Those of Horse-chestnut, Hickory, Maples,
and such trees, being the resting buds of winter, are conspicuous by
their protective covering of scales. These bud-scales, as will hereafter
be shown, are themselves a kind of leaves.
49. =Axillary Buds= were formed on these annual shoots early in the
summer. Occasionally they grow the same season into branches; at least,
some of them are pretty sure to do so whenever the growing terminal bud
at the end of the shoot is injured or destroyed. Otherwise they may lie
dormant until the following spring. In many trees or shrubs these
axillary buds do not show themselves until spring; but if searched for,
they may be detected, though of small size, hidden under the bark.
Sometimes, although early formed, they are concealed all summer long
under the base of the leaf-stalk, which is then hollowed out into a sort
of inverted cup, like a candle-extinguisher, to cover them; as in the
Locust, the Yellow-wood, or more strikingly in the Button-wood or
Plane-tree (Fig. 74).
[Illustration: Fig. 74. An axillary bud, concealed under the hollowed
base of the leaf-stalk, in Buttonwood or Plane-tree.]
50. The _leaf-scars_, so conspicuous in Fig. 72, 73, under each axillary
bud, mark the place where the stalk of the subtending leaf was attached
until it fell in autumn.
51. =Scaly Buds=, which are well represented in Fig. 72, 73, commonly
belong to trees and shrubs of countries in which growth is suspended
during winter. The scaly coverings protect the tender young parts
beneath, not so much by keeping out the cold, which of course would
penetrate the bud in time, as by shielding the interior from the effects
of sudden changes. There are all gradations between these and
52. =Naked Buds=, in which these scales are inconspicuous or wanting, as
in most herbs, at least above ground, and most tropical trees and
shrubs. But nearly related plants of the same climate may differ widely
in this respect. Rhododendrons have strong and scaly winter-buds; while
in Kalmia they are naked. One species of Viburnum, the Hobble-bush, has
completely naked buds, what would be a pair of scales developing into
the first leaves in spring; while another (the Snowball) has conspicuous
scaly buds.
53. =Vigor of Vegetation from strong buds.= Large and strong buds, like
those of the Horse-chestnut, Hickory, and the like, contain several
leaves, or pairs of leaves, ready formed, folded and packed away in
small compass, just as the seed-leaves of a strong embryo are packed
away in the seed: they may even contain all the blossoms of the ensuing
season, plainly visible as small buds. And the stems upon which these
buds rest are filled with abundant nourishment, which was deposited the
summer before in the wood or in the bark. Under the surface of the
soil, or on it covered with the fallen leaves of autumn, similar strong
buds of our perennial herbs may be found; while beneath are thick roots,
rootstocks, or tubers, charged with a great store of nourishment for
their use. This explains how it is that vegetation from such buds shoots
forth so vigorously in the spring of the year, and clothes the bare and
lately frozen surface of the soil, as well as the naked boughs of trees,
very promptly with a covering of fresh green, and often with brilliant
blossoms. Everything was prepared, and even formed, beforehand: the
short joints of stem in the bud have only to lengthen, and to separate
the leaves from each other so that they may unfold and grow. Only a
small part of the vegetation of the season comes directly from the seed,
and none of the earliest vernal vegetation. This is all from buds which
have lived through the winter.
54. =The Arrangement of Branches=, being that of axillary buds, answers
to that of the leaves. Now leaves principally are either _opposite_ or
_alternate_. Leaves are _opposite_ when there are two from the same
joint of stem, as in Maples (Fig. 20), the two being on opposite sides
of the stem; and so the axillary buds and branches are opposite, as in
Fig. 75. Leaves are _alternate_ when there is only one from each joint
of stem, as in the Oak, Lime-tree, Poplar, Button-wood (Fig. 74),
Morning-Glory (Fig. 45,--not counting the seed-leaves, which of course
are opposite, there being a pair of them); also in Indian Corn (Fig.
70), and Iris (Fig. 59). Consequently the axillary buds are also
alternate, as in Hickory (Fig. 73); and the branches they form
alternate,--making a different kind of spray from the other mode, one
branch shooting on one side of the stem and the next on some other. For
in the alternate arrangement no leaf is on the same side of the stem as
the one next above or next below it.
55. But the symmetry of branches (unlike that of the leaves) is rarely
complete. This is due to several causes, and most commonly to the
56. =Non-development of buds.= It never happens that all the buds grow.
If they did, there might be as many branches in any year as there were
leaves the year before. And of those which do begin to grow, a large
portion perish, sooner or later, for want of nourishment, or for want of
light, or because those which first begin to grow have an advantage,
which they are apt to keep, taking to themselves the nourishment of the
stem, and starving the weaker buds. In the Horse-chestnut (Fig. 72),
Hickory (Fig. 73), Magnolia, and most other trees with large scaly buds,
the terminal bud is the strongest, and has the advantage in growth; and
next in strength are the upper axillary buds: while the former continues
the shoot of the last year, some of the latter give rise to branches,
and the rest fail to grow. In the Lilac also (Fig. 75), the uppermost
axillary buds are stronger than the lower; but the terminal bud rarely
appears at all; in its place the uppermost pair of axillary buds grow,
and so each stem branches every year into two,--making a repeatedly
two-forked ramification, as in Fig. 76.
[Illustration: Fig. 75. Shoot of Lilac, with winter buds; the two
uppermost axillary ones strong; the terminal not developed. 76. Forking
ramification of Lilac; reduced in size.]
57. =Latent Buds.= Axillary buds that do not grow at the proper season,
and especially those which make no appearance externally, may long
remain latent, and at length upon a favorable occasion start into
growth, so forming branches apparently out of place as they are out of
time. The new shoots seen springing directly out of large stems may
sometimes originate from such latent buds, which have preserved their
life for years. But commonly these arise from
58. =Adventitious Buds.= These are buds which certain shrubs and trees
produce anywhere on the surface of the wood, especially where it has
been injured. They give rise to the slender twigs which often feather
the sides of great branches of our American Elms. They sometimes form on
the root, which naturally is destitute of buds; they are even found upon
some leaves; and they are sure to appear on the trunks and roots of
Willows, Poplars, and Chestnuts, when these are wounded or mutilated.
Indeed Osier-Willows are _pollarded_, or cut off, from time to time, by
the cultivator, for the purpose of producing a crop of slender
adventitious twigs, suitable for basket-work. Such branches, being
altogether irregular, of course interfere with the natural symmetry of
the tree. Another cause of irregularity, in certain trees and shrubs, is
the formation of what are called
[Illustration: Fig. 77. Tartarean Honeysuckle, with three accessory buds
in each axil.]
59. =Accessory or Supernumerary Buds.= There are cases where two, three,
or more buds spring from the axil of a leaf, instead of the single one
which is ordinarily found there. Sometimes they are placed one over the
other, as in the Aristolochia or Pipe-Vine, and in the Tartarean
Honeysuckle (Fig. 77); also in the Honey-Locust, and in the Walnut and
Butternut (Fig. 78), where the upper supernumerary bud is a good way
out of the axil and above the others. And this is here stronger than the
others, and grows into a branch which is considerably out of the axil,
while the lower and smaller ones commonly do not grow at all. In other
cases three buds stand side by side in the axil, as in the Hawthorn, and
the Red Maple (Fig. 79.) If these were all to grow into branches, they
would stifle each other. But some of them are commonly flower-buds: in
the Red Maple, only the middle one is a leaf-bud, and it does not grow
until after those on each side of it have expanded the blossoms they
contain.
[Illustration: Fig. 78. Butternut branch, with accessory buds, the
uppermost above the axil.]
[Illustration: Fig. 79. Red-Maple branch, with accessory buds placed
side by side. The annular lines toward the base in this and in Fig. 72
are scars of the bud-scales, and indicate the place of the winter-bud of
the preceding year.]
60. =Sorts of Buds.= It may be useful to enumerate the kinds of buds
which have been described or mentioned. They are
_Terminal_, when they occupy the summit of (or terminate) a stem,
_Lateral_, when they are borne on the side of a stem; of which the
regular kind is the
_Axillary_, situated in the axil of a leaf. These are
_Accessory_ or _Supernumerary_, when they are in addition to the normal
solitary bud; and these are _Collateral_, when side by side;
_Superposed_, when one above another;
_Extra-axillary_, when they appear above the axil, as some do when
superposed, and as occasionally is the case when single.
_Naked buds_; those which have no protecting scales.
_Scaly buds_; those which have protecting scales, which are altered
leaves or bases of leaves.
_Leaf-buds_, contain or give rise to leaves, and develop into a leafy
shoot.
_Flower-buds_, contain or consist of blossoms, and no leaves.
_Mixed buds_, contain both leaves and blossoms.
61. =Definite annual Growth= from winter buds is marked in most of the
shoots from strong buds, such as those of the Horse-chestnut and Hickory
(Fig. 72, 73). Such a bud generally contains, already formed in
miniature, all or a great part of the leaves and joints of stem it is to
produce, makes its whole growth in length in the course of a few weeks,
or sometimes even in a few days, and then forms and ripens its buds for
the next year's similar growth.
62. =Indefinite annual Growth=, on the other hand, is well marked in
such trees or shrubs as the Honey-Locust, Sumac, and in sterile shoots
of the Rose, Blackberry, and Raspberry. That is, these shoots are apt
to grow all summer long, until stopped by the frosts of autumn or some
other cause. Consequently they form and ripen no terminal bud protected
by scales, and the upper axillary buds are produced so late in the
season that they have no time to mature, nor has their wood time to
solidify and ripen. Such stems therefore commonly die back from the top
in winter, or at least all their upper buds are small and feeble; so the
growth of the succeeding year takes place mainly from the lower axillary
buds, which are more mature.
63. =Deliquescent and Excurrent Growth.= In the former case, and
wherever axillary buds take the lead, there is, of course, no single
main stem, continued year after year in a direct line, but the trunk is
soon lost in the branches. Trees so formed commonly have rounded or
spreading tops. Of such trees with _deliquescent_ stems,--that is, with
the trunk dissolved, as it were, into the successively divided
branches,--the common American Elm (Fig. 80) is a good illustration.
[Illustration: Fig. 80. An American Elm, with Spruce-trees, and on the
left Arbor Vitae.]
64. On the other hand, the main stem of Firs and Spruces, unless
destroyed by some injury, is carried on in a direct line throughout the
whole growth of the tree, by the development year after year of a
terminal bud: this forms a single, uninterrupted shaft,--an _excurrent_
trunk, which cannot be confounded with the branches that proceed from
it. Of such _spiry_ or _spire-shaped_ trees, the Firs or Spruces are
characteristic and familiar examples. There are all gradations between
the two modes.