BOTANY FOR YOUNG PEOPLE AND COMMON SCHOOLS
How Plants Grow
A Simple Introduction to Structural Botany
by Asa Gray
Originally published in 1858 and now in the public domain.
HOW PLANTS GROW, AND WHAT THEIR PARTS OR ORGANS ARE.
SECTION III. -- How Plants grow Year after Year.
51. They Grow on as they Began. The seedling has all the organs that any plant has, even the largest and oldest, excepting what belongs to blossoms; it has all it needs for its life and growth, that is, for vegetation.
It has only to go on and produce more of what it already has, more roots beneath to draw up more moisture from the soil, and more stem above, bearing more leaves, exposing a larger surface to the light and air, in which to digest what is taken in from the soil and the air, and turn it into real nourishment, that is, into the stuff which vegetables are made of. So, as fast as a young plant makes new vegetable material, it uses it for its growth; it adds to its root below, and to its stem above, and unfolds a new leaf or pair of leaves on every joint. Each joint of stem soon gets its full length, and its leaf or pair of leaves the full size; and now, instead of growing, they work, or prepare nourishment, for the growth of the younger parts forming above.
52. Simple Stems. In this way, piece by piece, the stem is carried up higher and higher, and its leaves increased in number; and the more it grows, the more it is able to grow, as we see in a young seedling, beginning feebly and growing slowly for a while, but pushing on more and more vigorously in proportion to the number of leaves and roots it has produced. In this way, by developing joint after joint, each from the summit of its predecessor, a Simple Stem is made. Many plants make only simple stems, at least until they blossom, or for the first year. The Lilies, figured on the first page, and corn-stalks are of this kind.
Fig. 51 is a sort of diagram of the simple stem of Indian Corn, divided into its component pieces, to show how it consists of a set of similar growths, each from the summit of the preceding one. There are old trees even, which consist of a simple, unbranched stem. Palm-trees, such as our Southern Palmetto (Fig. 79) are of this kind.
But more commonly, as stems grow they multiply themselves by forming
55. But the branches of stems spring only from particular places and are arranged on a regular plan. They arise from the Axil of a leaf and nowhere else, except in some few peculiar cases. The axil (from a Latin word meaning the armpit) of a leaf is the hollow or angle, on the upper side, where the leaf is attached to the stem.
As branches come only from the axils of leaves, and as leaves have a perfectly regular and uniform arrangement in each particular plant, the places where branches will appear are fixed beforehand by the places of the leaves, and they must follow their arrangement.
In the axils, commonly one in each, branches first appear in the form of
56. Buds. A Bud is an undeveloped stem or branch.
If large enough to have its parts distinguishable, these are seen to be undeveloped or forming leaves; and large buds which are to stand over winter are generally covered with protecting scales, a kind of dry, diminished leaves.
57. Terminal Bud.
So the plumule or first shoot of the embryo (see Fig. 22, &c.) is a bud. But this first bud makes the main stem, and its growth, week after week, or year after year, carries on the main stem.
Palms (as Fig. 79) grow in this way, by this bud only. Being always on the end of the stem, that is, terminating the stem, it is called the Terminal Bud.
58. Axillary Buds.
But the buds which are to form branches appear on the sides of the stem; and since they are situated in the axils of the leaves, as just explained (55), they are named Axillary Buds. (See Fig. 52, 53.)
These buds grow into branches, just as the first or terminal bud of the seedling grows to make the main stem.
59. The Arrangement of Branches, therefore, follows that of the axillary buds, and this that of the leaves.
Now leaves are placed on the stem in two principal ways; they are either alternate or opposite.
They are alternate when they follow one after another, there being only one to each joint of the stem, as in Morning-Glory (Fig. 4, all after the seed-leaves), and in the Linden or Basswood (Fig. 52), as well as the greater part of trees or plants.
They are opposite when there are two leaves upon each joint of stem, as in Horsechestnut, Lilac, and Maple (Fig. 31, 53); one leaf in such cases being always exactly on the opposite side of the stem from its fellow. Now in the axil of almost every leaf of these trees a bud is soon formed, and in general plainly shows itself before summer is over.
In Fig. 52, a,a,a,a, are the axillary buds on a twig of Basswood; they are alternate, like the leaves, and t is the terminal bud.
Fig. 53, a twig of Red Maple, has its axillary buds opposite, like the leaves; and on the very summit is the terminal bud.
Next spring or sooner, the former grow into alternate branches; the latter grow into opposite branches. These branches in their turn form buds in the axils of their leaves, to grow in time into a new generation of similar branches, and so on, year after year. So the reason is plain why the branching or spray of one tree or bush differs from that of another, each having its own plan, depending upon the way the leaves are arranged on the stem.
60. The spray (or ramification) of trees and shrubs is more noticeable in winter, when most leaves have fallen. Even then we can tell how the leaves were placed, as well as in summer. We have only to notice the leaf-scars, for each fallen leaf has left a scar to mark where its stalk separated from the stem.
And in most cases the bud above each scar is now apparent or conspicuous, ready to grow into branches in the spring, and showing plainly the arrangement which these are to have.
Here, for instance, is a last year's shoot of Horsechestnut (Fig. 54), with a large terminal bud on its summit, and with very conspicuous leaf-scars, l s, and just above each is an axillary bud, b. Here the leaves were opposite each other; so the buds are also, and so will the branches be, unless one of the buds on each joint should fail.
Fig. 55 is a similar shoot of a Hickory, with its leaf-scars (l s) and axillary buds (b) alternate, that is, single on the joints and one after another on different sides of the stem; and these buds when they grow will make alternate branches.
61. The branching would be more regular than it is, if all the buds grew.
But there is not room for all, so only the stronger ones grow. The rest stand ready to take their place, if those happen to be killed. Sometimes there are more buds than one from the same axil. There are three placed side by side on those shoots of Red Maple which are going to blossom. There are several in a row, one above another, on some shoots of Tartarean Honeysuckle.
63. The Duration of Plants of different kinds varies greatly. Some live only for a few months or a few weeks; others may endure for more than a thousand years. The most familiar division of plants according to their duration and character is into Herbs, Shrubs, and Trees.
64. Herbs are plants of soft texture, having little wood in their stems, and in our climate dying down to the ground, or else dying root and all, in or before winter.
65. Shrubs are plants with woody stems, which endure and grow year after year, but do not rise to any great height, say to not more than four or five times the height of a man. And if they reach this size, it is not as a single main trunk, but by a cluster of stems all starting from the ground.
66. Trees are woody plants rising by a trunk to a greater height than shrubs.
67. Herbs are divided, according to their character and duration, into Annuals, Biennials, and Perennials.
68. Annuals grow from the seed, blossom, and die all in the same season.
In this climate they generally spring from the seed in spring, and die in the autumn, or sooner if they have done blossoming and have ripened their seed. Oats, Barley, Mustard, and the common Morning-Glory (Fig. 4) are familiar annuals.
Plants of this kind have fibrous roots, i.e., composed of long and slender threads or fibres. Either the whole root is a cluster of such fibres, as in Indian Corn (Fig. 48), Barley (Fig. 56), and all such plants; or when there is a main or tap root, as in Mustard, the Morning-Glory, &c., this branches off into slender fibres.
It is these fibres and the slender root-hairs which are found on them, that mainly absorb moisture and other things from the soil; and the more numerous they are, the more the plant can absorb by its roots. As fast as nourishment is received and prepared by the roots and leaves, it is expended in new growth, particularly in new stems or branches and new leaves, and finally in flowers, fruit, and seed. The latter require a great deal of nourishment to bring them to perfection and give nothing back to the plant in return.
So blossoming and fruiting weaken the plant very much. Annual plants usually continue to bear flowers, often in great numbers upon every branch, until they exhaust themselves and die, but not until they have ripened seeds, and stored up in them (as in the mealy part of the grain of Corn, &c., (Fig. 44, 45) food enough for a new generation to begin growth with.
69. Biennials follow a somewhat different plan. These are herbs which do not blossom at all the first season, but live over the winter, flower the second year, and then die when they have ripened their seeds.
The Turnip, Carrot, and Parsnip, the Beet, the Radish (Fig. 57), and the Celandine are familiar examples of biennial plants.
70. The mode of life in biennials is to prepare and store up nourishment through the first season, and to expend it the next season in flowering and fruiting.
Accordingly, biennials for the first year are nearly all root and leaves, these being the organs by which the plant works and prepares the materials it lives on.
Stem they must have in order to bear leaves; for leaves do not grow on roots. But what stem they make is so very short-jointed that it rises hardly any, so that the leaves seem to spring from the top of the root, and all spread out in a cluster close to the ground.
As the plant grows, it merely sends out more and more branches of the root into the soil beneath, and adds more leaves to the cluster just above, close to the surface of the warm ground and well exposed to the light and heat of the sun.
Thus consisting of its two working organs only, root and leaves, the young biennial sets vigorously to work.
The moisture and air which the leaves take in from the atmosphere, and all that the roots take from the soil, are digested or changed into vegetable matter by the foliage while exposed to sunshine, and all that is not wanted by the leaves themselves is generally carried down into the body of the root and stored up there for next year's use.
So the biennial root becomes large and heavy, being a storehouse of nourishing matter, which man and animals are glad to use for food. In it, in the form of starch, sugar, mucilage, and in other nourishing and savory products, the plant (expending nothing in flowers or in show) has laid up the avails of its whole summer's work.
For what purpose? This plainly appears when the next season's growth begins. Then, fed by this great stock of nourishment, a stem shoots forth rapidly and strongly, divides into branches, bears flowers abundantly, and ripens seeds, almost wholly at the expense of the nourishment accumulated in the root, which is now light, empty, and dead; and so is the whole plant by the time the seeds are ripe.
71. By stopping the flowering, biennials can sometimes be made to live another year, or for many years, or annuals may be made into biennials. So a sort of biennial is made of wheat by sowing it in autumn, or even in the spring and keeping it fed down in summer. But here the nourishment is stored up in the leaves rather than in the roots.
72. The Cabbage is a familiar and more striking example of a biennial in which the store of nourishment, instead of being deposited in the root, is kept in the leaves and in the short stem or stalk. These accordingly become thick and nutritious in the Cabbage, just as the root does in the Turnip, or the base of the short stem alone in Kohlrabi, or even the flower-stalks in the Cauliflower, all of which belong to the same family and exhibit merely different ways of accomplishing the same result.
73. Perennials are plants which live on year after year.
Shrubs and trees are of course perennial. So are many herbs, but in these only a portion generally survives. Most of our perennial herbs die down to the ground before winter; in many species all but certain separate portions under ground die at the close of the year, but some parts of the stem containing buds are always kept alive to renew the growth for the next season.
And a stock of nourishment to begin the new growth with is also provided.
Sometimes this stock is laid up in the roots, as for instance in the Peony, the Dahlia (Fig. 58), and the Sweet Potato.
Here some thick roots, filled with food made by last year's vegetation, nourish in spring the buds on the base of the stem just above (a,a), enabling them to send up stout leafy stems, and send down new roots, in some of which a new stock of food is laid up during summer for the next spring, while the exhausted old ones die off; and so on, from year to year.
74. Sometimes this stock of food is laid up in particular portions of branches of the stem itself, formed under ground, which contain the buds, as in the Ground Artichoke and the Potato. Here these parts, with their buds, or eyes, are all that live over winter. These thickened ends of stems are called Tubers.
In Fig. 59 a is a tuber of last year, now exhausted and withering away, which grew in spring by one of its buds to make the stem (b) bearing the foliage of the season. This sends out some branches under ground, which in the course of the season thicken at the end as they receive a stock of nourishment prepared by this year's foliage, and become new tubers (c, a forming one; d,d, well-grown tubers of the season), to live over winter and make the next year's growth.
75. Because they live underground, these tubers are commonly supposed to be roots, but they are not, as any one may see. Their eyes are buds, and the little scales behind the eyes answer to leaves, while roots bear neither buds nor leaves.
The fibrous roots which grow from these subterranean branches are very different in appearance from under-ground stems, as is plain to see in the Potato-plant.
Fig. 60 shows a few of the real roots, as well as several branches of the stem, with potatoes forming in all stages at their tips.
Fig. 61 is one of these forming potatoes magnified, showing a little scale behind each eye which answers to a leaf.
Fig. 62 is a part of a slice through an eye, more magnified, to show that the eye is really a bud, covered with little scales.
76. In some perennial herbs, prostrate stems or branches under ground are thickened with this store of nourishment for their whole length, making stout Rootstocks, as they are called; as in Sweet Flag, Solomon's Seal (Fig. 63); and Iris, or Flower-de-Luce (Fig. 64).
These are perennial, and grow on a little way each year, dying off as much behind after a while; and the newer parts every year send out a new set of fibrous roots.
The buds which rootstalks produce, and the leaves or the scales they bear, or the scars or rings which mark where the old leaves or scales have fallen or decayed away, all plainly show that rootstocks are forms of stem, and not roots.
The large round scars on the rootstock of Solomon's Seal, which give the plant its name (from their looking like impressions of a seal) are the places from which the stalk bearing the leaves and flowers of each season has fallen off in autumn.
Fig. 63, a is the bud at the end, to make the growth above ground next spring; b is the bottom of the stalk of this season; c, the scar or place from which the stalk of last year fell; d, that of the year before; and , that of two years ago.
77. Finally, the nourishment for the next year's growth may be deposited in the leaves themselves. Sometimes it occupies all the leaf, as in the Houseleek (Fig. 65) and other fleshy plants. Here the close ranks of the thickened leaves are wholly above ground.
Sometimes the deposit is all in the lower end of the leaf, and on the ground, or underneath, as in common Bulbs.
Take a White Lily of the gardens, for example, in the fall, or in spring before it sends up the stalk of the season (Fig. 66). From the bottom of the bulb, roots descend into the soil to absorb moisture and other matters from it, while above it sends up leaves to digest and convert these matters into real nourishment. As fast as it is made, this nourishment is carried down to the bottom of each leaf, which is enlarged or thickened for containing it. These thick leaf-bases, or scales, crowded together, make up the bulb; all but its very short stem,, concealed within; which bears these scales above and sends down the roots from underneath.
Fig. 67 shows one of the leaves of the season, taken off, with its base cut across, that the thickness may be seen. After having done its work, the blade dies off, leaving the thick base as a bulb-scale. Every year one or more buds in the centre of the bulb grow, feeding on the food laid up in the scales, and making the stalk of the season, which bears the flowers, as in Fig. 1, 2.
78. An Onion is like a Lily-bulb, only each scale or leaf-base is so wide that it enwraps all within, making coat after coat.
79. In shrubs and trees a great quantity of nourishment, made the summer before, is stored up in the young wood and bark of the shoots, the trunk, and the roots.
Upon this the buds feed the next spring; and this enables them to develop vigorously, and clothe the naked branches with foliage in a few days; or with blossoms immediately following, as in the Horsechestnut; or with blossoms and foliage together, as in Sugar Maple; or with blossoms before the leaves appear, as in Red Maples and Elms.
The rich mucilage of the bark of Slippery Elm and the sweet spring sap of Maple-trees belong to this store, deposited in the wood the previous summer, and in spring dissolved and rapidly drawn into the buds, to supply the early and sudden leafing and blossoming.
80. In considering plants, as to "how they grow," it should be noticed that all of them, from the Lily of the field to the tree of the forest, teach the same lesson of industry and provident preparation. No great result is attained without effort, and long preceding labor.
Not only was the tender verdure which, after a few spring showers and sunny days, is so suddenly spread out over field and forest, all prepared beforehand, most of the leaves, even, made the summer before, and snugly packed away in winter-buds, but the nourishment which enables them to unfold and grow so fast was also prepared for this purpose by the foliage of the year before, and laid up until it was wanted.
The grain grows with vigor, because fed with the richest products of the mother plant, the results of a former year's vegetation.
The Lily-blossom develops in all its glory without toil of its own, because all its materials were gathered from the earth and the air long before, by the roots and the leaves, manufactured by the latter into vegetable matter, and this stored up for a year or two under ground in the bottoms of the leaves (as starch, jelly, sugar, &c.), and in many cases actually made into blossoms in the dark earth, where the flower-buds lie slumbering in the protecting bulb through the cold winter, and in summer promptly unfold in beauty for our delight.
Analysis of the Section.
51. The seedling is a complete plant on the simplest scale; in growth it merely increases its parts, and multiplies them in number, as fast as it makes materials for growth. 52. Simple stems, how formed and carried up, piece by piece. 53. Branches 54. of Roots, how they differ from those 55. of Stems. Where these arise from; in what form they appear. 56. Buds, what they are. 57. Terminal Bud, what it makes. 58. Axillary Buds: why so named; what they make. 59. How branches are arranged, and what their arrangement depends upon; alternate; opposite. 60. The spray and buds of shrubs and trees in winter; Leaf-scars. 61. Why branches are not as regular and as many as the buds or leaves.
62,63. The Duration and Character of Plants as affecting the way they grow. 64. Herbs. 65. Shrubs. 66. Trees. 67. Herbs are annuals, biennials, or perennials.
68. Annuals; their mode of life; character of their roots, intended only for absorbing; duration, &c.
69. Biennials; how defined; examples. 70. Character of their roots, and illustrations of their mode of life; the first year, food made and stored up; the second year, food expended, for what purpose. 71. How biennials may sometimes be made perennial, and annuals biennial. 72. The store of food may be kept in the leaves, or in the stems above ground; Cabbage, &c.
73. Perennials; what they are; mode of life of perennial herbs from year to year; accumulation of food in roots. 74. Accumulation of food in underground branches; Tubers, as of Ground Artichoke. 75. Potato illustrated. 76. Accumulation in whole stems or branches under ground; Rootstocks. 77. Accumulation of food in leaves, above ground, as in Houseleek or in the bottoms of leaves, usually under ground; Bulbs; as of Lily, and 78. of Onion.
79. Food, how stored up in shrubs and trees, and for what purpose; used in leafing and blossoming: in spring. 80. A lesson taught by vegetation.