Popular Science Monthly/Volume 19/August 1881/The Unit in Plant-Life
MAN, being himself distinctly individualized, endeavors to find the unit of existence in all other forms of life. He meets with no great difficulties among the higher animals, but is perplexed and sometimes discouraged when search is made for the individual in the lower animals and in plants.
A child is an easily-recognized unit of life at its birth, and is no more than a single individual when it has reached mature middle life, or the decline and decay of old age. A limb may have been lost on the field of battle, or an eye removed by a surgeon, but there is no replacement of the lost parts. The human individual may suffer division, but it is a mutilation, and not a multiplication of the living unit.
In some respects the seed of a plant is analogous to the young of the higher animals; it is the result of a sexual union and the starting-point of a continuation of the species. For these and other reasons the seed may be, and has been, called the unit of life in the higher plants. But what possibilities are contained within the coats of a single seed when the proper conditions for its growth and propagation are secured! When we look beyond the dry and inactive seed, which can be held upon the tip of the little finger, and note what it may produce; when we know that such a single seed has been the starting-point of a variety of fruit that now has its representatives as full-grown trees in thousands of orchards all through this broad land, we must either expand our idea of a plant-unit until it is too great and comprehensive to be of service, or seek some other basis of individuality than that which is in some respects analogous to the accepted one among the higher animals. The idea of the identity of the individual among the more complex and perfect forms of existence in the two kingdoms of life may be dismissed, because the methods of propagation in the two are far from the same.
If we take some common plant of the higher orders—any tree or shrub, or even a herb—it will be found, upon careful study of its structure, that there is an almost monotonous repetition of parts. It will also be observed that these parts may be grouped under three heads, to which the common names of root, stem, and leaf are applied. The root includes that portion of the plant, whether aërial or subterranean, the young growing extremities of which are protected by a layer of tissue called the root-cap. The stem is the axis of the plant, and is the part which bears the leaves, and includes those peculiar growths, such as thorns, runners, tendrils, etc., which serve a special purpose in the plant economy. The leaf is a lateral outgrowth from the stem, and is usually a flat, green expansion, but may assume the form of scales, highly-colored and strangely-shaped floral parts, etc. The fact that the largest tree and the smallest herb are alike made up of a greater or less number of these plant-members, as they are termed, leads naturally to the thought that any mass of plant-tissue having a root, a stem, and a leaf, may be a plant individual, and that, when a number of these members are intimately associated together, a community of plant individuals is formed. This is the modern conception of a tree or shrub— a living structure, which is the result of the combined, harmonious, silent working of many generations of individuals. Out of the three members are made all the multiplicity of forms and structures which meet our eyes as we look upon the higher forms of vegetation. They all have a common origin in the apical growing-points, and are indistinguishable in their earlier stages, but become differentiated as they develop, and at last assume their characteristic, mature forms. The growing-point of a stem (punctum, vegetationis) is a conical apex, a little below which the leaves appear first as very slight swellings. By their more rapid growth than the stem, they reach above the growing-point, and, folding over each other, cover it more or less completely. As the stem elongates, the leaves upon the older portion are gradually separated, and an ordinary stem, with its leaves arranged at regular intervals, results. A bud is simply a young stem, with its undeveloped leaves. A developed stem is a series of similar parts, those parts being a leaf with a portion of the stem above and below it, each borne upon its predecessor, and in turn bearing the next one in the series. These similar parts have received the name phyton, and are very generally considered as the individuals out of which a plant community is built up. The gardener divides the young branches of the verbena, salvia, etc., into these phytons, and places them in moist sand, where they soon begin an independent existence, and in time reproduce their kind. In the operation of grafting, a similar portion of a plant community of one variety is given a fitting place for growth in another, and by its growth and multiplication of phytons a new colony is established. With this view a tree or shrub may form an individual part of a landscape, but not in the same sense that one may speak of a cow or a horse. The tree more nearly resembles a swarm of bees ; there is a similarity of unity between a shrub and a hive. The larger part of the shrub is made up of foliar units, with ordinary leaves for the elaboration— gathering, so to speak, of the food for the whole community. These are the workers and the neuters of the vegetable "hive." Other plant individuals devote their energies to the production of new plant-units, namely, the stamens—the male units of the flowers—which perish as soon as their transient but important work of fertilizing the pistil is accomplished; these are the drones. The pistil is the central part of the flower, and around which all the work of propagation converges and the labors of the year culminate, and from which the new individuals, the seeds, go forth to develop into free and independent colonies. In several respects this pistil is queen of the congregated vegetable units.
There are some objections to the phytons being considered the unit of vegetable life. The division of a plant may be carried beyond it, and life and growth of the parts still be maintained. Thus buds may arise from petioles, or leaf-stalks, and from the veins of the leaf, as in the ordinary propagation of the begonia, and very strikingly in the bryophyllum. Buds may start from the woody bundles of roots, as in the sweet-potato, poplar, or the cut stems of the elm, willow, etc. These many cases of a seemingly spontaneous growth have led to another definition of the plant-unit which is formulated briefly as follows : A plant individual is that smallest part that can grow when separated from its former place in a plant community, and given the fitting conditions for growth by itself. In most cases this "smallest part" is the phyton, or a portion of the stem with its leaf, and the bud or growing point which it bears in its axil. This young lateral bud, which is frequently so small as to be unseen by the naked eye, is, in fact, the vitalized, undeveloped stem that is to increase in size if growth takes place. The writer is of the belief that in this growing-point the individuality of higher plants should be located. If there are two buds upon the phyton, it seems proper to say there are two individuals, as there are two distinct points of growth, and two branches may result therefrom.
If the phyton is to be considered as the plant-unit, we must seek for another unit of life for those plants in which no phyton elements exist. The unit of growth is the cell; in it, either alone or in connection with other cells, all the functions of life are performed. Cells compose the growing tissue of every plant; in them resides that vitalized substance called protoplasm, in which all life-changes take place, and from which all structures are built up.
Many of the lower plants are unicellular, as, for example, the common yeast-plant, bacteria, etc., among fungi, and the desmids and diatoms among algæ They increase in number by a simple division of the cell into two, each half increasing in size and dividing as did its parent. The individual among such plants is evidently the single cell. As we pass a little higher in the scale of vegetable life, it is found that though the cells are associated together in filaments, or laminæ, they are, in most respects, very independent, losing only a trifle of their originality by being associated in the simplest form of a community. As we pass upward in the scale, the differentiation increases, and there is a consequent division of labor, some of the cells being devoted to one kind of work, while others engage in a special labor for the community. Instead of the sum of the vital forces possessed by an individual being confined to a single cell, they are scattered through a large amount of growing tissue.
The seat of vitality is protoplasm, and wherever there is enough of this vitalized substance to grow and reproduce its kind, there we have an individual—a unit of life. It may be concentrated in a single cell, or distributed through many cells, the number and distribution being determined by the amount of dependence of the growing cells upon each other. The greater the division of labor, the higher the form of life, and the more difficult to recognize the individual; but, whenever it is found, it consists of a mass of protoplasm, usually contained in one or more cells, capable of growth under proper conditions, and ultimately reproducing its kind.