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Page:Popular Science Monthly Volume 19.djvu/561

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THE UNIT IN PLANT-LIFE.

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 com-