Page:Popular Science Monthly Volume 19.djvu/559

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543
THE UNIT IN PLANT-LIFE.

less fusible quality than the bitumens of France and Switzerland, and has not been produced in the compressed form. It is extensively made into mastic, the fabrication of which already amounts to ten thousand tons a year, and is rapidly increasing.—La Nature.

 
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THE UNIT IN PLANT-LIFE.
By BYRON D. HALSTED, Sc. D.

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 subter-