314 THE SCIENTIFIC MONTHLY
almost invariably adapted to keep their ehlorophyllic organs in the sun- light* in competition with other plants; and partly from the evolu- tion of their reproductive organs, which pass through the primitive spore stage into various forms of sexuality, with, finally, the develop- ment of the seed-habit and the dominance of the sporophyte.' It is a striking peculiarity of plants that the locomotive powers evolve chiefly in connection with their reproductive activities, namely, with the move- ments of the germ cells: in this respect and in their fundamentally different sources of energy they represent the widest contrast to ani- mal evolution. One of the most striking features of plant evolution is the development of a great variety of automatic migrating organs, especially in the seed and embryonic stages, by which they are mechani- cally propelled through the air or water. Plants are otherwise de- pendent on the motion of the atmosphere and of the water for the migration of their germs and embryos and of their adult forms into favorable conditions of environment.
In the absence of a nervous system the remarkable actions and re- actions which plants exhibit to stimuli are purely of a physico-chemi- cal nature. The interactions between different tissues of plants, which become extraordinarily complex in the higher and larger forms, are probably sustained through chemical catalysis and the circulation through the tissues of accelerating and retarding agents in the nature of enzymes or hormones. It is a very striking feature of plant develop- ment and evolution that, although entirely without the coordinating agency of a nervous system, all parts are kept in a condition of perfect correlation. This fact is consistent with the comparatively recent dis- covery that a large part of the coordination of animal organs and tissues which was formerly attributed to the nervous system is now known to be catalytic. Throughout the evolution of plants the fun- damental distinctions between the chromatin and the protoplasm are sustained exactly as among animals.
It would appear from the researches of de Vries* and other botan- ists that the sudden alterations of structure and function which may be known as mutations of de Vries^ are of far more general occurrence among plants than among animals. Such mutations are attributable to sudden alterations of molecular and atomic constitution. Sensi- tiveness to the biochemical reactions of the physical environment should theoretically be more evident in organisms like plants which derive their energy directly from inorganic compounds which are constantly changing their chemical formulae with the conditions of moisture, of
3 Wager, Harold, 1915, p. 468.
»M. A. Howe.
- de Vries, Hugo, 1901, 1903, 1905.
5 As distinguished from the earlier defined Mutaiions of Waagen, 332.
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