7. Parts which have become dissimilar by differentiation undergo a reduction, in which the intermediate forms are suppressed and at last only the qualitatively dissimilar forms with qualitatively dissimilar functions remain.
8. The environment in which plants live operates in different ways, directly as a stimulus or indirectly as a felt necessity and by this means lends to their forms and activities a definite expression of time and place, and thus brings about different adaptations. These become permanent through heredity, but are again gradually lost if other adaptations supersede them.
Laws 1 to 4 may be expressed as one—the law of combination: Similar parts that are wholly or partly separated have the tendency to unite more and more completely and intimately into one continuous tissue.
The laws of ampliation (5), differentiation (6), and reduction (7), may be summarized in one as follows: While increasing in size the similar parts of an ontogeny become internally dissimilar and the dissimilarity increases as the transition forms of the dissimilar parts vanish. Hence only the extreme forms remain.
It may also interest the reader to know that Nägeli was the first to propose the general theory of cell formation as accepted at the present day.
FOOTNOTES
[A] See Appendix, Translators' Notes.
[B] Nägeli makes his idioplasm ramify throughout the organism in unbroken continuity, much like a system of nerves in the higher animals. This idea with Nägeli was purely speculative. It was known that the protoplasm is in connection throughout the organism, but it has been proved more recently that only the somatic protoplasm is thus connected. The part in which the essential nature of the organism is contained is localized in the nucleus and hence might properly be designated as nucleoplasm, as Weismann suggests. If the idioplasm is localized in the nucleus, it cannot be continuous throughout the system, as Nägeli assumes. But this objection applies only to a detail of the theory and does not affect the fundamental conception,—that of a portion of the protoplasm which is differentiated from the rest and represents a definite molecular structure which determines the specific nature of the organism.—Trans.
[C] Hence, according to Nägeli, every cell of the organism has idioplasm of identical structure. This at once suggests the objection, how can the idioplasm, for instance, of a pollen grain be the same as that of a leaf? Identical idioplasms should always produce identical structures. Nägeli attempts to explain this difficulty by attributing the different results to different "conditions of tension and movement," i.e., a dynamical difference between the idioplasms of the different parts of the organism. (Abstammungslehre, p. 53.)