182 The Phenomena of Morphogenesis 



ferentiation often changes, not only as to the structure of the parts de- 

 veloped but as to their reactivity and developmental potency. 



An important aspect of the process of differentiation is that it seems 

 not to involve genetic diversity. The regeneration of an entire normal 

 plant is sometimes possible from a single cell, which may come from 

 almost any of the parts of the plant body, and from various tissues (p. 

 253), a fact which suggests that every cell of the plant is totipotent and 

 identical genetically with all the rest. This conclusion is supported by 

 the common observation that the number and character of the chromo- 

 somes, and thus presumably of the genes, are the same in all cells, save 

 for the occurrence of somatic polyploidy. Although the process of dif- 

 ferentiation is doubtless under genetic control, this cannot operate, as 

 Weismann and others once suggested, by a parceling out of genetic "de- 

 terminers" during development. The conclusion seems obvious that in 

 these processes that part of the cell must be involved which is not 

 identical everywhere in the body, namely, the cytoplasm. The origin of 

 structural diversity in the midst of genetic identity is the chief problem 

 that faces students of differentiation. 



GROWTH AND DIFFERENTIATION 



Although growth and differentiation usually proceed together, they 

 seem to be distinct processes, each more or less independent of the other. 

 Growth may occur without differentiation by a simple multiplicative 

 process, as in large parenchymatous masses such as the endosperm of a 

 seed, in the tissue of an amorphous gall, or in tissue culture. In the early 

 stages of many embryos, on the other hand, in the development of the 

 female gametophyte in certain lower vascular plants, and in similar cases 

 there is differentiation without growth. A notable example of this is fur- 

 nished by the Acrasiaceae, a family of slime molds (p. 223). Here the en- 

 tire vegetative growth occurs while the individuals are myxamoebae, and 

 the elaborate differentiation of the colonial sorocarp does not begin 

 until this vegetative phase is over. Animal embryology, particularly 

 in the early cleavage stages from large eggs, provides many similar 

 cases. 



The independence of these two major developmental processes is 

 further emphasized by the fact that conditions which favor one tend to 

 be different from those which favor the other. In general, abundance of 

 water and available nitrogen tend to induce growth, whereas abundance 

 of accumulated carbohydrates, with less nitrogen and water, promotes 

 differentiation (Loomis, 1932). Red rays of the spectrum tend to pro- 

 mote growth and blue rays differentiation (p. 313). Under one photo- 

 period a given species will produce nothing but vegetative growth 



