Differentiation 219 



a given environment, exceptionally favorable conditions will enable the 

 plant to realize developmental potencies which it never would display 

 otherwise. Thus Bloch (1935/?) has shown that in Tradescantia flumi- 

 nensis, which typically has bundle sheaths with only thin-walled cells, 

 wounding may so stimulate differentiation that thick-walled sheath cells, 

 similar to those in related species of Tradescantia, may be formed. Here 

 the ontogenetic cycle has been extended beyond its normal course, either 

 in reversion to a former evolutionary level or toward the realization of 

 developmental potencies not yet normally expressed by this species, 

 though common in related ones. 



Often the whole histological pattern may be affected. In air roots of 

 orchids growing freely, adventitious roots are produced on all sides; but 

 if the root is in contact with a support, these lateral roots are formed only 

 next the support, presumably because of differences in moisture or other 

 factors on the two sides (Bloch, 1935«). Anatomical differences are also 

 evident in these two root sectors. 



In differentiation, the role of specific substances, particularly growth 

 substances, is important (p. 390). Root-forming substances, shoot-forming 

 substances, flower-forming substances, and others have been postulated. 

 That a substance by itself has a specific organ-forming character is prob- 

 ably too naive a conception, but certainly auxin and various other hor- 

 mones and growth substances are effective as stimuli which call forth 

 specific morphogenetic responses in the plant. Auxin influences the growth 

 of cambium, the development of vessels, and other histological processes. 

 It also inhibits certain activities. Beneath epidermal cells that regenerate 

 new shoots (p. 245) vascular tissue often differentiates, presumably because 

 of a substance coming from the young bud, which thus is able to estab- 

 lish a connection with the main vascular system. Camus has shown that 

 buds grafted to pieces of fleshy root in tissue culture induce the differen- 

 tiation of vascular tissue in parenchyma cells beneath them, and Wetmore 

 found that auxin alone does the same thing ( p. 405 ) . 



In general, environmental factors seem chiefly to affect the later stages 

 in cellular differentiation and especially the character of the cell wall. The 

 fundamental pattern of a structure is less affected than are its quantitative 

 expression and the size and character of the cells that compose it. 



What a cell or tissue will do depends in part upon its innate genetic 

 potentialities and in part upon the environment in which it happens to be. 

 Cells possess different degrees of reactivity to environmental differences. 

 In some, this is small, and the fate of the cell is therefore rather fixed and 

 limited, regardless of its environment. In others its developmental reper- 

 toire is much wider, and it may be greatly influenced by the conditions 

 which surround it. It should be remembered that the degree of a cell's 

 reactivity is not constant but that it may vary with the position that the 



