Differentiation 217 



phylly is a necessary prerequisite for the ability of a plant to live in both 

 aquatic and terrestrial habitats. 



Pearsall and Hanbv (1925) have evidence that leaf variation in Pota- 

 mogeton is due, at least in part, to chemical differences in the soil, and 

 Gessner ( 1940 ) and Bauer ( 1952 ) relate it to rate of metabolism in the 

 buds. McCallum (1902) thought that in Proserpinaca the water type of 

 leaf arose primarily because of reduction in transpiration. H. Jones ( 1955) 

 has made extensive studies of the differences in development of the pri- 

 mordia that produce the linear and the ovate leaves of Callitriche and 

 the conditions under which these are formed. 



There are many instances where, instead of the permanent induction 

 of structures at certain ontogenetic levels, there may be reversion to earlier 

 stages under certain environmental conditions. This is especially frequent 

 in those cases where juvenile stages are adapted to different environments 

 than are the adult ones. A commonly cited example is that of Campanula 

 rotundifolia, which has rounded juvenile leaves adapted to weak light, 

 although the mature leaves are linear. A mature plant grown in low 

 illumination will often revert to the juvenile type of foliage. Seedlings, 

 even in strong light, however, bear nothing but juvenile foliage. Often 

 wounding will bring about such reversion, as in shoots growing from in- 

 jured regions of certain pines, which for a time bear foliage like that of 

 the seedling. With many perennials there is a partial return to the juvenile 

 stage at the beginning of each growing season. 



Frank and Renner (1956) found that in Hedera helix chemical treat- 

 ments of various sorts did not induce reversion to the juvenile state but 

 that cold shocks and X irradiation did so. De Zeeuw and Leopold ( 1956 ) 

 were able to induce flowering by auxin treatment in juvenile plants that 

 otherwise would not have flowered. They suggest that the completion of 

 the juvenile phase may be due to the accumulation of a sufficient auxin 

 level. Robbins ( 1957 ) has shown the effectiveness of gibberellin in ju- 

 venile reversion. Allsopp ( 1955 ) attributes heteroblastic differentiation 

 in general to changes, chiefly of size, in the shoot apex following alter- 

 ation of the water balance ( p. 332) . 



Environment and Internal Differentiation. Internal differentiation, also, 

 may be greatly affected by environmental factors. It is important to recog- 

 nize that changes that take place in this process are part of an underlying 

 pattern of relationships among the cells and between them and the en- 

 vironment. 



This fact is made clear whenever such relationships are disturbed. If 

 tissue like the cortex, for example, is exposed to the outside air by re- 

 moval of the outer cell layers, structures tend to differentiate at the new 

 surface which are characteristic for such a position. Thus when Vochting 



