332 Morphogenetic Factors 



between shoots grown in water and in air, especially in Proserpinaca. 

 Transpiration, seasonal differences, and reversion to juvenile stages may 

 be involved. Combes (1947) found that in Oenanthe low temperature 

 is effective in producing deeply incised leaves. Allsopp (1955) has 

 studied the water fern, Marsilea, grown under various conditions, and 

 has discussed the general problem of the structure of water plants. The 

 land form of leaf has four leaflets, and there are stomata on both sur- 

 faces, but the water form is merely lobed and lacks stomata in the lower 

 epidermis (Fig. 14-6). Raising the osmotic concentration of the culture 



5% U. E. 



I7.U.E. 



5% L.E. 



I%LE. 



Fig. 14-6. Comparable sporeling leaves of Marsilea Drummondii, an "amphibious" 

 plant. Form of leaf and structure of upper and lower epidermis in leaves grown in 

 media containing 5 per cent glucose (left) and 1 per cent (right). The former re- 

 semble typical land forms and the latter, water forms. ( From Allsopp. ) 



medium by adding glucose produces the land type of leaf. Whether this 

 is an osmotic or a nutritional effect is not certain. Allsopp concludes that 

 it is the water balance of the developing tissues, determined by the 

 osmotic pressure of the surrounding liquid, and the relative humidity of 

 the air or, in general, the diffusion pressure deficit of the water of the 

 environment, which produce the structural features distinctive of land 

 or water forms. Here, again, the morphogenetic problem involves much 

 more than the direct effect of a single environmental factor. 



Effect of Transpiration Stream. Another aspect of the problem of the 

 morphogenetic effects of water involves the influence of water supply 



