330 Morpho genetic Factors 



stomata vestigial or absent, and the cell walls thin. These traits are gen- 

 erally regarded as adaptations to an aquatic habitat. A few heterophyllous 

 water plants (p. 216) can live either submersed or growing in the air with 



Fig. 14-3. Diagram of cross sections of stems of Achyranthes, showing relative de- 

 velopment of tissues in air which is dry ( I ) , normal ( II ) , and humid ( III ) . b, xylem; 

 1, phloem; s, sclerenchyma; m, pith. Dry air tends to reduce size of pith and cortex 

 and to increase development of xylem and sclerenchyma. ( From Eberhardt. ) 



their roots in soil. Among these are Polygonum amphibium and the 

 water buttercup, Ranunculus aquatilis. In these plants the land form, 

 essentially mesophytic in structure, is very different from the water form 



3 



»=ss 



c 



I II 



Fig. 14-4. Outer cortex and epidermis of Aster sinensis grown in air that is of normal 

 humidity ( I ) and that is dry ( II ) . The latter shows greater wall thickness in collen- 

 chyma and larger bundles of sclerenchyma. E, epidermis; C, cortex; S, sclerenchyma. 

 ( From. Eberhardt. ) 



and in some cases was not at first recognized as belonging to the same 

 species. Occasionally, as in the buttercup, foliage transitional from one 

 to the other may be found. In plants like the mermaid weed, Proser- 

 pinaca palustris, leaves borne in the air are broad and little-lobed 



