LEAVES 



595 



a direct formative stimulus, but this is made improbable by the 

 fact that leaves just below the water surface differ strikingly from 

 those just above, there being no intergrading series corresponding with 

 the gradual decrease in light; besides, air leaves develop as readily 



i 865 " "" " 11 



Figs. 865-866. — 865, a shoot of the mermaid weed (Proserpinaca palustris) that has 

 been placed in water after growing for some time in the air ; note the transitional leaves (i) 

 between the air leaves (a) below and the water leaves (w) above; Proserpinaca has a 

 phyllotaxy of high rank, there being a number of orthostichies (p. 549). — After McCal- 

 LUM (drawn from a photographic reproduction); 866, a shoot of the mermaid weed 

 (Proserpinaca palustris) that has been grown successively in water, air, water, and air; 

 note the corresponding sets of leaves, iv, a, w', a'. — After McCallum (drawn from a 

 photographic reproduction). 



in shade as in full sunlight. Variations in temperature, or in the 

 amount of oxygen or carbon dioxid, seem to have little formative 

 significance. 



The factor which appears most directly related to leaf form in am- 

 phibious plants is transpiration. In Proserpinaca the water leaf can be 



