I 



1902] 



CHANG A OF FORM IN PR0SERP2NACA PALUSTRIS 



97 



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Fig. 5. — Cross section of aerial leaf. 



• . ■ 



plexity* of the conditions, regards light, or rather the lack of 



light, as the chief factor. Wachter^ substantially agrees with 

 Goebel. 



There seems to be thus an almost universal tendency among 



writers to attribute 

 the difference in 

 structure between 

 aerial and water 

 forms to differences 

 in light or nutrition, 

 especially the former. 

 These terms, how- 

 ev^er, are very gen- 

 eral, especially nu- 

 trition, which involves so much as to mean little in the way of 

 explanation. Ultimately, there must be a critical analysis of the 

 conditions present in the water medium, of the individual factors 

 there that can act upon the growing cells, and the effect of each 

 of these on the behavior of the primordia. 



Every living cell in a plant is surrounded at all times by a 

 thin membrane of water through which all substances entering 

 the cell must pass in 

 solution. No substance, 

 be it salt, organic com- 

 pound, or gas, can enter 

 or leave the cell in any 

 other way than by diffu- 

 sion as a solute through 

 this film of water. When 



Fig. 6. — Cross section of submerged leaf, 



a plant is submerged this membrane is extended, its thickness 



*GoEBEL. K., Pflanzenbiologische Schildenungen. Part II. 1891. 



Ueber die Einwicklung des Lichtes auf die Gestaltung der Kakteen und anderer 



Pfl 



zen. Flora 80: 96-116. figs. j. 1895. 

 Ueber der Einfluss des Lichtes auf die Lestaltung der Kakteen und anderer 

 Pflanzen. Flora 82 : 1-13.^^..^. 1899. 



1,-, ^^^'^'^"■^E^. W., Beitrage zur Kenntniss einige Wasserpflanzen. Flora 83: 

 j43 340. 1897. 



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