324 Introduction to Botany. 



or carbon dioxide. Therefore we find them modified in 

 such a way as to facihtate the entrance, circulation, and 

 storage of gases; and to this end the surfaces of leaves and 

 stems are increased, and in the parts which are not sub- 

 merged stomata are abundantly provided. 



The yellow water lily, Neliimbo lutea, serves as a good 

 example of hydrophytes which are partly submerged (Figs. 



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i^^j-./^^i^s-^ 



69 and 173). Its underground stems grow horizontally a 

 few inches below the surface of the mud, in relatively 

 shallow and still water. Leaf and flower stems grow verti- 

 cally from the horizontal stems. The round, peltate leaves 

 which are first produced float on the surface of the water, 

 while most of the later leaves rise for some distance above 

 it, so that the light can pass under and strike those which 

 are floating. The stomata are very numerous on the upper 

 side of the leaf, but are found very rarely on the under 



