308 Introduction to Botany. 





 of floating leaves, such as those of the water lily, and a 



large increase in the extent of the intercellular spaces for 

 the conduction of air from the leaves through the stems 

 into the underground parts. 



In plants entirely submerged in the water we usually find 

 the stems and leaves very much branched for the purpose of 

 exposing a large surface for the absorption of oxygen and 

 carbon dioxide from the water, and at the same time there 

 is a reduction in or an entire obliteration of the roots, since 

 such plants throughout their whole surface can absorb 

 whatever the water affords them. There is also a re- 

 duction in the amount of waterproofing, a diminution or 

 disappearance of the stomata, and a decrease in the water- 

 conducting elements. 



191. Light. Although light is quite as important as 

 water for the life of the higher independent plants, the 

 variations in its quality and intensity over the earth are 

 not so great as to cause or to require the profound modi- 

 fications which we find produced by variations in the water 

 supply. It is a commonly observed fact that plants which 

 sprout in cellars have long internodes and reduced leaves. 

 Experiments have shown that if the intensity of the light 

 be increased up to a certain point, the size of the leaves 

 produced increases ; but that passing above a certain 

 degree of intensity of illumination the leaves attain a less 

 and less size. 



192. Effect of Dim Light. Experiments with Scolopen- 

 drium officinarum showed that in the dark the leaves 

 reached a length of 76 millimeters and a breadth of n 

 millimeters (Fig. 161, i), while in a light about one-eighth 

 as intense as direct sunlight the leaves grew to be 228 

 millimeters long and 25 millimeters broad (2); and in a still 

 stronger light having about one-third the intensity of full 



