26 THE STORY OF PLANT LIFE 



the margin upturned. A few are ribbon-like, floating 

 on the surface. Many have two types of leaves, sub- 

 merged and floating. 



The aquatic plant can thus absorb freely, but 

 cannot transpire. The water is allowed to escape 

 into the air-spaces. This renders aeration difficult, 

 especially where the water is not running and so 

 continually filled with fresh air. In stagnant water 

 the oxygen is used up in the process of oxidation or 

 decomposition of rotting matter. The difficulty of 

 obtaining light is due to the breaking or deflection of 

 the rays by the surface, and it is soon absorbed and 

 altered, the red and yellow rays becoming quickly 

 absorbed. The submerged aquatics are thus shade 

 plants with long internodes, etc. 



Similar in kind, but not in degree, are the adapta- 

 tions of hygrophiles and mesophytes to water supply. 

 Plants that exhibit one set of characters in summer 

 and others in winter are called tropophytes. They 

 are evergreen when they exhibit adaptations to 

 physiological drought in winter, through cold, and 

 have the characters in summer, though they are 

 useful only in winter. Deciduous types throw off 

 their summer characteristics in winter and become 

 xerophytic. 



The air or atmosphere serves as the main element 

 in which land plants live. Aquatic plants live in 

 water, and so get their air indirectly, whereas land 

 plants get it directly. 



