ECOLOGY OF LEAVES 



115 



.^m^k 



leaf of an unknown plant whether it is an alpine, a desert, 

 or a seaside species. This is because of the impoitance 

 of leaves in disposing of the water taken into the plant 

 (Chapter Xlll). 



135. Leaves of Hydrophytes — Not nearly all hydrophytes 

 are aquatics, but some merely prefer very moist soil or 

 moist air. Of the truly aquatic species some have their 

 leaves wholly submerged; 

 others, such as the duck- 

 weeds and pond-lilies, have 

 them floating; and still 

 others, like the sedges, the 

 bur reeds, the cat-tails, and 

 the pickerel weeds, have 

 their leaves freely exposed 

 to the air. A few plants 

 have both water leaves and 

 air leaves (Fig. 79). It is 

 generally supposed that the 

 thread-like form of sub- 

 merged leaves in so many 

 species of aquatics gives 

 them greater capacity to 

 absorb dissolved gases from the water which surrounds them. 



136. Leaves of Xerophytes. — In regions where the great- 

 est dangers to vegetation arise from long droughts and 

 the excessive heat of the sun, the leaves of plants usu- 

 ally oifer much less surface to the sun and air than is the 

 case in temperate climates, as shown in the Australian 

 blackberry (Fig. 80). Sometimes the blade of the leaf is 

 absent and the expanded petiole answers the purpose of a 

 blade, or, again, foliage leaves are altogether lacking, as in 



Fig. 79. Submerged and Aerial 

 Leaves of a European Crowfoot 

 {Ranunculus Purshii) . The leaf 

 with thread-like divisions is the 

 submerged one. 



