Factors in Evolution 41 



leaves; very narrow submersed ones with no blade, 

 and large aerial leaves borne on stout stalks with 

 broad arrow-shaped blade. In various other 

 aquatics, like the water crow-foot, there are sub- 

 mersed leaves which are finely divided, while the 

 aerial leaves are broad. The general tendency in 

 submersed leaves is to assume a very long slender 

 form, or to be divided into many fine segments. 

 These finely divided leaves remind one of the fila- 

 mentous structure of many algae, and probably this 

 adaptation is efficient in exposing a larger surface 

 for the absorption of CO 2 and oxygen in the water, 

 where these substances are less abundant than they 

 are in the atmosphere. 



So long as the plants are completely submersed, 

 loss of water by evaporation is impossible; but so 

 soon as they are exposed to the air evaporation 

 is very rapid and the algse or other aquatic 

 plants wither very quickly when thus exposed. 

 Where, ho.wever, algae are regularly exposed for a 

 certain time, as happens with many of the seaweeds 

 that grow between tide marks, they usually de- 

 velop gelatinous or mucilaginous substances which 

 retain water with great tenacity, and such algae 

 may be exposed to the air or even to the hot sun 

 for several hours without losing sufficient water to 

 injure them. When, however, a plant is habitually 

 exposed to the air, as is the case with a typical land 

 plant, all of the exposed surfaces develop a more or 

 less thickened cuticle which is impervious to water. 



