THE LEAF 



79 



floating, leaves. In general, submerged plants possess 



long and narrow, or linear, leaves (Fig. 



63). Or, they may have leaves of a more 



or less rounded form, but much divided, 



or dissected, into linear parts (Fig. 64). 



Since submerged plants of many widely 



separated families in common show this 



type of leaf, or these types, the form 



must in some way be due to the circum- 

 stances of life in water. In exactly what 



respect these cir- 

 cumstances call for 

 linear leaf forms 

 is, however, an 

 open question. 

 They may be ad- 

 vantageous from 

 any one or all 

 of the following 

 causes. First, 

 light diminishes 

 rapidly as depth 



of water increases. It will, therefore, be an advantage 



for the blade to reach upward as far as possible in its 



growth ; that is, to take a linear form. 



131. Secondly, the narrow and dissected forms have 

 been attributed to the scarcity of carbon dioxide and 

 oxygen in water. The amount of these necessary sub- 

 stances that will be absorbed by a leaf, other things 

 being equal, is proportional to the extent of the surface 

 in contact with the water. The more divisions the leaf 

 has, or the longer and narrower it is, the greater the 

 surface for any given quantity of tissue ; and hence the 

 more rapid the absorption of the dissolved gases. 



132. In the third place, Sir John Lubbock has suggested 

 that, while the forms under discussion do offer a large 

 amount of surface relatively to the total mass of the leaf, 

 we must not forget that- the buoyancy of the water favors 



64. One of the submerged 

 leaves of Cabomba, a 

 near relative of the 

 Water Lily. 



63. Fresh water 



Eelgrass. 



