THi; LEAF 



79 



floating, leaves. In ,t;'eneral, submerged plants possess 



long and narrow, or linear, leaves (Fig- 



t)3). Or, they may have leaves of a more 



or less rounded form, but much divided, 



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



Since submerged jilants of man}'^ 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 exact!}' what 



respect these cir- 

 cumstances call for 

 linear leaf forms 

 is, however, an 

 open question. 

 The)' may be ad- 

 vantageous from 

 any one or all 

 of the f olid wing 

 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 sulj- 

 stanccs tliat will l)C absor1)ed by a leaf, other things 

 Ijcing equal, is proportional to tlie extent of the surface 

 ill contact with the water. Tlie more divisions tlic leaf 

 lias, or the longer and narrower it is, tlie greater the 

 surface for any given quantity of tissue ; and hence the 

 more rapid the abs(n-ption of the dissolved gases. 



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

 that, while the forms under discussion do offer a large 

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

 we must not forget tliat the buoyancy of the water favors 



04. Oue of tlie sulimerj^cd 

 leaves c)f Galjuiiibii, a 

 near relative of the 

 Water Lily. 



63. Fresli water 

 Eelsrass. 



