270 Adjustment to Conditions of Aquatic Life 



b. Reduction of wood vessels and of wood fibers 

 and other mechanical tissues. In the denser 

 medium of the water these are not needed, as 

 they are in the air, to support the body. Pliancy, 

 not rigidity, is required in the water. 



c. Enlargement of air spaces. This is prevalent 

 and most striking. One may grasp a handful 

 of any aquatic stems beneath the water and 

 squeeze a cloud of bubbles out of them. 



d. Concentration of vessels near the center of 

 the stem where they are least liable to injury 

 by bending. 



e. A general tendency toward slenderness and 

 pliancy in manner of growth, brought about 

 usually by elongation of the internodes. 



3. In leaves: — many adaptive characters; among 

 them these: 



a. Thinness of epidermis, with absence of cuticle 

 and of ordinary epidermal hairs. This favors 

 absorption through the general surfaces. 



b. Reduction of stomates, which can no longer 

 serve for intake of air. 



c. Development of chlorophyl in the epidermis, 

 which, losing the characters which fit it for 

 control of evaporation, takes on an assimilatory 

 function. 



d. Isolateral development, i. e., lack of differ- 

 entiation between the two surfaces. 



e. Absence of petioles. 



/. Alteration of leaf form with two general ten- 

 dencies manifest: Those growing in the most 

 stagnant waters become much dissected (blad- 

 derworts, milfoils, hornworts, crowfoots, etc.). 



Those growing in the more open and turbu- 

 lent waters become long, ribbonlike, and very 

 flexible (eelgrass, etc.). 



