"74 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



aquatic plant, the foliage leaves of which emerge 

 from the water. The blade of these leaves (fig. 

 40, F) is shaped like an arrow-head, and the barbs 

 facilitate rain-water drainage. The petiole is very 

 long, and terminates in a sheathed base derived 

 from the hypopod. 



The apical leaves (fig. 4, G) are represented by 

 transparent membraneous scales, which serve as a 

 protection to the floral buds. 



The basilar leaves, which either float on the 

 surface of the water or else are entirely sub- 

 merged, have undergone both development and 

 degeneration. Degeneration is more noticeable on 

 glancing downwards from the foliage leaves to 

 those which protect the winter buds. 1 



1 (a) The leaves nearest to the foliage leaves float upon the 

 surface of the water (fig. 40, E). The lateral barbs, 

 being no longer required, have begun to degenerate. 



(b) The floating leaves immediately beneath exhibit barbs 



which are still more reduced (fig. 40, D), and there are 

 no stomata except those upon the upper surface. 



(c) The next leaves are completely submerged and lengthened 



out into long ribbons which broaden into flat blades 

 at their upper ends, which are not separated from the 

 hypopod by petioles (fig. 40, c). In this case the blade 

 is very much smaller, and the petiole has disappeared. 



(d) The next leaves to these are submerged, and are ribbon- 



like in shape. They spring immediately from the 

 primordial leaf without differentiating into hypopod, 

 mesopod, and epipod. 



(c) After these leaves follow leaves consisting solely of a hypo- 

 pod, which is much reduced in size (fig. 40, A). These 

 leaves are folded round, and serve as a protection to the 

 bud. 



