i68 



SUBMERGED LEAVES 



[CH. 



Fig. 109. Myriophyllum spicatum, L. 

 T.S. through a segment of the leaf of 

 the water form. The epidermis contains 

 chloroplasts and the mesophyll is laden 

 with large starch grains, onlj^ indicated 

 in a few cells. (X157.) TSchenck, H. 

 (1886).] 



spongy parenchyma. In many cases the assimilatory activity 

 seems, in great measure, confined to the epidermis, the meso- 

 phyll serving rather for storage 

 purposes. Myriophyllum^ shows 

 this distinction clearly; the 

 epidermis is rich in chloro- 

 phyll, while the mesophyll 

 contains large starch grains 

 (Fig. 109). This leaf is a good 

 example of the subdivided, 

 submerged type, each limb of 

 which exhibits a tendency to 

 a radial arrangement of the 

 tissues. Littorella^ Utricularia 

 minor (Fig. 74, p. 108) and 

 CeratophyllumdiW show the same 

 approximately radial type of 

 leaf anatomy. The effect of 

 environment upon this kind of leaf, is illustrated by a com- 

 parison between the land and water forms of Myriophyllum. 

 In the case of M. alternifolium^ the land form, when growing 

 in sunny situations, has shorter and thicker leaf segments than 

 the water form; they are also dorsiventral and elliptical, 

 instead of radial and cylindrical, while the xylem is more 

 highly developed than in the water form. The epidermis 

 contains only a few small chlorophyll grains, and stomates 

 occur. The epidermal cells also have the sinuous outline which 

 is lacking in the water form. The absence of marked dorsi- 

 ventrality in the leaves of many submerged plants, such as 

 Myriophyllum^ may in part be attributed to the fact that they 

 are perpetually being moved about by water currents, and thus 

 they do not retain any constant position in relation to the 

 incident light. 



The very young submerged leaves of Myriophyllum verticil- 

 latum and M. spicatum show a peculiarity which has repeatedly 



iSchenck, H. (1886). 



