xi] SUBMERGED LEAVES 141 



Other types of submerged, radical leaf are the small, almost 

 cylindrical leaves of Lobelia Dortmanna and Littorella lacustris 

 (Fig. 142, p. 218), and the linear serrate leaves of Stratiotes 

 aloides (Fig. 32, p. 53), which are too firm and stiff to be called 

 ribbon leaves. In Lobelia and Littorella^ the shortness of the 

 leaves obviates the necessity of pliability to the motion of the 

 water, while in Stratiotes the need for flexibility is diminished 

 by the partially free-swimming habit of the plant and its pen- 

 chant for quiet waters. 



FIG. 90. Sagittaria sagittifolia, L. Young plant produced from a tuber (T) and 



bearing ribbon leaves only; tuber, with axis and scale leaves, and roots, indicated 



in solid black. Drift at bank of Cam, May 31, 1911. (Nat. size.) [A. A.] 



When the leaves, instead of being radical, are borne on a 

 pliable, elongated stem, the function of flexibility seems to be 

 taken over by the axis and the leaves are generally small and 

 simple, as in the case of Elodea canadensis. In Hippuris vulgaris, 

 however, the whorled, submerged leaves may reach a consider- 

 able length. 



The finely divided type of submerged leaf takes two different 

 forms, according to whether the species to which it belongs 

 is Dicotyledonous or Monocotyledonous. There are numerous 

 examples of dissected, submerged leaves among the Dicoty- 



