ii] THE ARROWHEAD 13 



plants growing from tubers, among the drift at the edge of a 

 river, with a varying number of ribbon-Hke leaves, succeeded 

 in some cases by one or two of slightly spathulate form (Fig. 4). 

 Fig. 5, p. 14 represents a young plant found in July which shows 

 a series of leaf stages between the early band-like form and the 



Fig. 4. Sagittaria sagittifolia, L. Plant with soft submerged leaves growing from 

 a tuber, t; from river drift at the edge of the Cam near Waterbeach, May 31, 1911. 



( nat. size.) [A. A.] 



mature ' arrowhead ' type. The significance of this heterophylly 

 and its relation to the environment will be discussed in Chapter xi. 

 Sagittaria^ like the other Alismaceae, is characterised by the 

 presence of mucilage-secreting trichomes, in the form of scales, 

 in the axils of the leaves. In a paper published a few years ago, 



