96 



A. H. Graves, 



the leaf-base they can better perform their office of protection of 

 the axillary structures. In the case of the grasses, where the sheath 

 is also retained, Strasburger (1908, pp. 29, 30) states that be- 

 sides protecting the soft lower part of the internodes when inter- 

 calary growth takes place, the sheaths also give the stem rigidity. 

 Possibly this latter strengthening function applies also in some 

 degree to the sheaths of water plants. 



3. Axillary Scales. 



a. General Characters and Anatomy. 



In the axils of all the leaves are two small, ovate, scaly forma- 

 tions, one on each side of the median line of the leaf (PI. Ill, fig. 8 ; 

 PL VII, fig. 43, as). These structures, common in water plants, 

 and first shown by Irmisch (1858, p. 12) to be of general occurrence 

 throughout the Potamogetonaceae, consist of generally two layers 

 of cells rounded in cross-section, and loosely joined together, con- 

 taining large nuclei and a large quantity of C3'toplasm (Text-fig. 17). 

 A longitudinal section of a single scale (Text-fig. 16), shows that 



Figure 16. — Longitudmal section 



through axillary scale, showing 



arrangement and shape of cells. 



X 170. 



Figure 17.— Cells from cross 



section through axillary scale, 



showing cell structure. 



X 1400. 



these cells are long and arranged more or less in rows in the 

 upper part of the scale. Prillieux (1864) has found similar struc- 



