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 (Pl. III, fig. 8; 
Pl. VU, fig. 48, 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 cytoplasm (Text-fig. 17). 
A longitudinal section of a single scale (Text-fig. 16), shows that 

Figure 16.—Longitudinal section Figure 17.—Cells from cross 
through axillary scale, showing section through axillary scale, 
arrangement and shape of cells. showing cell structure. 
> in0: >< 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- 
