234 PROF. R. J. HARVEY GIBSON ON THE AXILLARY 
is first deposited in the radial walls of the basal cells and gradu- 
ally extends to the other walls of these and neighbouring cells. 
The general appearance recalls the sheathing-cells of the glosso- 
` podium of the ligules of Selaginella. The same phenomenon is 
noticeable in the basal cells of other genera, notably Triglochin 
and Aponogeton. 
VALLISNERIA. 
Two to five somewhat massive scales occur in the axil of each 
leaf of Vallisneria spiralis. Each arises from a ridge three cells 
in thickness, rapidly dividing so that the scale becomes finally 
about 5-6 cells thick. This thickness it maintains for most of 
its length. Müller (12) speaks of the development of the scale 
as being due to the activity of an apical cell-row. The mode of 
growth, however, appears to me to be, as in other forms, in- 
variably basal. I have never seen any evidence of apical growth 
in any of the axillary seales I have examined. 
The Morphology and Physiology of Axillary Scales. 
Speculations as to the morphological value and physiological 
purpose of axillary scales are not wanting. The subject was 
discussed by Irmisch (6) in 1858, afterwards by Prillieux (13) 
and by Balfour (10), and yet in 1882 Buchenau writes: “Ueber 
ihre morphologische und physiologische Bedeutung sind wir noch 
immer im Unklaren." Irmisch (6) holds that the axillary scales 
are trichomic, and considers that they are protective organs to 
the merismatie structures in their vicinity. Prillieux, on the 
other hand, inclines to the belief in a stipular homology for 
them. Balfour (10) steers a median course, suggesting that 
from “position and development” they may be in Halophila 
ovalis designated as “ stipules,” but doubtfully so in H. stipu- 
lacea, which possesses membranous wings on either side of the 
petiole in addition to the axillary scales. He decides to retain 
Irmisch’s name “as indicating a special form of structure, what- 
ever be its morphological value.” 
My own view is that these axillary scales are homologous with 
the more specialised and solitary ligules of Selaginella, of Tsoétes, 
and of such fossil forms of Pteridophyta as Lepidodendron, 
where the presence of a ligule was demonstrated by Solms- 
Laubach (15) and by Hovelacque (2), and in Lepidostrobus, 
where a ligule is described by Maslen (3). 
