SCALES OF AQUATIC MONOCOTYLEDONS. 235 
Without unduly emphasising minute anatomical detail, I 
would draw attention first to the close resemblance between the 
figures showing early developmental stages in the scales of Potamo- 
geton and Alisma (Pl. 5. fig. 3 & Pl. 6. figs. 7, S, and those of 
corresponding stages in the ligules of Selaginella figured both by 
Bower (18) and by myself (1). In the paper referred to (1) I 
laid stress on certain peculiarities seen in the ligules of Selagi- 
nella, more especially (a) the close association of the ligules with 
the young leaves and growing apex; (b) the deeply stainable con- 
tents of the ligular cells in the young condition especially ; (e) the 
ultimate isolation of the ligule by cutinization of its basal cells ; 
(4) its early appearance aud early decay; (e)its variable form 
and state of development; and lastly (f) its probable homology 
with a ramentum. In all these characters the scales described 
above show a strong family likeness. The scales are almost 
simultaneous in development with the leaves, their cells are 
deeply stainable, they contain abundant protoplasm, they show 
the same merismatie basal layer, the same development of an 
isolating cutinization in the basal cells when their merismatic 
functions have ceased. There are, it is true, points of difference, 
the chief of which are (a) the non-differentiation of a special 
glossopodium, and (b) the non-development of any vascular 
dilatation of the leaf-trace below their point of origin. These 
differences, however, do not appear to me to form insuperable 
dillieulties in establishing the homology I desire to make out; 
since in the first place histologically differentiation has not 
reached a very high level in aquatic monocotyledons, and in the 
second place, being aquatie and in the young condition even 
submerged plants, there would appear to be no necessity for any 
enlargement of the, in itself, feebly developed leaf-trace in the 
vicinity of the leaf-rudiments. If the term “ stipule” has been 
made to include, as Balfour says (10. p. 14), both “a lateral 
branch of a leaf arising at its very point of insertion” as well as 
“structures organically connected with the stem,” or, in a word, 
* any small appendieular structures found in the vicinity of the 
base of the leaf,” we are, I think, in danger, by over-dependence 
on the connotation of a term, to lose sight of what may ulti- 
mately prove to be important homologies. The discovery of the 
occurrence of a ligule in the Lepidodendres leads Scoit (4) to 
assert the probable affinity of that group “ with Selaginella or 
Isoctes, rather than with Lycopodium, among recent members of 
