148 Gibson . — Contributions towards a Knowledge of 
In section (Fig. 1 7) the ligular epidermis is followed by a very distinct 
palisade-layer, one or, in many places, two cells deep. The main mass 
of the leaf is occupied by reticulate mesophyll, absent however at the 
margins and apex. The vascular bundle consists, in section, of three 
strands of spiral tracheides, the central strand being surrounded by 
phloem, and all three imbedded in small-celled parenchyma without 
intercellular spaces. 
Section II. Comparative Anatomy. 
1. The distribution of the Stomata. 
In comparing the different species, one is struck by the 
great variation in the distribution of the stomata on the leaf. 
As has been pointed out in the discussion of the individual 
species, stomata may occur on the ligular or aligular surfaces, 
or on both ; generally over the midrib, but frequently on the 
marginal lamellae or even on the margin itself. A glance at 
the following tabular statement shows that out of fifty-two 
species examined, fourteen species and one variety have stomata 
on the aligular face only of both dorsal and ventral leaves ; 
ten species and two varieties have stomata also on the ligular 
face of the ventral leaves ; eight have stomata on the ligular 
face of the dorsal but not on the corresponding face of the 
ventral leaf ; ten species have stomata on both ligular and 
aligular faces of both types of leaf ; whilst twelve species have 
stomata actually on the margin either of the dorsal or ventral 
leaf, or not removed more than one cell from it. One can say 
therefore, though only in very general terms, that the 
stomata in the genus are distributed as a rule on the aligular 
epidermis. 
2. Structure and development of the Stomata (Figs. 12, 13). 
I have followed out the stages in the development of the 
stomata in several species — e. g. 5 . uncinata (Figs. 12, 13), 
5 . Kraussiana , &c. — and find no essential differences between 
them. The stomata appear to be quite of the normal phane- 
rogamic type, with two guard-cells, each provided with 
a nucleus and a variable number of chloroplastids, not infre- 
