328 
DUNCAN S. JOHNSON 
the top and toward the base of the petiole are found the simple or 
branched, bristle-like trichomes (figs. 1,2). There are three, nearly 
equal, vascular bundles in the petiole, one median and two marginal. 
Of the latter, one arises on that side of the xylem strand which is on 
the side of the stem opposite the leaf. The other marginal bundle, 
together with the median bundle of the petiole and the bundle from 
the axillary branch, arises from the other xylem strand of the stele. 
Each bundle of the petiole consists of 40 or 50 cells in cross section, 
and includes 5 or 6 thickened ducts and a similar number of phloem 
elements. These bundles are surrounded, in the cross section, by 
about 100 parenchymatous cells, 25 to 60 ^ in diameter and 40 to 
70 iu long, between which are numerous, small air-canals (figs. 14, 15). 
The blade of the leaf has a pinnately arranged, sparingly branched 
vascular system, that supports a delicate but bristly lamina of but 3 
or 4 cells in thickness (figs, i, 2, 17). The epidermis of the upper 
surface consists of irregularly polygonal cells 40 to 60 ^ across, from 
among which arise the ten- or fifteen-celled, bristle-like trichomes 
(fig. 3). Each trichome rests upon a ring of 5 or 6 surface cells, that 
is raised slightly above the upper surface of the leaf and is free from 
the underlying palisade layer. Stomata are wanting from the upper 
side of the leaf. Oil-cells are about as abundant as on the lower side. 
Hydathodes are rather less abundant than below. The epidermis of 
the lower side consists chiefly of wavy-margined cells, often 100 ju 
long, sparsely interspeised with stomata, oil-cells and hydathodes 
(fig. 18). The epidermal cells of the margin and of the lower surface 
of the veins are much elongated, and have oil-cells only scattered 
among them, except at the glandular tip of the leaf, where hydathodes 
may occur also (fig. 18). The cutin layer of the epidermal cells is 
often striate like that of the stem, especially in cells at the edge of the 
leaf. The average number of stomata in the areas between the veins 
is about 40 per square millimeter (fig. 18). The form of the guard 
cells as seen from the surface is of a type rather common among angio- 
sperms (fig. 19). When viewed in cross section the guard cells are 
seen to be peculiar in having a thin, sharp-edged cutin lip bordering 
the stoma, much like that shown by Haberlandt ('87, and '09, p. 424, 
427) to be characteristic of aquatics or land plants of very moist 
habitats, such as Lemna, Trianea, Fegatella, etc. (fig. 20). Moreover, 
this lip is so broad, and the ventral walls of the guard cells next the 
slit so widely separated in all sections seen, that it seems likely that 
