330 
DUNCAN S. JOHNSON 
have the mesophyll structure of the aerial, dicotyledonous leaf reduced 
to about its simplest possible terms. 
The vascular system of the lamina of Peperomia hispidula is 
derived from the three vascular bundles of the petiole, which enter 
the blade separately. The medium bundle forms the midrib, which 
gives rise from its upper half to two pairs of lateral veins (fig. i). 
Each of the two lateral bundles from the petiole forks soon after 
entering the blade, and the two divisions so formed are connected 
with each other and with the branches from the midrib in the manner 
shown in figure I. The submarginal vein arising from these fusions 
contains, for most of its length, only three or four vascular elements, 
and the leaf is but slightly increased in thickness in this region (fig. 17). 
Each of the three veins entering the base of the blade contains, on the 
contrary, several times this amount of vascular tissue, and each, with 
the tissue about it, forms a rib two or three times the thickness of 
the rest of the lamina. The tissue of these ribs, outside the vascular 
strands, consists generally of slightly modified parenchyma, surrounded 
by an epidermis with a few oil cells scattered through it (fig. 16). At 
the end of the midrib, however, and at one other point on each lateral 
margin of the blade, there is a broader, more massive thickening of the 
tissue about the veins. At these points the thickening projects chiefly 
on the upper side of the lamina (figs. I, 18, 21). These swellings may 
be ID or II cells thick, and consist of rounded parenchymatous cells 
surrounded by intercellular spaces connecting quite directly with those 
immediately below the numerous stomata (figs. 18, 21). The latter 
are grouped, without regularity of orientation, over the whole of this 
glandular swelling (fig. 21). The rounded cells of the swelling have 
few chloroplasts, large vacuoles and rather large nuclei. The whole 
structure of the thickenings is closely similar to that described for the 
water-secreting glands of Tropaeolum. Unfortunately the plants 
brought to Baltimore for the study of these structures in the living 
Peperomia did not live, and I therefore have no experimental evidence 
concerning the function of these swellings. 
Considering the structure of the leaf as a whole, with its non- 
wetable bristles, its delicate texture, thin cuticle, limited vascular 
supply, and its well-developed glands evidently fitted for water 
secretion, it is clear that we have in this Peperomia a pretty highly 
specialized plant. This leaf structure, together with the type of 
root structure and stem structure noted above, make this species well 
fitted for the dense, reeking forest which it inhabits. 
