STUDIES OF THE DEVELOPMENT OF PEPEROMIA HISPIDULA 327 
below (figs. 10, 11). The transverse section of the stele consists of 
only about 150 cells, which vary in diameter from 5 to 25 and in length 
from 20 to ICQ iJL. There are usually two distinct groups of primary 
xylem, one near each edge of the flattened bundle (fig. 11). Each 
group contains from 4 to as many as 8 or 10 ducts which are ringed, or 
less often spirally thickened. These ducts vary from 10 to 30 /x in 
diameter and are often 500 ijl in length. Between, and arranged around 
these ducts are thin-walled parenchymatous cells, which may become 
75 fjL long. A group of 20 to 30 phloem cells is found outside each group 
of xylem and on the same radius (fig. 11). Each of the phloem groups 
consists of 6 to 8 sieve tubes, and of rather more numerous cambiform 
cells with surrounding parenchyma. 
In the stem, as in the root, no indication was found of the formation 
of a cambium ring nor even of any formation of secondary xylem or 
phloem. The small amount of water-conducting tissue formed in the 
root and stem of this Peperomia is a fact evidently related to its moist 
habitat. So small a plant, creeping close to the soil, and growing in 
a very humid air, must lose very little water by transspiration and 
thus needs to carry but little water in its conducting system. More- 
over, that little water needs to be carried but a short distance, 
because new groups of roots are formed frequently along the stem 
as it grows, so that every internode or pair of internodes, except those 
near the apex, has its own direct connection with the water supply 
in the soil. 
As contrasted with the type of conducting system just described 
we find that the stem of xerophytic species of Peperomia, such as 
P. reflexa or P. verticillata, have much more highly developed bundles. 
In the latter, for example, there are 10 or 12 bundles in two concentric 
rings. Even in the rather delicate, creeping P. filiformis, growing in 
the same damp forests with P. hispidtda, there is a ring of four bundles 
in the stele. 
The internal structure of the leaf of Peperomia hispidula is also 
simpler than that of any other species of the genus known to the writer. 
The petiole, as seen in cross section (fig. 14), is somewhat flattened 
dorsiventrally and has a slight wing at each edge. The epidermis 
consists of rather irregular cells with slightly thickened outer walls, 
interspersed with scattered oil-cells and hydathodes. The wings of 
the petiole are made up solely of two layers of the epidermis, or, in some 
portions, of these and a third layer pushed between them (fig. 14). At 
