Aug. s. 1918 
Anatomy of the Potato Plant 
243 
internal differentiation of the leaf is complete. When the leaf is still 
small, the hairs are very numerous and the epidermis over the veins is 
covered with a dense mat of them; with the elongation of the individual 
organs the thickness of this mat decreases, since between the already 
existing hairs no new ones are formed. On the mature leaf the hairs 
are widely scattered. 
Tissue differentiation in petiole, midrib, and veins follows the same 
general order as described for the stem, but the location and the relative 
amount and size of the elements is somewhat different. Sections through 
the middle of a leaf primordium show a small crescent-shaped mass of 
procambial tissue surrounded by large cells of the fundamental meri- 
stem. At first only slight differentiation is noticeable; in about the 
middle of the procambium a very few protoxylem elements appear. 
A little farther back from the growing point specialization in the periph¬ 
eral procambium is going on; the cells are increasing in number and 
their nuclei are large. Those near the protoxylem at the same time 
expand and become arranged more or less in tangential rows. 
Near the base of a leaf primordium a well-developed procambium area 
with about eight protoxylem elements is found. At the periphery of the 
procambium, which has now become semicircular in section, phloem 
initials appear, forming a band of smaller cells. The first differentiation 
of the phloem into groups is noticeable in the internal phloem (on the 
upper side of the procambium). These groups are few and large, and 
make up a large proportion of the vascular tissue. Here and there the 
outer phloem (that toward the lower side of the leaf) which had appeared 
from the procambium as a nearly continuous band, is separated into 
very small groups. There is, however, considerable variation in the 
condition found in different leaves of the same size. Often the procam¬ 
bium, though specialized at the periphery, has not as yet formed phloem 
group initials, whereas in other leaves of the same size almost complete 
differentiation of the phloem groups has occurred. 
In sections through older leaf primordia an increase in the number of 
protoxylem elements occurs, and the first metaxylem initials, which can 
be readily distinguished by their large size, have also begun to appear. 
The cells of that portion of the procambium which has not yet become 
specialized to form either phloem or xylem, increase in number and 
size. Those which lie along the convex side of the vascular semicircle 
have a somewhat regular arrangement thus foreshadowing the appear¬ 
ance of a cambium. Between the outer phloem initials large oval cells 
become noticeable (PI. 36, A). Their appearance and location is so 
characteristic and constant that they would seem to deserve greater 
attention. Serial sections, however, show nothing suggesting function 
or structure different from that of the cortical cells. Sections through 
more advanced stages show both outer and inner phloem arranged in 
groups which are now quite distinct, and only in the region of the ends 
