244 
Journal of Agricultural Research 
Vol. XIV, No. 6 
of the crescent do these groups unite, thereby establishing a vascular 
connection between inner and outer phloem (PL 36, A, C). A cambium 
is developed throughout the crescent of vascular tissue, being more 
prominent in the region where the procambium has given rise to the 
largest amount of vascular tissue and less in those regions where speciali¬ 
zation is very slight. This cambium later gives rise to some secondary 
growth (PL 44, D). 
With the appearance of the first distinct phloem groups, differenti¬ 
ation in the leaf blade begins. Up to this time the tissue of the leaf 
blade is homogeneous in structure, being made up of brick-shaped cells 
(Pl. 36, B). An epidermis soon becomes distinguishable and stomata 
develop. The first cells of the blade to change are those just beneath the 
upper epidermis. These cells elongate to twice their former length and 
remain closely packed, forming the palisade initials (Pl. 36, D); in the 
lower tissue intercellular spaces begin to form, but the size and shape of 
the cells remain nearly the same. The leaf blade is fully developed, and 
the cells have completely matured before differentiation in the veins has 
ceased. The mature leaf blade (PL 36, E) has a well-developed epidermis, 
a palisade tissue one cell deep on the upper side, and three to five rows 
of spongy parenchyma the cells of which are separated by numerous 
and large intercellular spaces. 
THE) ROOT 
In plants grown from tubers all of the roots are fibrous in nature, and 
arise endogenously from the nodal pericycle of the subterranean part of 
the stem (PL 37, A). Transverse sections through this region in a young 
sprout developing beneath the ground show cells of the pericycle oppo¬ 
site the protoxylem groups richer in content and undergoing division. 
The cells first elongate radially, and then divide tangentially. The meri- 
stematic masses so formed are the root primordia; the central cells of this 
tissue become the vascular tissue of the young rootlet. The endodermis 
just opposite the root increases in extent, pushing out as a lobe ahead of 
the developing rootlet. The latter pushes its way mechanically through 
the cortex, and is aided by the dissolving action of enzyms which are 
probably secreted by the cells of the endodermis. Just before the epi¬ 
dermis of the stem is broken the cells of the endodermis cease division 
and are ruptured, giving way to the rootlet, which then penetrates to the 
surface and develops independently in the soil. 
Transverse sections near the tip of a rootlet which has just broken 
through the cortex show several distinct zones of tissue. The innermost 
region, which occupies only a small area, is a solid strand of primary 
vascular tissue separated from the thick cortex by an endodermis. The 
epidermis at this stage is specialized, in that many of its cells are elongated 
to form the root hairs. 
The vascular tissue is arranged radially, as is usual in roots, the xylem 
and phloem in separate strands. These strands alternate with one 
