240 
Journal of Agricultural Research 
Vol. XIV, No. 6 
and 102 in the outer region. The number of protoxylem elements at 
this stage is about 130. 
On taking the whole of a cross-sectional area at this stage, it is notice¬ 
able that there has been an unequal differentiation in the procambium 
cylinder, with greater development and specialization in the region of 
the procatnbial projections. These regions have developed to an extent 
su<ch that under low power six groups of vascular tissue and gaps sepa¬ 
rating them may be readily differentiated. Gradually, however, differ¬ 
entiation extends also to these interfascicular regions, being initiated by 
the appearance of a cambium and primary phloem group initials. Plate 
31, A, shows the development of such an interfascicular cambium. It 
does not form simultaneously throughout, but arises in different places, 
gradually uniting and thereby bridging the gaps between the large 
groups of vascular strands. Phloem groups are seen outside and inside 
the cambium. The outer groups are small and closely arranged (PI. 29, 
A, C), the inner more or less scattered and usually more distant from 
the cambium, forming, together with those of the inside of the pro¬ 
jections, a more or less symmetrical figure. 
In the region of the larger vascular groups large xylem initials are 
formed (PI. 29, C; 42, A). The thin delicate primary wall of these cells 
soon becomes strengthened by secondary thickenings in the form of 
scalariform and reticulate bands. In vertical section these cells are 
cylindrical with somewhat sloping end walls. The type of secondary 
thickening, unlike that found in the protoxylem elements, is such that- 
further elongation of the cell is impossible. This fact, together with 
difference in size and location, serves to distinguish between protoxylem 
and these later-formed, larger cells of the primary xylem which are known 
as metaxylem. 
The nature and amount of differentiation which we have thus far fol¬ 
lowed in the young potato sprout relate to the procambium, which, as 
we have seen, gives rise to all the primary vascular tissue of the stele. 
Simultaneous changes in the other meristematic tissues consist chiefly 
of cell enlargement without marked differentiation. In the cortical 
region the peripheral tissue undergoes qualitative but not quantitative 
changes. The two outermost layers are of cells, rather regularly arranged, 
the elements themselves being more or less rectangular and vertically 
elongated. Within these we have two or three layers of cells, polyhedral 
in cross section, and tracheid-like in tangential view. The walls of these 
cells, however, are thin and of cellulose. With the appearance of the 
potato sprout above the surface of the soil, these prosenchymatous cells 
of the cortex develop secondary thickenings in a characteristic manner. 
Wall thickenings occur in the four comers, sometimes also along the 
radial walls (PI. 29, A, D). This specialized tissue of the cortex, the 
collenchyma, serves as the supporting tissue of delicate, growing organs. 
Differentiation of the vascular cylinder continues. The first formed 
metaxylem elements are now mature; new ones appear continuously. 
