248 
Journal of Agricultural Research 
Vol. XIV, No. 6 
tubers of this stage are quite small, not over 5 to 10 mm. thick. The 
tuber increases both in length and in thickness; the latter interests us 
chiefly. From this stage on the parenchyma cells of the perimedullary 
zone, and to some extent those of the cortex also, contribute mostly 
to the formation of the tuber tissue. The vascular cylinder is forced 
more and more outward by excessive cell divisions in the regions of the 
perimedullary zone and pith. However, all of the vascular tissue does 
not partake in this move. The inner phloem groups become separated 
with the increase in the tissue of the perimedullary zone and peripheral 
pith, and gradually split up into numerous strands which traverse 
these regions of active growth in all directions (PI. 38, B, C, D). 
An examination of the mature potato tuber leads to the conclusion 
that most of the tuber tissue is derived from the parenchyma of the peri- 
medullary zone of the stele, to a less extent from the parenchyma of 
the external phloem, the cortex, and the pith. The amount of secondary 
elements added as a result of the activity of the cambium is insignificant 
and is limited to a few vessels and some wood parenchyma. 
Reed (jj), who first followed in detail the development of the potato 
tuber, maintains that the pith and the perimedullary zone contribute 
equally to the formation of new tuber tissue; but this does not seem to 
be the case, since even mature potatoes (in stained sections) show phloem 
strands in the region near the center—that is, all of the tissue of the tuber 
except that of the most central part and of the cortex is traversed by 
phloem strands. The view of De Vries and others that the tuber is 
formed largely by the activity of the cambium is no longer tenable. 
The young tuber has a more or less distinct epidermis with scattered 
stomata (PI. 39, A, B). Upon the enlargement of the tuber the epi¬ 
dermis undergoes marked changes. At first anticlinal walls appear in 
a few of the epidermal cells, probably as a sequence of the tension caused 
by the expansion of the organs (PI. 39, C). Eater periclinal walls also 
appear (PI. 39, D). Simultaneously with the division in the cells of 
the epidermis division walls also appear in the subepidermal layer. 
Cell division in this region continues until a layer of tissue is pro¬ 
duced which takes over the protective function of the epidermis; 
this is generally known as periderm or “cork” (PI. 39, F). The forma¬ 
tion and constant regeneration of the periderm are due to the activity 
of the meristematic cell layer known as the phellogen. In the potato 
tuber the phellogen consists of a single layer of thin-walled cells which 
divide tangentially and which constitute the inner row of daughter cells 
produced by the first division of the cells of the hypodermal layer (PI. 
39, D, E). While most of the periderm arises from the phellogen derived 
from the hypodermis, the epidermis gives rise to a superficial periderm 
usually three to four cells in extent. Both layers of periderm tissue, 
though adjacent to each other, are distinct. The periderm is perforated 
by a number of lenticel-like structures which arise immediately beneath 
the stomata, the function of which they assume. 
