8 i8 
Journal of Agricultural Research voi. xxvii.no. n 
and the center of the tuber naturally widens, the pith retains its con¬ 
nection with the eyes and its buds. Thus we see in the mature tuber 
branches radiating from the pith toward the periphery of the tuber 
(PI. i, A. B). In tangential section these branches appear as com¬ 
pressed hollow cylinders inclosing groups of phloem tissue. These pith 
branches, however, are not always symmetrical, since, on account of 
irregular growth activity, groups of phloem cells become more and more 
subdivided and spread out, causing a temporary or permanent breaking 
up and subdivision of these pith branches—a condition which becomes 
more conspicuous toward the periphery. In certain varieties the pith 
branches are large and uniform; in others, notably the Triumph, the 
branches are fine and almost indistinct; the cells are almost free from 
starch. The mass of tuber tissue inside the vascular ring is of two 
kinds—the storage parenchyma derived from cell division in the Mark- 
krone and containing groups of internal phloem, and the pith with its 
lateral branches. The degree to which either of the tissues contributes to 
the formation of the tuber varies greatly with different varieties, but on 
the whole the pith occupies a smaller area than does the storage 
parenchyma. 
The structure and growth of the potato tuber has been considered in 
earlier publications, the different writers giving their own interpretation. 
In the growth of the tuber, as in any other organ, two factors are of 
importance—cell division and cell enlargement (PI. 4, A, B, C). The 
latter factor is of special significance, since, as Esmarch (5) has shown, 
the cells of the young pith and cortex may increase to 4 times their 
original diameter, thereby increasing the volume to 64 times that of the 
young tissues. Cell division, however, is not equally rapid and extensive 
in the various tissues, and the investigators have been at variance as to 
the relative importance of each. 
De Vries (20) considered most tuber tissue as potentially vascular and 
formed by a cambium, differing from the normal development in that 
almost all of the potential wood matured into thin-walled storage paren¬ 
chyma. Conclusions from ontogenetic studies of the tuber, as developed 
in this paper, are in harmony with De Vries’s view that the major part of 
the tuber is procambial in origin, but differ in attributing little activity to 
the cambium as such. The position of the protoxylem (fig. 2) in close 
proximity to the vascular ring shows how very little new tissue has 
actually been formed by the cambium. 
Reed (15) showed that the tuber is built up by the activity of three 
tissues—pith, phloem parenchyma, and cortex. He clearly states that 
the cells between the internal phloem and the protoxylem begin to divide 
very early, causing a spreading out of the phloem groups. While this is 
in agreement with the writer’s observations, it is difficult to assume that 
numerous cell divisions occur also in the parenchyma of the phloem 
groups and that through this activity a large amount of tissue is added 
to the growing tuber. It appears rather that the phloem parenchyma 
must be considered an integral part of the phloem groups and that its 
cells at maturity do not become storage parenchyma. The cell divisions 
which contribute to the increased size of the tuber must be sought in 
other tissues, and, as the writer has shown, most likely among the undif¬ 
ferentiated cells of the procambium. 
Esmarch (5) regards the enlargement of the pith as the chief factor 
in tuber growth. He considers the internal phloem groups to be of 
