250 
Journal of Agricultural Research 
Vol. XIV, No. 6 
the tangential stretching resulting from an expansion of the vascular 
tissue which is not accompanied by a corresponding enlargement of the 
cortex itself. Whenever the expansion is too great to be compensated 
for by passive stretching of the cells, some of them become meristematic 
and divide by the formation of anticlinal walls. In the interfascicular 
region the expansion is much less, and consequently the cells of the 
cortex retain more or less their original shape. The boundaries of both 
regions may show transition stages, depending on the amount of second¬ 
ary growth in the interfascicular region. 
The secondary growth which is found in organs other than the stem 
has received consideration in the chapters on the ontogeny and will not 
be treated further. 
GENERAL DISCUSSION 
In the study of the anatomy and the ontogeny of the potato plant 
as presented above there are given a number of features sufficiently 
striking to justify reconsideration and discussion. 
The protoxylem matures before the protophloem; it is perhaps even 
first to differentiate. It is usually stated that the phloem precedes the 
xylem in appearance in the higher vascular plants. The phloem initials 
appear at first in the inner, then in the outer region, and not in the 
reverse order, as stated by Weiss (jo), for the Solanaceae. The maturing 
of the elements follows the same sequence. The primary phloem groups 
do not arise by the division of single initial cells, but from groups of 
small cells set off by the enlargement of surrounding procambial cells. 
Once set off, these groups enlarge by the formation of new elements. 
The bicollateral condition, of course, is characteristic of the Solana¬ 
ceae, but the inner phloem groups are usually limited to the peripheral 
region of the pith; sometimes, however, they are found farther away 
and near the center of the stem, a condition often observed in the potato 
plant. These innermost phloem groups clearly belong to the stele proper, 
and do not represent the vestigial remains of a second set of vascular 
bundles, as is thought by Worsdell (12) to be the case in the cucurbits. 
Their position near the center of the pith is accounted for as follows: A 
number of the parenchymatous cells of the perimedullary zone divide 
repeatedly in the radial direction, causing some of the innermost groups 
to be deflected from their straight course and take a position far within 
the pith. This type of tissue increase is characteristic of the thickening 
of the tuber and results in this case in the formation of the extensive 
sheath of parenchymatous cells which is traversed by numerous small 
phloem strands. 
Of special Interest also is the question of the extent of union of the 
individual phloem groups as they traverse the stem. An examination 
of Plates 34, A and B, and 35, A and B, shows that these groups branch 
and anastomose freely. Owing to this, connection is established between 
the individual groups of both the inner and the outer phloem. A similar 
