542 Reed. — On the Anatomy of some Tubers . 
parenchyma cells pursue a very irregular course (PI. XLII, P'ig. 6). This is 
because the isolated strands are surrounded by meristematic cells which are 
not dividing at a uniform rate, so that in one region the strands will be 
pushed in one way and elsewhere in a different direction. 
It is interesting to find that the medullary phloem is apparently used 
for the conduction of carbohydrates from the parent plant to the developing 
tuber. This conclusion can hardly be resisted when one considers the 
manner in which the phloem elements ramify amongst the tuberous elements 
of the medulla. 
From the above it is seen that the tuber of X. tuberosum is built up 
from three kinds of tissue : (i) medulla, (2) phloem parenchyma, and (3) to 
a smaller extent, cortex. Xylem parenchyma does not contribute any 
considerable portion to the tuber. In some few cases it may be seen that 
the protoxylem is partly separated from the later-formed xylem by a wedge 
of parenchyma, but this is not generally the case (PL XLII, Fig. 4). This 
fact is interesting when we compare it with what obtains in Helianthus 
tuberosus , where it is found that the tuber is built up largely from xylem 
parenchyma together with medullary parenchyma. 
It is also worth noticing that the tuber forms few, if any, new wood 
elements. In ripening fruits, &c., L. Jost (8) states that under natural 
conditions the amount of mechanical tissue increases in proportion to the 
increase in weight. The case of these tubers is quite in harmony with this 
observation, for although there is an increase in the size and the weight of 
the tuber as growth continues, this weight has not to be borne by the plant, 
since the tubers are subterranean. 
It will be recalled that the stolon which gives rise to the tuber was 
covered with a single-layered epidermis which has a slightly developed 
cuticle (PI. XLII, Fig. 1, Ep). Almost as soon as the stolon commences to 
dilate to form the tuber the cells of the hypoderma begin to divide tangen¬ 
tially and give rise to a cork cambium (PI. XLII, Fig.5, Hyp). In the case 
of S. Dulcamara , De Bary ( 4 ) states that the periderm arises in the epi¬ 
dermis, and Solereder (12) makes the same statement for some species of 
Solauum , but does not include any species of Solanum in the group in which 
the periderm arises in the sub-epidermal layer. It may possibly be that the 
more deep-seated origin of the cork cambium in the tuber of S. tuberosum 
is due to the fact that since the tuber is a subterranean structure, the 
epidermis would be liable to injury. 
The cork cambium gives rise to several layers of cork on the outside 
and phelloderm on the inner side (PI. XLII, Fig. 5 a , ck.,P.) The develop¬ 
ment of a covering of cork for such a structure as the potato is almost a 
necessity, since the tuber contains a high percentage of water. 
