Physiological Studies in Plant Anatomy 35 
the region. This phenomenon seems explicable on the grounds that, 
in the region of the cut surface, the nutrient sap, restricted normally 
within the endodermis, is able to irrigate the protoplasts of the cortex. 
The unusual quantity of solutes suitable for growth then supplied to 
the cortex not only provides for the activities of the cork phellogen, 
but leads to the development of the branch shoots. 
We are led, indeed, by our experimental results to this conclusion: 
the sap supply in the vascular strand, with its accompanying organic 
solutes, is necessary for the vigorous development of the tissues of a 
vascular plant. When the sap is retained within a functional endo¬ 
dermis only the parenchymatous tissues within that endodermis are 
capable of active growth. If, however, the sap is able to irrigate the 
cortical parenchyma, exogenous meristems are able to develop, and 
these give rise either to lateral shoots or leaves. In the uninjured 
stem the functional endodermis completely retains this nutrient sap 
within it, and on the basis of the previous argument this condition 
should lead to an active formation of endogenous roots from the stem 
(Priestley and Pearsall (20)). The most superficial observation shows 
that this is in fact the case. If the etiolated stems are growing in 
relatively dry air, these endogenous roots appear to be unable to 
break through the stem surface, but can be detected in the region of 
the nodes as whitish spots below the cuticle. If the plants are grown 
in a humid atmosphere these roots readily break through the stem 
surface. They then develop in great numbers as the stems grow, arising 
in acropetal succession, not only from the nodes, but frequently in 
the internodal regions also. An exaggerated development of roots 
may be obtained by allowing the buds upon a potato tuber to develop 
in the dark under water. Development will not continue long under 
such abnormal conditions, but it will continue long enough to give 
a group of short shoots with two or three internodes, which bristle 
with lateral roots. It is interesting to note that none of these roots 
arise directly from the tuber. The development of the tuber is being 
studied in this laboratory, and its appearance is coincident with a 
disappearance of the endodermis present in the slender underground 
tuberiferous stem. 
Finally, reference may be made to the axillary branching that is 
frequently to be seen in etiolated potato plants, and occasionally in 
etiolated broad beans. Considered in the light of the experiments 
just described and the observations just recorded, it appears that the 
branching can hardly be considered as analogous to the normal pro¬ 
duction and development of axillary buds. In these etiolated plants, 
3—2 
