Nature of aerial Tubers in Dioscorea sativa , Linn. 495 
circumstances to be described later *, the arrangement of the 
structures in the leaf-axils was as follows. Between the 
main stem and the subtending leaf, and nearest to the stem, 
was one median axillary branch, immediately below and in front 
of this was the tuber on which were three buds. One, which 
was most developed, was just at the point of attachment of 
the tuber. The other two were also close to the point of 
attachment, one on the anterior side and the other on the 
posterior. 
Two well-developed roots were formed on some tubers, one 
on each side of the biggest bud of the tuber (Fig. 5). 
These buds, and the axillary branch, appear to be homo- 
logous with the impaired median buds in those leaf-axils 
which bear peduncles as well as tubers, but when flowering 
branches are produced the median vegetative branch remains 
undeveloped. 
In a quite small tuber, about two mm. in diameter, the 
tissues are becoming differentiated. Within the epidermis 
is a cortex of parenchymatous cells containing chlorophyll. 
Many of these cells are enlarged and contain raphides. 
Beneath the cortex is a meristematic zone, which is most 
marked on the posterior side of the tuber. The central part 
consists of parenchymatous cells which already contain a con- 
siderable quantity of starch. Some enlarged cells are filled 
with raphides. 
The two smaller buds are still, and remain for an indefinite 
time, quite rudimentary. They are in some cases entirely, 
in other cases partially, enclosed by the scale-like covering 
(Fig. 2). 
A tuber about the size of a pea is covered with a single 
layer of epidermal cells beneath which is a thick layer, about 
twelve or fifteen cells deep, of cortical parenchyma which is 
brown in colour and beginning to lose its cell-contents. Then 
follows a zone of meristematic cells, separating the cortex 
from the central part which is made up of parenchymatous 
1 See p. 497. 
