founded on the Structure of their Seedlings . 37 
on it becomes a question of importance, particularly as the 
first foliage leaves are in general the earliest assimilating 
organs — the cotyledon commonly remaining within the seed. 
An early development of the plumule is usual, and when this 
occurs some plumular traces are commonly found in the 
hypocotyl. (Cf. Yucca gloriosa and Y. aloifolia with Y. 
arborescensi) This arrangement is useful in two ways : it 
secures uninterrupted communication between the plumule 
and the primary root, and it increases the girth of the latter. 
Among Monocotyledons a root which is to attain to any 
considerable diameter must be planned on a generous scale 
from the beginning, for there is no secondary thickening 
to provide fresh vascular tissue as it is needed. 
Similar co-operation between plumule and cotyledon is 
found in the two species of Asparagus which I have cut, 
A. officinalis and A. decumbens. In both the hypocotyl con- 
tains both plumular and cotyledonary traces, which are all 
alike and behave in the same way during the transition. 
The central double bundle found in the cotyledons of Yucca 
gloriosa and F. aloifolia probably represents the two cotyle- 
donary bundles of Cordyline , but it is possible that there may 
be no morphological connexion here. In Dracaena the 
three double bundles are no doubt formed for physiological 
reasons, and the double bundles of Yucca may arise in a 
similar way. 
Among the Aloineae I have examined six . species of Aloe 
and two of Gas ter ia. Their seedlings resemble each other 
very closely both in external form and internal structure. 
The adaptation of these seedlings to life in a dry hot climate 
is very clear, and has profoundly affected their vascular struc- 
ture. The cotyledon is modified to serve as a sucking organ 
in the upper part and as a water-jacket lower down. It con- 
tains two distinct bundles with massive phloem groups. 
The vascular symmetry of the very short transitional region 
is most simply explained by supposing that of the root-stele 
to depend solely on the three or four plumular traces which 
