founded on the Structure of their Seedlings . 9 
This region is commonly very short in Monocotyledons. 
In bulbous or tuberous species it rarely exceeds *5 mm., and 
in many cases can hardly be said to exist except by a sort of 
legal fiction. Even in herbaceous and arborescent species the 
hypocotyl as defined above rarely attains a length of 3 mm. 
The symmetry of the root-stele is sometimes determined 
by the cotyledonary traces only : sometimes by cotyledonary 
and plumular traces together : and in a few exceptional cases 
it appears to depend on the plumular traces only ( Aloineae , 
Bulbine , Tamus ). The systematic indications are most con- 
stant in the first case : that is, when the cotyledonary traces 
only are continued into the primary root, and its symmetry is 
unaffected by the insertion of the plumular traces. I have 
learnt to look on this as the primitive arrangement. It is 
found in species which are somewhat tardy in developing 
the plumular leaves. The root-stele in such species is fully 
differentiated, while the plumular traces are still embryonic. 
The early or late development of the plumule is of course 
a question chiefly of the habit of the species, and this is deter- 
mined by the external conditions to which it is exposed. 
The seedlings of climbers and arborescent species, for example, 
commonly develop the plumule early, and the primary root of 
such species is comparatively long-lived. It must therefore 
be polyarch in order to attain sufficient girth, and as a rule in 
plants of this habit many plumular traces are continued down- 
wards into the root-stele, side by side with the cotyledonary 
traces. 
In such cases I have found the features of the transition to 
vary considerably, even among individuals of the same species. 
The primitive characters are swamped among those which 
are more or less dependent on external conditions, and these 
are necessarily variable. 
1. Tribe Scilleae. 
Seventeen species, representing ten genera, have been 
examined from this tribe. 
Albuca Nelsoni. The seedling is slender, and attains some 
