38 Sargant . — Theory of the Origin of Monocotyledons 
enter it. The cotyledonary traces are inserted on them, 
sometimes before the transition to a root-structure begins, 
but more often while it is still in progress. The root is either 
triarch or tetrarch. In Aloe Buchanii the cotyledonary traces 
seem occasionally to exercise some influence on the symmetry 
of the root-stele. Four seedlings were examined from that 
species, and in two of them the root is diarch. 
The method of transition is commonly but not invariably 
Van Tieghem’s type 3. That is, the xylem groups rotate 
in sitiij while the phloem groups branch to right and left 
of them, and the branches from adjacent groups unite with 
each other in pairs. 
B. Monocotyledons not included in the order 
Liliaceae. 
In working out the seedling structure of the Liliaceae 
I have paid most attention to the four great tribes, Aspho- 
deleae, Allieae, Scilleae, and Tulipeae, which may be considered 
as including the typical representatives of the order. The ob- 
servations made on species from outlying groups are scattered, 
and serve for the most part merely to show that the vascular 
structure of their seedlings is not dissimilar from one or other 
of the central types. 
The seedlings of Allium and Zygademis , for example, are 
figured on the same plate and can readily be compared 
(PI. V). The structure of the cotyledon is the same in 
both, except that the cotyledon of Allium contains two 
bundles united by a common protoxylem group (PL V, Figs. 
1 and 3), while that of Zygadenus has a single bundle (Figs. 
7 and 8). 
Now the most salient feature distinguishing the vascular 
symmetry of the cotyledon in most Liliaceous seedlings is 
the absence of a midrib. In the first foliage leaf there are 
commonly three bundles, of which the central one or midrib 
is by far the best developed. There may be five, seven, nine, 
or even more bundles in each foliage leaf, but the number is 
