io Sargant . — Theory of the Origin of Monocotyledons 
size before the position of the plumule is defined externally 
as a swelling near the base of the cotyledon (A 5 in PL I, 
Fig. i). The cylindrical cotyledon is green, and acts as the 
first assimilating organ. Its apex carries the seed out of 
the ground, and remains within it until the supply of food 
in the endosperm is exhausted. The primary root is still 
unbranched in A 5 , and no other has appeared. The upper 
limit of the root is defined externally by a sudden increase in 
diameter. Later on, when the base of the cotyledon has 
expanded into a sheath which envelops the growing plumule 
(B x in Fig. i, PI. I), this abrupt increase in girth is not 
so clear, but there is a distinct constriction separating the 
base of the bulb from the top of the root. 
The two seedlings figured (A 5 and Bj) have been dissected. 
In each there are two massive bundles running the whole 
length of the cotyledon. Their phloem groups are very well 
developed, as is generally the case when the cotyledon serves 
as a sucking organ for some time after germination. The 
cotyledonary sheath of A 5 possesses in addition five lateral 
bundles, which are formed near the top of the sheath by the 
branching of the two main bundles. Near the base of the 
sheath they join the plumular traces above the level at which 
these are inserted on the main cotyledonary traces. The far 
larger sheath of the older seedling B x possesses no less than 
twelve of these minor bundles : all given off by branching 
from the main bundles, and all ultimately merged in the 
plumular stele, or ending blindly before the transition from 
root to stem begins. 
The presence of lateral bundles in the cotyledonary sheath, 
though not confined to the Scilleae, is very characteristic 
of the tribe. They do not always arise by the branching 
of others, but are sometimes formed independently, and dis- 
appear without joining other traces. When this is the case 
they probably serve merely to stiffen the sheath. 
The seedling A 3 , from which the sections drawn in Figs. 2-4 
were cut, is younger than A 5 , and there are only two slender 
lateral strands in the cotyledonary sheath in addition to the 
