48 Sargctnt. — Theory of the Origin of Monocotyledons 
endosperm. The lower part of the cotyledon plays many 
parts, and is modified in shape and texture to suit the habit 
of the seedling. 
In Thrinax excelsa the plumular axis is connected with the 
seed by the long and rather slender petiole of the cotyledon. 
This expands at the base into a tough membranous sheath 
which completely surrounds the axis at its insertion. The 
four cotyledonary bundles run downwards through the sheath, 
in the same direction as the plumular traces, but outside them. 
Just above the node these four bundles form the outermost 
of a series of concentric circles in which all the traces of the 
axis are arranged, and they are equidistant from each other. 
At the first node the traces from the cotyledon run inwards — 
from the four points of the compass, as it were. Four plumular 
traces alternate with them in the stele of the hypocotyl, and 
the remainder are inserted on one or other of the circle of 
eight traces. An octarch root-stele follows quite regularly, 
apparently according to Van Tieghem’s type i, by branching 
of the xylem groups. 
In Desmoncus sp ., D. minor , Areca sapid a, and Acantho- 
phoenix crinita , the cotyledon has a short petiole and a thick 
fleshy^sheath which is continuous with the primary root. The 
plumule is inclined to the cotyledon — sometimes almost at 
a right angle— and its traces are commonly continued down- 
wards into one or more cauline roots, which penetrate the 
fleshy tissue of the cotyledonary sheath. 
The apex of the cotyledon in the four species we are 
considering contains from ten to twelve bundles, irregularly 
disposed in a circle and without any trace of a midrib. Near 
the base of the petiole these become reduced to four by fusion 
with each other. In the sheath they approach each other in 
pairs, and when the plumular traces appear in the section the 
cotyledonary traces have united to form two massive bundles 
facing each other. 
Sections which cut the cotyledonary traces transversely 
must of course pass through the plumular traces obliquely, 
and this distinction enables us to follow the course of the 
