6 THE AMERICAN MIDLAND NATURALIST 
to the end of the cotyledon where they gradually disappear. 
This bundle of the hypocotyl is seen about to divide in figure 
12,f. The epicotyl (f) has at first no definite vascular bundles, 
but later a number of closed collateral bundles are formed most 
of which are in connection with the secondary roots that are 
sent out directly from the epicotyl. There is, however, some 
further development of conducting tissue leading from the hypo- 
cotyl into the epicotyl as lateral branches of the original bundle 
(Fig. 12,f). In the cross section of the hypocotyl (Fig. 12) the main 
bundle (c) with its branches (d) are seen in the form of a semi- 
circle but as they approach the epicotyl they seem to take the 
form of an almost complete circle. 
THE PRIMARY ROOT (FIG. II). 
The wood bundle of the primary root is of the radial type 
with alternating phloem (leptome) (f) and xylem (hadrome) (g) 
rays and is most often triarch but sometimes diarch or tetrarch 
as in the illustration. The xylem growth is exarch, and the ducts 
are of the ring and spiral types. The bundle is surrounded by a 
pericycle (e) of small flattened cells and an endodermis (d) which 
although not composed of thick walled cells is nevertheless very 
distinctive. ‘The cortex (c) consists of about seven to ten irregular 
layers of short cylindrical parenchymatous cells which contain 
some raphides (h). The outer cortical layer is composed of larger 
cells (b), and these have a marked palisaded appearance. ‘The 
epiblema cells (a) are small and rather thin walled even on the 
outer side. 
THE HYPOCOTYL (FIG. 12). 
The bundle (f) leading to the cotyledon shows in cross section 
as an irregular double row of xylem ducts with the phloem grouped 
about the ends of the row and on the side next the epidermis. 
The bundle (c) leading into the epicotyl is of the collateral type 
with what seems to be a nearly semicircular cambium strand (d) 
extending out a considerable distance on each side. This strand 
takes on the form of an almost perfect circle as it nears the epicotyl. 
In this semi-zone secondary bundles are later formed leading 
from the hypocotyl into the epicotyl. The space between these 
bundles is occupied by large cylindrical parenchyma cells (e), 
and the surrounding cortex (b) is of similar structure. The whole 
