48 Thompson—The Structure and Development of 
They bend outward along a radius of the bundle, and ina 
definite position, at about one half of the distance from the 
periphery to the mid-line of the bundle. After the passage of 
these strands, there is no further trace of internal phloem in 
the petiole or leaf. 
HIsTOLOGY OF THE Root. 
The structure of a very young root, in transverse section, is 
illustrated in Plate IX, Fig. 3. The loose-celled starch-bear- 
ing cortex, about seven to eight cells deep, is separated by a 
thin-walled endodermis from the axial vascular cylinder. The 
bundle is typically diarch. The two groups of the protoxylem 
consist each of about six spiral trachez, and between them 
at the sides of the bundle lie two small patches of phloem, 
separated from the protoxylem by the procambium, a layer 
of large prominently nucleated cells. Outside the xylem and 
phloem elements, and just within the endodermis, is the 
pericambial zone. Later, by secondary growth, the xylem is 
united into a central cylinder, surrounded externally by a ring 
of phloem, but internal phloem is entirely absent in the root. 
On older roots irregular warts or swellings are frequently 
found, which, when sectioned, reveal a vigorous fungoid growth. 
The fungoid hyphe ramify through the cells of the inner and 
especially the middle cortex, and in some places large cavities 
occur, resulting from the breaking down of the cortex cells. 
These are filled with the coiled hyphz and the fructifications 
of the fungus. Starch is usually absent in the cells inhabited 
by the fungus. In the root of a seedling about six weeks old, 
the fungus was already well established in many cortex cells. 
HISTOLOGY OF THE SEEDLING. 
The diarch condition of the root is continued in the hypocotyl, 
and it may at once be stated that the median plane of the two 
protoxylem masses corresponds to the median plane of the 
cotyledons. The spiral trachez of each end have at first a 
Y-shaped arrangement, the arms of the Y pointing toward 
