246 
Journal of Agricultural Research 
Vol. XIV, No. 6 
found in the xylem; their number, of course, is very small. Phloem 
fibers, as might be expected, are not found in the root. 
The cells of the cortex and endodermis are of the type described in 
detail for the stem. The outer cells of this tissue, since they ate not 
protected by a specialized dermal layer, become somewhat tom and their 
walls suberized. 
THIS HYPOCOTYL, 
Since the change from the exarch condition of the root to the endarch 
condition of the stem takes place in the region of the hypocotyl, seed¬ 
lings instead of sprouts grown from tubers had to furnish the material 
for investigation. The primary vascular tissue of the root develops cen- 
tripetally, the latest protoxylem elements to mature being found near 
the center (fig. 4, a). 
Fig. 4. —Solanum tuberosum: Diagrammatic drawings of a series of sections through hypocotyl 
showing the position of the primary xylem and phloem groups, the changes from exarch to endarch, and 
the behavior of the phloem. 
In the stem the condition is reversed, for here the maturing of the 
elements takes place centrifugally, and consequently the smallest pro¬ 
toxylem elements are found near the center. In the change from the 
exarch to the endarch condition it is noticed first, that the two protoxy¬ 
lem groups of a diarch root begin to swing outward, one group following 
a left, the other a right curve (fig. 4, b). In the region just below the 
cotyledons the bending has progressed so far that the protoxylem groups 
instead of forming a radial row, come to lie in a tangential plane. In 
the region above the cotyledons the change from the exarch to the 
endarch condition is complete. 
