238 
Journal of Agricultural Research 
Vo!. XIV, No. 6 
projections (fig. 3, D). More advanced stages in the differentiation of 
the procambium show these calls to be cambium initials. 
While these changes are taking place the number of xylem elements 
is increased. The later-formed cells appear progressively farther and 
farther away from the pith, thereby indicating that the development 
of the protoxylem is proceeding centrifugally (fig. 3, C). Aside from a 
difference in position, the later-formed protoxylem is characterized by 
larger size and by a different type of secondary thickening in the form 
of loose spirals and close rings. 
Fig. 3 .—Solatium tuberosum: Diagram illustrating the mode or origin, orientation, and development of the 
vascular tissue of the stem. A, B, transverse sections through the distal end of a potato sprout; C, radial 
section through such a sprout showing vascular connection with mother tuber; D, E, transverse sections 
farther away from the growing point. 
Growth and differentiation become more and more evident; these 
consist of further changes in the regions already set off, and of speciali¬ 
zation in the still undifferentiated part of the procambium. In the 
external region distinct differentiation also now occurs. The changes, 
though similar, are less extensive than those noted at an earlier period 
in the procambial region adjacent to the pith. Thus, a small number 
of phloem group initials is set off; the number of cells in each of these 
is also small (fig. 3, D; PI. 42, B). 
Three types of primary vascular tissue are thus early differentiated 
from the procambium: protoxylem, cambium, and phloem. The phloem 
groups appear in the innermost and outerhiost regions of the procam¬ 
bium; the bundles so formed are therefore bicollateral (Pi. 41, B). 
