Aug. 5/1918 
Anatomy of the Potato Plant 
237 
ONTOGENY 
THE STEM 
A section taken from the tip of a growing potato sprout shows that this 
region displays no trace of the complicated structure of the older portion 
of the stem, but is entirely made up of thin-walled cells which are rich 
in content. Close behind the meristem or true growing region, the 
uniform cell mass becomes differentiated into distinct layers. The cells, 
however, retain the abundance of protoplasm, the thinness of the wall, 
and also the power of division. Farther away from the growing point, 
the distinctive characters of the tissues become more and more apparent, 
the organ gradually attaining the differentiation of the mature plant. 
The first differentiation in the distal end of a developing sprout con¬ 
sists in the setting off of three distinct regions: the dermatogen, the 
procambium, and fundamental tissue, including cortex and pith (Pi. 41, 
A, D). The procambium forms an unbroken hollow cylinder, with small 
projections into the pith. It is made up of small and elongated thin- 
walled cells, with abundant protoplasmic content, thereby differing from 
the surrounding cells of cortex and pith, which are much larger and 
short cylindrical in vertical section. Almost simultaneous with the 
setting off of these tissue regions, the first elements of mature vascular 
tissue appear. They are found most commonly in the small inner pro¬ 
jections of the procambium cylinder (Pi. 42, A, D), and are recognized 
by their slightly larger size and by the secondary thickening of the wall. 
Longitudinal sections show that these cells are longer than those of the 
procambium, and further, that the secondary thickenings consist of 
simple rings located rather distantly from one another. The youngest 
material examined showed six to eight such elements in one cross section 
(fig. 3, A). These cells, the first of the protoxylem, then, are the first 
vascular elements to differentiate from the procambium. It is, however, 
generally held that the phloem is differentiated at an even earlier period 
than the xylem, but if such is the case, the phloem cells are not dis¬ 
tinguishable from the surrounding procambium cells. 
A period during which both growth and differentiation reach their 
greatest intensity now ensues. The changes consist chiefly of pro¬ 
gressive growth and maturation in the procambium cylinder. The 
latter increases in actual size, both by cell division and cell enlargement. 
This increase in size of certain of the elements is most marked in the 
procambium cells at the periphery of the pith, resulting in the setting 
off of small groups of cells which have not enlarged; the latter cells are 
the internal phloem group initials (fig. 3, B; Pi. 41, C; 42, B, D). A 
number of cells near the middle of the procambium cylinder stand out 
clearly because of their more regular size and orderly arrangement in 
the form of a tangential band one cell wide. This band, however, is not 
continuous, but is evident only in those places opposite the procambium 
64611°—18-2 
