Aug. s, 1918 
Anatomy of the Potato Plant 
249 
SECONDARY GROWTH 
As soon as the intercalary cambium has united the large vascular 
groups, sometimes even earlier, secondary growth becomes markedly 
evident. In its beginning the activity of the cambium is noticed only 
in the region of the large bundles of the stem and is extended only grad¬ 
ually to the interfascicular region, as described in the early part of this 
paper. 
A fully mature stem shows a cylinder of vascular tissue which in the 
region of the large corner bundles is often more than 2 mm. thick (Pl. 
43, B). The xylem in this region contains many large, porous vessels 
which are more or less regularly arranged. The spaces between the 
vessels are occupied by tracheids and wood parenchyma. The latter 
tissue, however, is small in extent, the cells being most commonly found 
around the large vessels (PI. 44, A), as is the case in the roots. Medullary 
rays are very numerous, but the individual ray is narrow, being rarely 
more than one or two cells wide. 
The interfascicular xylem differs from that found in the comers, in that 
it contains no large vessels, but is made up chiefly of a uniformly arranged 
mass of tracheids traversed by uniseriate medullary rays (PI. 44, B). The 
first-formed xylem elements of this region have smaller lumina and much 
thicker walls than those later formed. A section through this region 
resembles strikingly a section of the xylem of a woody stem showing 
spring and summer growth reversed (PI. 44, B). 
The cambium gives rise on the outside to a comparatively broad ring 
of phloem which consists mainly of sieve tubes and medullary rays 
(PI. 45, A; 47, A). The amount of this secondary phloem varies with 
the individual and with the place where the section is taken. In the 
region of the node (PI. 43, A) the amount usually exceeds that found in 
the intemode (PI. 43, B); and in a given section the largest amount is 
found on the face of the large comer bundles. The medullary rays of the 
phloem widen as they approach the endodermis, and they often bend 
toward each other in pairs at their tips, inclosing a triangular area of 
tissue which is made up almost entirely of secondary sieve tubes (PI. 33, 
A; 45, B). The primary phloem groups remain functional even after the 
formation of secondary phloem, and continue active up to the time of 
maturity of the plant. Their delicate walls, of course, become slightly 
thickened and occasionally calluses close a sieve plate; the latter, however, 
occurs only rarely and is probably a pathological condition (PI. 46, A, B; 
47, B)- 
The cambium gradually diminishes in extent. By the time the vines 
die, the cambium is in places entirely disposed of—that is, the cells have 
matured as either phloem or xylem cells. 
With an increase in the amount of vascular tissue, the cortex and the 
pith undergo structural changes. In the region of the large bundles the 
cells of the cortex parenchyma have become flattened radially owing to 
