Aug. 5,1918 
Anatomy of the Potato Plant 
245 
another, and the xylem abuts directly on the endodermis. The number 
of these varies; lateral roots are usually, perhaps always, diarch (Pl. 
37, C); others may possess more than these. The maturing of the vas¬ 
cular elements, however, has proceeded in a different direction than was 
observed in the stem. The first protoxylem elements to mature are those 
farthest away from the center; the development then is centripetal. 
The later-formed protoxylem elements of the two groups approach each 
other more and more closely with the increasing age of the region; those 
last formed me^t in the center. Sometimes, however, a few cells in the 
center do not become vascular tissue, but remain parenchymatous, form¬ 
ing a pith. 
The protoxylem forms two small groups of tissue, somewhat oval in 
shape, which are separated from the diarch xylem mass by a paren¬ 
chymatous sheath. Very early a cambium appears; this forms a com¬ 
plete cylinder lying outside the xylem and inside the phloem. Cells are 
produced by this cambium more rapidly in the latter region and the 
cylinder soon becomes symmetrical. This secondary tissue is collateral, 
whereas the primary is radial. 
With the continuance of secondary growth, the amount of vascular 
tissue relative to that of the cortex increases rapidly. Instead of the 
small vascular cylinder and the huge cortex of the very young root (Pl. 
37, C), in the old root there is a large amount of vascular tissue with a 
comparatively narrow cortex (Pl. 37, D). Most of the tissue of the root 
is, then, secondary in origin. 
The secondary wood of the root consists largely of vessels and wood 
parenchyma. The vessels are porous and either very large or very small. 
The large ones are arranged more or less in tadial rows; the spaces 
between them are chiefly filled by the smaller type of vessel. The wood 
parenchyma, although somewhat scattered, is usually found around the 
large vessels—that is, it is vasicentric. Occasionally the lumen of one 
of the large vessels is blocked by bladder-shaped intrusions derived from 
the membranes of the pits between the vessels and the adjoining paren¬ 
chyma cells. These vesicles, which are known as tyloses, are very com¬ 
mon in many plants, but in the normal potato plant they are not often 
observed except in the root. The medullary rays of the mature root are 
few and uniseriate (Pl. 37, D). Typical tracheids are absent, but a 
few thin-walled fibers are found scattered between the vessels. 
Just as most of the xylem is secondary in origin, so is the phloem made 
up almost entirely of secondary elements. The primary phloem is not 
recognizable in the mature organ; its cells have become nonfunctional 
and later are crushed. Most of the phloem cells formed by the cambium 
are sieve tubes (Pl. 44, C), with their respective companion cells. The 
sieve tubes are larger than those found in the stem; in general propor¬ 
tion and arrangement, however, they do not deviate from the latter. The 
cells of the medullary rays of the phloem are slightly broader than those 
