Mar. 24, 1923 
Anatomical Studies on Potato-Wart 
965 
The phloem groups abut directly on the cortex. There is no endo- 
dermis; and only groups of small-celled tissue, the homolog of a pericycle, 
may delimit the phloem from the storage tissue. The phloem groups are 
small and widely scattered; sometimes a sieve tube with a single cambi- 
form cell may constitute an entire group. Often, however, the groups are 
larger and attain the size of the inner phloem of the tuber. The relative 
number of sieve tubes and parenchyma cells is approximately that found 
in the phloem of the underground organs and is small compared with the 
aerial parts. When stained with chlorozinciodid the side walls of the 
sieve tubes in contact with cortical cells show a network of fine, dark- 
staining bands inclosing areas of lighter color. Since there is an obvious 
need for rapid movement of food from the storage parenchyma to the 
growing region of the wart, the extensive pitting of the sieve tube with 
these storage cells appears as a functional adaptation; the converse rela¬ 
tion, however, may also hold. 
The xylem is composed of narrow, porous elements with secondary 
thickenings in the form of rings and close spirals. The end walls of the 
cells are chiefly oblique, being sometimes, however, strictly transverse. 
Since the course of the vascular tissue is very irregular and since branch¬ 
ing and anastomosing takes place frequently, the xylem cells are com¬ 
monly atypical. This diversity of form finds expression in abnormal wall 
sculpture, excessive slope of the end walls, and a general irregular form 
of the element as a whole. Typical fibers and pitted vessels are wanting. 
Narrow parenchymatous elements of variable length often separate the 
xylem from the cortex. Besides being smaller, these elements differ 
further from typical cortical cells in being devoid of starch. The walls 
of the xylem elements remain cellulose for a long period. This fact 
and the conspicuous absence of fibers and tracheids shows that the tissue 
is adapted primarily for conduction and not at all for support. Such 
qualitative reductions in vascular tissues are common in simple galls and 
appear to be adaptations for changes in function. Bally (3) believes that 
the adaptation in case of the wart is for water storage rather than for con¬ 
duction. It might be questioned, however, whether conduction is second 
in importance to water storage in wart tissue; and may we not, from 
morphological considerations, expect only simple protoxylem elements 
in a tissue as simple, as well protected and as rapidly growing as that of the 
wart? Both xylem and phloem cells advance independently close to the 
peripheral region of the wart, a fact which further tends to emphasize the 
necessity for rapid movement of plastic materials as well as of water. In¬ 
dividual sieve tubes and protoxylem cells may be observed even in small 
warts, which makes it difficult to tell which of the vascular elements are 
differentiated first and whether phloem cells or xylem cells are more im¬ 
portant in the early growth of the organ. Although normally elements 
that are essentially vascular rarely terminate a bundle, in the wart tissue 
individual sieve tubes and also ringed protoxylem cells have been ob¬ 
served to advance to the periphery of the organ unaccompanied by 
parenchyma cells. 
The older basal part of the wart and more especially the transition 
region show a gradual approach to the normal structure. There is a 
steady increase in supporting tissue and improved provision for con¬ 
duction of larger quantities of water. We may then expect to find fibers 
and pitted vessels in increasing numbers, and indeed we do. Withal 
there is a complete though gradual transition from the normal to the 
wart tissue, and although in its simplest form the wart shows both 
