964 
Journal of Agricultural Research 
Vol. XXIII, No. 12 
The color of the wart below ground is that of the stolon and young 
tuber, commonly an ivory white. In varieties which produce highly 
colored tubers, like the Australian Blue, the color of the wart will also be 
that of the tuber. When developing above ground, the parts exposed to 
the sun develop chlorophyll and aid in assimilation. Toward the end of 
the growing season, or earlier, the warts show signs of decay. The color 
changes from white to a dark brown and finally to a dirty black. At the 
time the potatoes are ordinarily dug, most large warts have disintegrated, 
leaving only unsightly vestiges on the infected tubers. Small warts, 
according to Johnson (6), 3 may enter a dormant state and form the source 
for new infections the following season. 
ANATOMY 
A cross section of a young wart or of the peripheral part of an older one 
shows a centrally located vascular strand (PI. 2, D) and a broad band of 
cortical tissue whose peripheral cells harbor the resting spores of the 
parasite (PI. 4, B). The epidermis, where still intact, is composed of 
delicate cells isodiametric or with the longer axis in the radial direction. 
A periderm is not developed, although occasionally a shorter or longer 
band of cork cells is noticeable immediately below the zone of spore- 
containing cells, or may surround in concentric layers some deep seated 
resting spores. The walls of the cells containing the resting spores, as 
well as the neighboring cells, become lignified and suberized, and since the 
heavy walls of the resting spores undergo like changes, a lignin stain 
applied to a section of the wart makes the fungus infected area discernible 
to the naked eye and permits of ready differentiation with the aid of the 
microscope. 
The cortical tissue (PI. 5), which comprises by far the largest part of the 
wart, is composed of simple, large parenchyma cells. The cells are 
crowded with starch grains. Sugar is always present in varying amounts, 
and also tannins, primarily in newly infected regions, where, under the 
influence of the parasite, cellular activity is at its height. Other cell 
inclusions are more conspicuous by their absence. Calcium oxalate and 
protein crystals which are usually found in the peripheral region of the 
normal tuber are usually wanting. Protein crystals have been observed 
only in the outer cortical cells of the host—never in typical wart tissue. 
Modifications of the normal type of storage cells of the wart are fre¬ 
quently observed and consist in their transformation into sclereids. They 
have been found abundantly in the aerial parts of the potato plant 
affected with blackleg (2) and normally in small numbers in the cortex of 
the underground stem and in the parenchyma tissue of dormant eyes. 
The vascular tissue is centrally located, and only in transition regions 
is there found a siphonostele inclosing a large pith. The relative position 
of xylem and phloem is, at best, only approximate. Especially where 
the vascular elements are greatly reduced, the phloem and xylem appear 
independent of each other and with no definite orientation with respect 
to the axis of the organ. In small branches of the wart, however, phloem 
groups may be observed in close proximity to the xylem and may even 
surround it so completely as to give the effect of a typical amphicribral 
bundle. While the arrangement of the vascular tissue is certainly a de¬ 
parture from the normal type, it must be remembered that similar, if not 
identical, conditions exist in normal organs, such as the tuber (1). 
* Reference is made by number (italic) to “ Literature cited," p. 967. 
