New York Agricultural Experiment Station. 345 
against harmful substances in solution in the soil, and also keeps them 
from drying out, whether the tubers are in or out of the ground. 
This surface membrane, or skin, is made up of several layers of regular 
flat cells, fitting together so closely as to leave no spaces between the 
corners, and without a deposit of starch as in the other tissues of the 
tuber. Chemical tests show that the substance of the cell-walls is 
different from that of the walls of the other tissues; in short the skin 
of the potato is a thin layer of delicate* cork. Figure 1 shows a magni- 
fied section of the skin and some of the tissues just beneath it. 
A microscopic examination of scab spots of different ages discloses 
the fact that they are produced 
by the outer cells of the corky 
layer of the skin dying, and sep- 
arating to form the loose brown 
dead matter which contributes 
so much to the rough appearance 
of the diseased tubers. Long 
soaking of scabbed potatoes 
permits this dead part to be 
completely washed away, leav- 
ing the corky skin beneath. As 
the outer cells of the skin die 
at the scabbed spots, new ones 
are formed within at the ex- 
pense of the other tissues 
immediately beneath. Thus the 
scab eats its way deeper and 
deeper into the tuber. 
It is scarcely possible to saj 
FlGUEE 1. 
Trail 
showir 
lapsed 
th 
If-grown tuber, 
figure the col- 
3 surface of the 
rectai 
potat< 
cells 
the tn 
DroKeri ceus iron 
3Ugh the middle the broad baud of 
r cork cells forming the skin of the 
I at the lower part the starch-bearing 
iiose which form the larger part of 
Magnified 125 diameters. Original. 
at present wherein the true cause 
resides that brings about this abnormal production of cork cells and 
the consequent loss of surface cells to form the scab. Over twenty- 
three years ago Dr. Nobbe,* in raising some seedling potatoes by 
water culture, observed that the tubers growing in the confined air of 
vessel above the water were normally smooth, while those under the' 
the water were covered with glistening white warts produced by the 
excessive growth of lenticels, and he suggested that this "might 
possibly contribute to the explanation of the formation of scab in the 
field." 
Lentieels are normal structures which serve for the ready exchange 
of gases (oxygen, carbonic dioxide, etc.), between the inside and out- 
Land wirthsch. Versuchs-Stationen, VI, 1864, p.^60. 
44 
