Colorado Experiment vStation. 
14 
Figure 7. — The stoma or 
breathing pore of a potato leaf; a 
structure designed to dissipate 
rather than conserve moisture. 
Drawing by Ada Hayden. 
pore of an alfalfa leaf for contrast with 
that of the potato. A structure designed 
to conserve moisture. Drawing by Ada 
Hayden. 
I.aboratory studies of the potato leaf are of use in studying 
the needs of the plant. Farmers know that in dark, misty weather 
potato leaves grow broad and full at the edges and that the crop 
fairly leaps forward under such conditions. Moisture and tem¬ 
perature conditions in the soil are apt to be right at such times, but 
the microscope shows that misty weather is particularly good for 
the leaf itself, and it has been shown, moTeover, that starch forma¬ 
tion is favored by the less intense light which accompanies such 
weather. To prove that a damp atmosphere is good for potatoes, 
we have only to remember the climates of Scotland and of upper 
The stomata, or breathing pores, of the potato leaf (Figure 7) 
project like little craters and are adapted to the ready escape, on 
every breeze that passes, of moisture from the scantily protected 
inner tissues of the leaf. In marked contrast are the stomata of 
the alfalfa plant (Figure 8) which we know is an age-long native 
of arid regions. Its leaf openings are protected by being sunken 
and by having an upper protecting ring about the tiny well. These 
stomata are adapted to the conservation of moisture. 
The evidence of root and leaf seems to prove that the potato 
plant, if native to mountain regions, must have grown where the 
soil was deep and open, free from soakage by heavy rains, and 
where there was much misty weather. That the leaf diseases of 
the potato—early and late blight—both develop faster in damp 
weather is evidence of the same sort. The best soil temperatures 
for potatoes as herein shown have a very narrow range above and 
below 70 degrees F., and this is very close to the optimum temper¬ 
ature for late blight, which is believed to have lived for ages with 
