Feb. 28,1916 
Relation of Stomatal Movement to Infection 
1017 
Mature leaves from Colorado have approximately the same number of 
stomata per unit area as old mature leaves from Wisconsin, although the 
stomatal pore lengths are less in the former than in the latter. This 
would seem to be due in part to the fact that the stomata read in Colo¬ 
rado were not open as widely as those read in Wisconsin, and thus their 
maximum pore length would not be attained when observed. However, 
the stomata which were well open in Colorado often had a pore length 
equal to the average in Wisconsin. The Wisconsin records include the 
readings made only early in the season on one day under favorable 
environmental conditions when the stomata were generally wide open. 
On the other hand, the Colorado records include readings made on various 
days throughout the season and often under unfavorable environmental 
conditions when the stomata were only slightly open, and thus they 
exhibited a short pore length. In such a case the stomatal numbers 
offer a safer criterion of leaf maturity than the stomatal pore lengths. 
The number of stomata per unit area were also read on leaves from a 
normal mother beet plant growing in the field at Madison, Wis., on July 
30, 1914, and the results obtained were entirely comparable to those from 
the first-year beets, in that leaf maturity could be indicated by the same 
stomatal numbers. The increase in number of stomata from the oldest, 
or basal, leaves to those occurring near the tips of the stalks, or the 
younger leaves, is shown in the following tabulation: 
Length of 
leaf. 
Width of 
leaf. 
Average 
number of 
stomata 
per square 
millimeter 
of upper leaf 
surface. 
Number of 
readings. 
Cm. 
Cm. 
20 
17 
107.9 
2 
9 
5 
121. 2 
3 
9 
5 
137 - 8 
3 
6 
3-5 
187. 6 
3 
4-5 
2 
204. 2 
3 
3 
I * 3 
240. 7 
2 
LEAP MATURITY AND STOMATAL MOVEMENT 
Observations made at different times and on many plants have shown 
that the degree of stomatal movement is greatly influenced by leaf 
maturity. In the detailed tests reported, the readings of the stomatal 
.pore widths on leaves of different maturities were made in the field at 
Madison, Wis., on a day when the sunlight was fairly strong and con¬ 
stant, the temperatures comparatively high, and the relative humidities 
above 60 per cent (fig. 1). This combination of factors was favorable 
for stomatal opening, as will be shown later under “Environmental 
factors.” The leaves used in this test were the same as those from 
which the stomatal numbers and pore lengths have been given in Tables 
I, II, and III. 
22535°—16 - 2 
