1012 
Journal of Agricultural Research 
Vol. V, No. 32 
young leaves are not susceptible to infection, and that young mature 
leaves are oply slightly so, while mature leaves show the greatest sus¬ 
ceptibility. It has also been found that old leaves past their maximum 
development have for the most part lost their susceptibility, for they 
seldom show an increase in the number of leaf spots present. Thus the 
greatest susceptibility to infection becomes concomitant with the greatest 
stomatal movement, as they both occur on the leaves of the same degree 
of maturity. 
With the varied host and environmental factors favorable, as might 
be indicated by the stomata on mature leaves remaining open for a 
period of from five to eight day hours and with vigorous viable conidia 
of the fungus present, infection would be practically assured. 
FACTORS INFLUENCING STOMATAL MOVEMENT 
LEAF maturity 
A study of the stomata on leaves of different maturities has indicated 
certain specific characters that might be used to determine the compara¬ 
tive development of different leaves. The number of stomata per 
square millimeter of leaf surface and the stomatal pore lengths have been 
found to give a good indication of leaf maturity as determined by the 
* size, condition, and position of a leaf on a normal plant. By using the 
stomatal numbers and pore lengths as a means of measurement, the 
degree of maturity of any leaf on a heavily infected or otherwise abnor¬ 
mal plant may be determined, regardless of the degree of development 
indicated by its size and position. This becomes of especial value in 
the study of the leaves on a plant heavily infected by Cercospora beticola , 
for the young leaves may be mature, though their size and position 
would indicate immaturity. 
Lloyd’s 1 (7) method 2 for observing stomata in situ has been used 
throughout the study in determining the stomatal numbers and pore open¬ 
ings. Microscopic examinations were made near the middle of the blade 
of leaves which were taken directly from the plants to the stage of the 
microscope. Readings were continued not longer than two minutes, the 
stomata remaining unchanged during that time. 
On a normally developed sugar-beet plant, pronounced differences 
are usually found to exist between the central, or heart, leaves, those 
occupying a midway position on the plant (here designated as mature 
leaves) and those occurring at the extreme outer portions of the leaf 
growth (old leaves). On leaves growing in such relative positions read- 
1 Reference is made by number to 41 Literature cited,” p. 1038. 
* Lloyd’s stomatoscope (shown in PI. LXXX, fig. 1), which was devised later, was kindly lent by the 
inventor for the studies which were made in Colorado in 1913. Two characters of this instrument, which 
make it exceedingly valuable for leaf study, are the long stage and the modified condenser, which serves 
also as a cooling chamber. The instrument also has a basal screw for tripod attachment. In a letter to 
the authors he has suggested (1) that the objective should be corrected for use without a cover glass, (2) 
that the focus of the condenser should be capable of being placed 5 mm. above the stage level for proper 
use in the case of thick leaves, and (3) that smoked glasses should be provided to shield the eyes. 
