490 
Journal of Agricultural Research 
Vol. XI, No. 10 
or stomata by certain parasites. In this connection it is interesting to 
note the results of Pool and McKay (. 28 ), who have studied in some 
detail the relation of stomatal movement to infection with Cercospora 
beticola Sacc. These investigators showed that the germ tubes entered 
only when the stomata were open, the movement of which was closely 
associated with a relatively high humidity, warm temperature, and 
light. Their results further showed that mature leaves were more readily 
infected and possessed a pore opening almost twice as large as young 
immature ones. The writer obtained similar results with the Tima bean 
fungus. It was found, as already pointed out, that young immature 
pods were not infected without wounding while mature pods could 
easily be. This difference may be due at least in part to the difference 
in the size of the stomatal aperture but more likely to the fact that im¬ 
mature stomata do not open, thus preventing the entrance of the hyphae. 
Under humid conditions the spores germinate and produce a germ tube 
Fig. 10 .—Diaporthe phaseolorum: Some of the germ tubes of the pycnospores penetrating the stomata two 
days after the pods were sprayed, and other germinated spores lying alongside the stomata and growing 
toward or away from the aperture. X 260. 
20 or 30 times the length of the spore in 24 to 48 hours. This is rather 
a feeble growth but the spores do not germinate readily in water, and 
then only a small percentage at that. On pods that had been sprayed 
with a spore suspension in water the germ tubes were found penetrating 
the stomata as shown by figure 10. 
It is evidently a matter of chance when a germ tube finds its way into a 
stoma, since, if there was any attractive force, the germ tubes might be 
expected to head that way, which is apparently not the case. The germ 
tubes, as often observed by the writer and illustrated by figure 10, grow 
just as frequently alongside or away from the stomata as toward them, 
showing that there is no appreciable chemotactic influence. Entrance of 
the stoma would be easy so far as relative size of germ tube and stom¬ 
atal aperature are concerned. In fact, it would not be impossible and 
it may actually happen that the spores fall directly through the pore 
into the intercellular space beneath. Measurements of the pores showed 
them to vary in length from 22 to 28 ji and in width from 5 to 7 fx when 
open, the germ tube of the spore in width from 2 to 3.5 /*, and the spore 
itself from 6 to 8.6 p by 2.4 to 4.1 ju. Considering the relative size of 
