Jan. 37 # 1933 
Production of Conidia in Philippine Sclerosporas 245 
That this is not due to any character inherent in the fungus is shown by 
the fact that both species of conidial Philippine Sclerosporas behave in 
this way when growing on 5. spontaneum and yet develop conidiophores 
on both sides of the leaves when growing on maize. On the contrary, 
the restriction to the lower surface of the leaves of sugar cane and the 
related wild Saccharum is attributable to the fact that in this genus by 
far the greater proportion of stomata are on the under surface. Dickhoff 
(6) and others have found this condition in sugar cane, and the writer 
has corroborated it in the wild Saccharum. It is of interest to note also 
that Miyake ( 1 5) reports that the Formosan downy mildew forms conidio¬ 
phores only on the under surface of the leaf in sugar cane but on both 
surfaces in maize and teosinte. 
When the production of conidiophores and conidia by the Sclerospora 
mycelium in a leaf first begins, it is not usually abundant. It starts on 
the first night with the protrusion of a few scattered conidiophores from 
the surface of a badly affected area usually along or adjacent to the midrib. 
During succeeding nights more and more conidiophores appear until 
finally vast numbers are produced nightly. As a rule, conidiophores 
appear first on the lowest leaves and leaf sheaths, which are markedly 
affected, and later develop progressively upward on the younger leaves 
of the plant. 
After discovering that conidiophores were produced by the downy 
mildew only at night, the writer naturally endeavored to determine what 
factors operative during the night served to induce the mycelium of the 
fungus to send out conidiophores through the stomata of the host leaves. 
The conditions of the environment which prevail at night are darkness, 
lowered temperature, and the deposition of a layer of moisture on the 
surface of the host plant; the plant's stomata are nearly or quite closed, 
and metabolism is filtered after the cessation of the photosynthesis which 
went on during the day. Hence, changes follow in the chemical content 
of the host cells and even of the gases in the substomatal chambers. 
An attempt was made to determine by experiment which of these 
factors, or combination of interacting factors, was involved. It was 
found that darkness was not operative alone, nor in conjunction with the 
closed stomata and the metabolic changes following cessation of photo¬ 
synthesis. No conidia were produced during the day time on infected 
plants kept in dark chambers, either at normal temperatures or cooled by 
ice; nor at night on plants which were cooled naturally and which had been 
undergoing normal photosynthesis in the sun during the day. Hence, it 
seems logical to conclude that neither darkness alone, nor darkness and 
lowered temperature combined, nor darkness and lowered temperature 
following normal exposure to the sun were the essential factors involved. 
Because in the first two instances the stomata were closed in plants in the 
dark chambers, and because in the third case the stomata were closed and 
the condition of the cells was the normal one after photosynthesis had 
ceased, it seems doubtful if these factors also are of any particular influ¬ 
ence in conidiophore production. 
The remaining factor, however, the presence of a layer of moisture 
deposited on the surface of the infected plant, does seem to be directly 
connected with conidiophore production. The writer found by a number 
of experiments, some of which have already been described earlier in this 
paper, that apparently it is the dew or other moisture on the surface of 
