264 
Journal of Agricultural Research 
Vol. XXIII, No. 4 
an hour possess an average limit of distribution of 330 kilometers (206.25 
miles). Similarly even in the case of Pinus sylvestris pollen, which has 
as great a diameter as 48 ju, and as rapid a fall as 5.3 cm. a second, he 
computes that as many as four-tenths of the number originally shed 
would be carried 4 kilometers by a wind of 36 kilometers an hour; while 
two-tenths would reach 13 kilometers; one-tenth would reach 20 kilo¬ 
meters; and one-hundredth would reach 36 kilometers. The work of 
Hesselman (12) shows that these calculations represent the conditions 
attained in nature. He found that conifer pollen grains 30 to 60 in 
diameter were deposited in relatively vast numbers in dishes on light 
ships 30 kilometers (18.6 miles) and 55 kilometers (34 miles) from land; 
while pollen of grasses also, in spite of being liberated in far less abund¬ 
ance, nearer the earth, and unequipped with appendages for buoyancy, 
was caught in considerable quantities. 
It seems justifiable to conclude from the foregoing evidence that the 
conidia of Sclerospora also are easily and effectively distributed by 
air currents. The writer's observations also bear this out. Small, 
infected maize plants, known to be producing abundantly, were covered 
at dusk with large cans or bell jars the edges of which were pressed 
into the earth and sealed with clay. The nocturnal fall of innumerable 
conidia then took place undisturbed except by such slight convection 
currents as were operative within the can. As a result Petri dishes 
placed on the ground under the plant, or, even the ground itself, showed 
a heavy deposit of spores which in some cases was sufficient to whiten 
the earth conspicuously. When, however, the plants were left exposed 
to the wind, no such abundant spore deposit was found under them 
even after apparently calm nights; and after nights with gusty winds 
almost all the conidia were carried away, few, if any, being caught in 
the exposed Petri dishes under the plant. Further observations and 
experiments concerning the distribution of conidia by winds of different 
velocities will be given in the following sections. 
The dispersal of the conidia by wind is of three main types; first, 
by very slight local air currents; second, by more general breezes of 
greater strength; and third, by violent and extensive typhoons. 
BY SUGHT AIR CURRENTS 
The slight air currents, although very gentle and necessarily restricted 
in extent, are very important, because of their invariable occurrence, 
in distributing the conidia locally. Even during nights when the air 
is apparently absolutely still, careful scrutiny will show that here and 
there a corn leaf is moving slightly in air currents otherwise imper¬ 
ceptible. These nightly driftings of the air are necessarily confined to 
restricted areas and are influenced in direction and extent by very local 
conditions. At the College of Agriculture of the Philippines where, for 
the most part, the dispersal of conidia was studied, the influence of 
Mount Makiling was the most noticeable factor. By this heavily for¬ 
ested mountain, which rises to a height of over 3,300 feet, the air is 
cooled at night and flows down the slopes to the plain. The presence 
and direction of this air flow were determined under varying night 
conditions by repeatedly observing the course and velocity of buoyant 
seeds, such as those of Ceiba or Saccharum, which were thrown into 
the air and followed by the beam of an electric torch. Field observa- 
