LIFE-HISTORY OF HEMILEIA VASTATRIX. 805 
cellular mycelium has attained a eertain development, its branches 
have blocked up many of the lacune, and having sent haustoria into 
the cells bounding them, the tissues of the affected area become 
paler in hue, the contents of the cells are disorganized, and a 
yellowish discoloration becomes visible on the exterior of the 
leaf. This discoloration (the incipient * disease-spot"") is readily 
detected after a little experience, though during the first or 
second day it may be but slightly perceptible. 
This stage, the first evidence of the presence of disease to the 
unaided eye, may be looked upon as a distinct one in the cycle 
of development of the fungus ; and I will give one or two examples 
to show how constant and well-defined is the period at which a 
vigorous mycelium may be known to be present, as evinced by the 
yellowish incipient “ disease-spot " seen from without. 
&. Spores were sown in the manner described on the living 
coffee-leaf on July 24th ; the pale spot appeared on August 7th, 
i. e. 14 days after. 
B. Spores were sown on January 30th; the spot appeared on 
February 13th, also 14 days after. 
Y. Spores were sown as above on October 20th; spot first 
visible November 3rd, also 14 days after. 
Henee we see that a certain constancy appears observable in 
the rate at which the mycelium proceeds in its work of destruc- 
tion. These examples, chosen simply to illustrate this fact, might 
be multiplied. However, it is not true that the “ disease-spot "" 
always appears on the 14th day ; as will be shown, it may happen 
that one or two days more or less are required, according to cir- 
cumstances to be examined ; an average of nearly 14 days is very 
common, however, as shown by the following Table (I.), which 
summarizes shortly a number of experiments, selected from a 
large series, made to determine (1) the time occupied in the 
germination of a spore, (2) how soon afterwards the ''disease- 
spot ” appears on the leaf, and (3) how long the mycelium may 
continue to produce spores. In each case a vigorous young 
plant was selected which had been grown for some months 
in a sheltered situation, and was clean and healthy. On a re- 
cently-formed leaf a sowing of spores was made, kept moist for 
24 to 48 hours, and then (the damp cell having been removed) 
placed in a carefully -cleaned Wardian case, well lighted, sheltered, 
and kept at an average temperature of about 78? Fahr. In all 
cases the spores were found to germinate in 24 hours and the 
