LIFE-HISTORY OF HEMILEIA VASTATRIX. 329 
July 1st. Each series of spots on the left-hand leaf could now 
be distinguished as follows:—One large and old spot, with a 
black patch in the centre, and few spores—that of June 3rd. A 
number (35) of very active spots which are just commencing to 
become brown in the centre—those of June 29th. A smaller 
number (12) of new active spots, and only just tinged yellow 
above—those of July 15th. 
On July 23rd the leaf was evidently becoming destroyed by the 
numerous (48) virulent spots draining it, and curious green rings 
around the spots of June 29th alone represented the normal 
colour of the leaf; by July 26th these had faded, and the leaf 
was quite yellow and exhausted, and it fell during the night of 
that date. 
As to the right-hand leaf, its history is very similar. On June 
15th there were two Hemileia-spots on it, a new one having 
appeared in addition to that of June 3rd; on the 29th June 
appeared 57 new ones, which spread rapidly, and covered the 
greater part of the leaf by the 6th July; on July 15th were 13 
still newer spots: the three generations were quite evident on 
July 27th, when the leaf was yellow. Before the last day of July 
this leaf also had fallen. 
It is clear from the foregoing that what the planters term an 
“attack ” of leaf-disease, 7. e. a sudden outburst of the “rust,” 
results from the coming to maturity at or about the same time 
of a series of mycelia which have been formed from the successful 
sowing of a certain number of spores ; and since all were exposed 
to similar conditions, we must look for the origin of the rust to 
the conditions previously present. It is clear, however, that we 
cannot say exactly when a given disease-spot commenced to form ; 
we can only argue from the known data. I do not think that 
any mycelium takes less than one week, or more than three 
weeks to form, as a rule, however ; and hencethe above argument 
may be widely applied. 
Here appears direct proof of the cumulative power of the 
fungus. We cannot well doubt that the earliest spots on the 
leaves arose from odd spores blown by the wind from fallen 
leaves or other coffee-trees, and that these sowed their spores in 
turn, to produce spots in June, while further spores from the 
last-formed spots produced the July outbreak, and so on. 
The above-recorded history of a pair of coffec-leaves may be 
summarized thus :— 
