314 MR. H. MARSHALL WARD ON THE 
used in all cases, a method which almost ensures immediate and 
successful germination, as had been largely experienced with 
other sowings. 
Nor do I think that slight differences in the soil, affeeting the 
supply of food and water to the plant, unduly influenced the two 
sets of experiments. Nevertheless, while insisting on the fact that 
the great difference between the leaves in which the mycelium 
produced the spots in 10 days or so and those in which it re- 
quired 15 days on the average, was that in the former the cell- 
walls were thin and soft and less numerous, yet it must be 
allowed so far that slight differences in the circumstances (light, 
air, and possibly temperature and moisture) existed, while un- 
known differences may have been presented in the quality and 
quantity of food and water absorbed by the roots, in the vigour 
of the plants generally, and (though experiments do not support 
this idea) in the rapidity of actual infection—all of which might 
more or less, though very slightly in the individual cases, affect 
the physiological activity of the mycelium, and therefore the 
accuracy of the conclusions. 
Passing to the consideration of the question, How long may a 
"* disease-spot"' continue to produce spores?, I may refer for 
details to the tables and to results already published. These were 
obtained by (1) marking certain leaves in the open, and noting 
when the development of spores ceased : and (2) making experi- 
mental sowings as above detailed, and watching the progress of 
the spots. By these means it was shown that the spores may be 
continuously produced for from 7 to 11 weeks; and there can be 
no doubt that the extreme limits are not here stated. 
One interesting and important observation may be recorded. 
On a “ disease-spot," produced on the leaf of a protected plant 
ina Wardian case, the successively developed spores hung in 
clusters in the perfectly still, moist air. From certain known data 
as to the number and length of these clusters, and the average 
size of the single spore, I was able to estimate the quantity of 
spores present; these were probably more than 150,000. Now, 
since 127 disease-spots have been counted on one pair of leaves, 
some idea of the enormous quantity of spores produced may be 
readily obtained. 
The slightest shake causes these spores to fall; and it may be 
proved directly that others are formed very rapidly by the same 
spore-heads. By gently brushing off the spores with a camel- 
hair pencil no injury is done, and in a few hours others are found 
