LIFE-HISTORY OF HEMILEIA VASTATRIX. 307 
tubes had commenced to block up the stomata within 48 hours. 
For all examples, also, the following holds good: the “ disease- 
spots ” appeared on the leaf on which the sowing was made, and 
within the area of sowing, and nowhere else on the plant. More- 
over, no more *' disease " appeared on the same plant, even after 
keeping it for several (six to eight) months, unless a fresh sowing 
was made and kept moist for 24 to 48 hours as before. 
These facts prove (1) that the “ disease-spots ” and “ rust ” re- 
sult from the tubes and mycelium traced by the microscope from 
the spore; (2) that the fungus corresponds in area with the 
disease-spot; (3) that even if spores be present on the leaf, no 
" disease ” results unless the conditions (moisture, &c.) for ger- 
mination be also present. Finally, taken in conjunction with the 
results of microscopic analysis, they prove that the disease-spot 
is due to the action of an organism derived from without, which 
passes through definite changes and has a limited term of life. 
It is therefore true that a certain approach to constancy is ex- 
hibited by the successive phases above described; and it is abun- 
dantly proved that the yellow “ disease-spot ” is the outward 
sign of internal injuries caused by the mycelium, and coextensive 
with it. 
As soon as the yellow “ disease-spot ” has become obvious, the 
reproduction of the fungus takes place, as described elsewhere * ; 
and the first appearance of orange-coloured “rust” or spores 
Occurs on the outside. The outcome of all the observations shows 
that this takes place, on the average, about the third day after the 
first external signs of the yellow spot are evident. 
Now it is plain that, as the above facts became established, one 
could predict, more or less accurately, when and where a disease- 
Spot would appear on a given leaf on which spores had been sown 
and kept moist. 
To illustrate this important point more fully, a few more expe- 
riments, as carried out, may be quoted. 
Spores were sown, as described above, on aleafof Coffea arabica 
on Oct. 15. From the known data, the disease-spot due to the 
action of the produced mycelium should be visible on or about 
Oct. 29-30, assuming rapid germination: the spot actually be- 
came visible on Oct. 29, in the area of the sowing, and nowhere 
else on the plant. Several other experiments gave similar 
results. 
* Quart. Journ. Mieros. Sci., January 1882. 
