LIFE-HISTORY OF HEMILEIA VASTATRIX. 333 
of the above-named effects could be produced; and, as a matter 
of experience, coffee-trees in richer soil, or better treated than 
others, may support more crop. This has given rise to another 
popular error with regard to “ leaf-disease,” that manuring Ze 
diminish or cure it. 
I have elsewhere * discussed this at some length, and shown 
that the reason that manured or favourably situated coffee sup- 
ports more crop is due to no direct action on the fungus at all 
(it is clear that circumstances which favour the host must rather 
benefit the parasite than otherwise), but that such trees support 
more crop because they can spare more food for that crop after 
paying the tax demanded by the fungus. Nay, it is certain that 
many trees support more crop and more fungus-mycelium at the 
same time than do others with which they are compared. 
Closely connected with the above fallacy is another—that cer- 
tain trees are “ predisposed ” to the disease or to infection. A 
refutation of this view has been matter of experiment: I have 
purposely waived all arguments from analogy, and tried to infect 
every description of plant—young and old, West-Indian and 
East-Indian, Coffea arabica and C. liberica ; and there is no ground 
for supposing one more easily infected than another. This has 
nothing to do with the rapidity with which the mycelium pro- 
duces the yellow spot. We have seen that a tender young leaf 
may succumb more rapidly (as a whole) than an older and more 
leathery one; and it must be evident that moisture, temperature, 
and other simple events may affect this. 
We here face another difficulty, viz., Why do some trees suffer 
more than their neighbours? Let us shortly examine how com- 
plieated are the causes which aid in determining the amount of 
fungus-mycelium &c. on a eoffee-tree. We may do this by 
assuming that two trees, side by side, are equally diseased. This 
involves one or the other of two comprehensive assumptions :— 
Either (a) that the two trees were at the outset equal in all 
respects, that their root-masses, areas of leaf-surface, &c. were 
alike in extent and exposure, and that the relations of these to 
the soil, moisture, air, and light, &c. were equal in all respects— 
that equal quantities of food-materials were present in each, and 
that the expenditure and income connected with these remained 
equal in each case. It must be further assumed that each tree 
* Third Report, 1881. 
LINN. JOURN.— BOTANY, VOL. XIX. 2r 
