January 2, 1882 
THE TROPICAL AGRICULTURIST. 
647 
complaints have reached me to the effect that some 
point* in my reports have been misunderstood by lay 
readers, an I ii may be possible, therefore, to render 
service by re-stating the important conclusions in 
slightly different language. 
The chief points touched upon more or less in the 
critici ma referred to are the following, and I will take 
them in order after mentioning them as below : — 
(1) The general conclusions to which the whole 
research has impelled me, viz : —that "leaf-disease" is 
caused by the ravages of a vegetable parasite derived 
from w ii hout; 
(2) The statement in my 3rd report to the effect 
that " no special predisposition on the part of the 
coffee is required for its infection" ; 
(3) The statement that " manure cannot be properly 
looked upon as a cure for the disease," and 
(4) The conclusions arrived at respecting the use 
With regard to the first point, I need not occupy 
time and space with a lengthy argument to repeat what 
shortly 
sunmiai 
still in 
from a 
from b 
equally 
Hemi 
and, as 
propagi 
plant.. 
rWua. 
as 
position for the sake of the few who 
such expressions as " the fungus arises 
state of the tree," "the disease arose 
ring," it is "in the sap," and other 
Sri'SB is a parasitic fungus with a definite, 
know, very simple life-history, and is 
means of spores as are other similar 
iie£ peculiarity about this particular 
r, is that its mycelium, or vegetative 
somewhat like roots and stems of 
squires the organized materials found 
l spreadmg°hi Th" loose passages be- 
•y tissues of the leaves, and imbibing 
.he c-.lls which compose these tissues, 
rl food substances, instead of going 
the voracious appeiite of t he fungus, 
have goue to benefit the coffee-tree 
supplying the materials necessary for- 
es where required — it might be leaves, 
Bra or fruit &c. 
ie that a direct robbery is perpe- 
ngus, in appropriating food-materials 
of the coffee-leaf had, with great 
crgy, manufactured for the use of the 
it might be flowe 
Ilenco we sei 
trated by the fin 
which the cells 
expenditure of em 
coff c-tree as a whole 
Now such materials are manufactured or elaborated 
in the green parts of the coffee-plant, and especially 
in the cells of the leaf exposed to light and air, thence 
to pass into the branches, stems &c. to be distributed 
ns required. To replace quickly the food-materials 
of which it has been robbed, therefore, the coffee leaf 
would require at least not less manufacturing capabil- 
ity than it had before ; but it has legs, because the 
spreading mycelium, occupying space on and in the 
leaf, destroys eventually the little hardworking cells 
which would have made more food stuffs. Hence the 
fungus further does injury by destroying the tissues, 
and occupying .'pace wbich should have benefited the 
coffee tree. 
In doing thrse injuries to the leaf, I he fungus shortens 
its period of usefulness, and the coffee-tree must pro- 
vide mops haves in a given tint", if it is to obtain 
ih iame amount of manufactured material that it 
normally derived from the activity of the leaves. 
gnJessil can do this, or an less the total amount of 
active leaves 4ro. is more than the tree requires, the 
Witity to produetverop must be diminished. In attempt- 
Kg to meet this difficulty (so to speak) the functions 
of the coffee-tree as a whole becomo still more dis- 
turbed, and it mast be plain to all that, if inch a 
Condition of things la-ts for a long lime, or r curs 
often, the ability to manufacture and distribute the 
I large quantities of food-material necessary for the 
production of the amount of crop looked for must 
diminish — the periodic losses of matter and local diss 
turhance of function affect the whole tree, arid it i 
enid to he "diseased." 
There can be no difficulty, therefore, in under- 
standing the phenomena of " leaf-disease" which 
have so long aud so generally puzzled the plant- 
ing community : a tree cannot he expected to 
produce leaves, flowers aid fruil &e. , in the same 
proportions when thus afflicted, as it did when the 
numerous drains and disturbances were absent. In the 
local actions at numerous centres of the parasite we 
see a vera causa for "leaf-disease," and it becomes 
unnecessary to seek refuge in vague expressions. 
I would now call attention to still further proof that 
the fungus causes the disease. In proportion as coffee 
plants are sheltered from the spores of the fungus do 
they remain free from the disease; while exposed plants 
become diseased as the fungus obtains a hold on their 
tissues. On sowing spores of Hemileia on certain leaves 
I found that these leaves became diseased : not only 
so, but the "disease-spots" appeared on the area 
where the sowing was made, and nowhere else on the 
plant, unless spores were again sown. If a leaf has 
only one "disease-spot" on it, the damage done is 
little; but if many spots occur, the leaf is soon de- 
stroyed ; so, too, with the whole tree. If only a few 
"disease-spots" occur here and there, the harm is 
small : it is when many leaves (in proportion to the 
whole) are afflicted with many " spots, " that the 
rapidly denuded tree suffers so much. 
I need not here detail the experiments which led to 
these results, nor the numerous actual "infections" 
perform' d; but will simply refer to the reports published 
during the past two years. When, however, it is 
demonstrated that, of two plants grown from West 
Indian seed, in baked earth or in earth from England, 
the one which is sheltered from spores does not become 
diseased, while that which is "infected " (by sowing 
spores) does become diseased in due course, we have 
no longer any excuse for doubting that the fungus is 
the gr> at cause of the disease. There is, however, much 
less doubt expressed regarding this point, than with 
respect to the next, viz : — 
(2) " No special predisposition on the part of the 
coffee is required for its infection " Having discovered 
that a coffee-leaf could be " infected with leaf-disease " 
by means of the spores ; and having proved that natural 
infection occurs by the germinatiou of wind-blown 
spores &c, it became necessary to ascertaiu exactly 
under what condition this takes place. 
The first circumstance of importance is clearly that 
water or moist air be present, for in a dry atmosphere 
no spore can germinate : the spore remains on the leaf 
apparently unchanged for an indefinite period, unless 
sutlieient aqueous vapour be provided, and in such 
The other two conditions are always present 
(with rare exceptions when the temperature falls 
very low): they are, the presence of atmospheric 
air or oxygen, and that the temperature be suitable 
for the development of the fungus. Provided 
these three conditions of the environment he not 
absent, I find no difficulty in bringing about tho 
germination ot the fuugus spore ; and if tbia takes 
place On the stoma hearing skin or epidermis which 
covers tho leaf of coffee, the littl» germ-tube enters 
tho stoma or spore, grows and branches inside the 
leaf, forming a mycelium or"faogai plant" in the cavities 
which drains the leaf cells of their food-substances as 
already described. 
Now, putting axide for tho moment the question 
whi ther a coffee-leaf which is rich in f ood-materiala of 
the reverse best suits such a " fungus plant," I pro- 
ceeded to make experiments by sowiug the spores of 
