6 4 8 
THE TROPICAL AGRICULTURIST. [January 2, 1882. 
Hemileia on all kinds of coffee-leaves, old and young, 
weak and strong, &c, to see if any special predisposition 
to disease existed ; or, in more exact words, to see if 
any one kind was more " predispos'd" than another. 
The answer to these questioning experiments is a 
defini'e one. In all cases 1 found that the leaf could 
be infected ; that the germ tubes entered the stoma'a, 
and the tissues became " diseased. " I here, in fact, 
asked nature herself " Are any kinds of coffee more 
predisposed toinfection than the other?" And nature said: 
"No!" Such is the true interpretation of the successful 
experiments on coffee from Ceylon, Java, India, and 
Jamaica, of several varieties and at various ages. 
Hence it will be seen that my statements in no 
way of the nature of an opinion. I was biassed by 
no predetermined idea when commencing these en- 
quiries ; or, if any assumption was tacitly admitted, it 
was to the effect that some kinds might be more 
prone to "disease" than others. 
It may be replied, although the above has been 
proved, it is surely true that when once inside the 
leaf the fungus does not do the same amount of damage 
in all cases. I will not here multiply examples, but 
will shortly examine the results of many observations 
bearing on this difficult matter. 
We have seen that Hemileia differs from many other 
plants in being unable to construct its own food- 
substances from the air and earth &c, as does the 
coffee and similar plants : since, however, Hemileia 
requires these substances ready made, it steals them 
from the coffee. In a certain sense, therefore, we may 
look upon the coffee leaf as the soil in which the 
fungus-plant lives ; and we shall not be surprized to 
find that according to the nutritive quality of this 
living soil, so to speak, the little fungus-plants flourish 
well or ill, much as coffee plants thrive or the reverse 
in good or bad vegetable mould, &c. It is important 
to bear in mind, however, that the roots of a plant 
like coffee send crude sap up into the leaves, and it is 
not until the materials have become elaborated in 
the leaves that they serve for food : the fungus, 
therefore, is more accurately compared with the crop 
than with the whole tree in this respect. 
• It becomes probable from the foregoing that a fungus- 
plant or mycelium might be found to grow more 
rapidly in some leaves than in others. I find that 
the finest specimens, botanically speaking, of Hemileia 
vastatrix are grown on those leaves from which the 
fungus can obtain its food most readily i., e. on the 
succulent, vigorous, well-developed leaves of a fine 
tree, where the cells are readily broken into, and the 
contents rich in food-stuffs. Such leaves abound in 
the height of the growing season (April to June, and 
September to November near Kandy), and I need. not 
remark further how terribly they suffer from the 
abundant mycelium about July and December as a rule. 
It tlius becomes evident that the fungus-plant can 
be placed in any kind of leaf with equal facility, just 
as c -ffee seedlings can be planted in any kind of soil ; 
and that the former flourishes best in a well-nourished, 
succulent leaf, from which it can most readily obtain 
ricb food, much as the latter thrives in deep, luxuri- 
ant mould. In Loth cases, of course, other things are 
supposed equal, since differences in temperature, the 
amount of moisture, intensity of light, etc. etc. have 
their due effects. 
And now I come to tbe most difficult point. It 
will probably be replied that good coffee suffers least 
even though the fungus be plentiful, and that it is 
the poor, weak, "shuck" tree which exhibits the 
ravage* of the pest so lamentably. But we must here 
take care lest we argue in a circle. In many cases 
known to me it is simply because the coffee 
has tiot yet become the prey of the fungus that it 
looko so well, Good, deep soil suffers little, although 
it supports many luxuriant plants : eo, too, good strong 
coffee can afford to feed many little fungus plants, 
and yet its large leaf- surface supply material to produce 
crop as well. Much " good coffee," in fact, does 
support large quantities of mycelium as well as of crop. 
But I have thus far allowed the assumption that 
other things are. equal, while as a matter of fact, other 
things are commonly very unequal where coffee and its 
"disease" in one place, are compared wiih those at 
a distance. My reports abundantly show what are 
the chief factors in this connection ; but I may call 
attention to one or two points which appear to be 
either unknown or ignored by many of the critics 
who so readily compare unequal things together, with 
the somewhat naive surprize that the results are not 
equal. To expect two coffee trees on an estate (and 
the illustration may be extended to masses) to be 
equally "diseased"' simply because they stand near 
one another is, to say the least, daring, since it in- 
volves one or other of the following comprehensive 
assumptions. It must be assumed either — (a) that 
the two trees were at the outset equal in all respects 
— that their root-masses, leaf-surfaces, &c. were alike 
in extent and exposure, and that the relations of 
these to the soil and air, light, &c. were similar in 
all respects — that equal quantities of food-materials 
were in the two trees, and that the supplies and de- 
mands connected with this were steadily equal, &c, &c. 
It must further be assumed that each tree received 
at the outset the same quotum of disease-producing 
fungus-spores, which developed with equal energy and 
effect, and were equally related, actively and passively 
in the two cases. 
Or, if the above formidable details be not assumed, 
it must be allowed (b) that the various complex re- 
lations between coffee and its surroundings on the 
one hand, and Hemileia and its environment on the 
other, though differing in details in all possible degrees 
among themselves, amounted to the same final result 
in the cases selected:— that, a'though the trees were 
dissimilarly related to earth, air, light and the fungus, 
&c. as far as quantity goes, yet they became in the 
end diseased to the same extent. 
Either of these assumptions would be rash in the 
extreme ; and it is a bold argument to infer that 
because one of two trees, apparently similar in many 
respects, is "worse diseased " than another, that the 
reason is outsirle what has been stated. 
In cases where trees or groups of trees are pointed 
out as " disease-proof or practically so, " on what 
grounds are the assertions made ? Havs those who 
make the statements satisfied thenifelves that there 
are no uther reasons than those they give for the 
comparative immunity from " disease spots," at the 
time of the particular trees referred to? 'Have they 
even proved that the trees rem air "free from disease " 
for a year ; or that the relations between the number 
of diseased and healthy leaves are not different in the 
cases cited. And yet, surely the onus of proof lies with 
those who controvert record of observed facts with unsup- 
ported statements : and it is, to say the least, rash to 
affirm that a tree is less "diseased" than its neighbours, 
without being able to say that the tree was carefully 
and closely observed for a sufficiently long time. But 
even admitting that " a tree here and there shows fewer 
" disease-spots " during a given period than surround- 
ing specimens, who will undertake to prove that it 
has had as much chance of becoming infected as 
another ? There is nothing remarkable, for instance, 
in more spores being distributed successfully on the 
leaves of one tree than on those of /mother : nor 
could we bo surprized if many spores fall from the 
haves altogether. The following simple _ illustration 
may make my meaning clearer, though it docs not 
cover the whole question. Suppose several boys bath- 
ing in a stream near tbe banks, s-nd suppose leeches 
to be borne down by the stream, and falling from 
