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M.—AGRICULTURE. 263 
astounding; in Britain it is probably at least 10 percent. of the total 
value of the crops and the loss is probably some 12,000,000/. sterling per 
annum; in some countries it is considerably more. Indeed, the number 
of insect pests and of harmful fungi and bacteria that skilled entomologists 
and mycologists have found in our fields might almost lead us to despair 
of ever raising a single crop, but fortunately the young plant, like the 
human child, grows up in spite of the vast number of possible deaths. 
The saving fact seems to be that the pest does harm only when three sets 
of conditions happen to occur together: the pest must be present in the 
attacking state; the plant must be in a sufficiently receptive state ; and 
the conditions must be favourable to the development of the pest. It is 
because this favourable conjunction of conditions comes but rarely that 
crops manage to survive. And this gives us the key to control if only 
we knew how to use it. Complete control of any of these three conditions 
would end all plant diseases. Unfortunately, control is never complete 
even in glasshouse culture, still less out of doors. But even partial control 
would be very helpful. All these pests go through life cycles, which are 
being studied in great detail all over the world, and especially in the United 
States. Somewhere there occurs a stage which is weaker or more easily 
controlled than others, and the pest would become harmless if the chain 
could be broken here or if the cycle could be sufficiently retarded to give 
the plant a chance of passing the susceptible stage before it is attacked. 
The plants themselves, as we have just seen, are in some degree under 
control, and if they could be pushed through the susceptible stages before 
the pest was ready they would escape attack. Barley in England is some- 
times considerably injured by the gout fly (Chlorops teniopus). The larvee 
emerge in spring from the eggs laid on the leaves and invariably crawl 
downwards, entering the young ear if, as usually happens, it still remains 
ensheathed in leaves. J. G. H. Frew, at Rothamsted, has shown that early 
sowing and suitable manuring cause the ear to grow quickly above the 
track of the larvae, and thus to escape injury. E. A. Andrews, in India, has 
found that tea bushes well supplied with potassic fertiliser escape attack 
from the mosquito bug (Helopeltis) for the rest of the season, apparently 
because bushes so treated become unsuitable as food to the pest. And 
further, the conditions are alterable. H.H. King, in the Sudan, has effected 
some degree of control of the cotton thrips (Heliothrips indicus) by giving 
the plant protection against the drying North wind and so maintaining 
a rather more humid atmosphere—a condition in which the plant flourishes 
more than the pest. Tomatoes in England suffered greatly from Verticillium 
wilt till it was found that a small alteration of temperature threw the attack 
out of joint. They are also much affected by stripe disease (B. lathyri), 
but they become more resistant when the supply of potash is increased 
relative to the nitrogen. It has recently been maintained, though the proof 
is not yet sufficient, that an altered method of cultivating wheat in England 
will afford a good protection against bunt. These cultural methods of 
dealing with plant diseases and pests offer great possibilities, and a close 
study jointly by plant physiologists and pathologists of the responses of 
the plant to its surroundings, and the relationships between the physio- 
logical conditions of the plant and the attacks of its various parasites, 
would undoubtedly yield results of great value for the control of plant 
diseases. Again, however, the plant bresder can save a world of trouble 
