ECONOMIC MYCOLOGY. 
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The problems are extremely complicated. There is first the 
necessity to establish the pathogenic nature of the attack, to isolate 
the causal organism, and to carry out infection experiments, where 
many variable factors of unknown nature enter in to baffle the in- 
vestigator. Some remedial treatment must also be sought, and large 
questions are involved which demand the application of fundamental 
principles of physiology and plant hygiene. The relation of host to 
parasite, the reaction of both to internal and external conditions, open 
up a wide field of research. The therapeutics of the plant must be 
considered from the same point of view as animal therapeutics ; and 
conditions of environment, predisposition, and questions affecting 
infection and immunity must all form the subject of definite scientific 
investigation. 
A close study of the life-history of a fungus often reveals some weak 
spot where it is specially vulnerable, and a knowledge of methods of 
natural infection and of conditions favouring the spread of the disease 
will often lead to an effective means of prevention. 
Telluric conditions, though little under control, play a most im- 
portant part in the spread of fungoid diseases. Thus the vapour 
pressure of the atmosphere is a determining factor in the spread of 
Phytophthora infestans, a damp, warm atmosphere enabling the conidia 
to germinate and also rendering more easy the passage of the germ- 
tube into the plant, while a dry atmosphere places the epidermal walls 
of the host in a position to resist the entrance of the germ-tube and 
at the same time it may be fatal to the conidia. A sudden fall of 
temperature renders many host-plants liable to fungus attack, possibly 
by modifying the constituents of the cell-contents. For example, 
Melhus has shown, in experimenting with radish plants, that chilling 
produced a marked effect upon the degree of infection secured, 95 per 
cent, of the seedlings becoming infected after chilling, but only 5 per 
cent, in those which were unchilled. In correlation with this observa- 
tion, it has been noted that rose-trees often show signs of a mildew 
attack some ten to fourteen days after a chilling wind. Melhus 
attributes this to the greater power of spore germination owing to the 
chilling process rather than to any effect in making the host more sus- 
ceptible, but he considers this point awaits further investigation. 
In this connexion attention may be drawn to the fact that in 
the potato, and to a greater extent in the so-called "starch-trees" 
(Betula, Tilia, &c.), a low temperature induces the transformation 
of starch into sugar, and that this process is reversed on the return 
to a higher temperature. Possibly other changes may take place in 
the plant-cell which also influence its susceptibility. 
Soil aeration is one of the very important factors in plant growth 
too often left out of account. While it is generally recognized that 
a plant respires, the application of this general principle to the roots 
is frequently neglected. The plant-root cannot properly avail itself 
of any food supply unless the necessary energy is available for this 
purpose, and this energy is mainly derived from the respiration of the 
