306 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH.. 



These and very many other spreading diseases in plants can only 

 with difficulty be controlled when their spores are given off in large 

 numbers, and when the vegetation on which they alight is damp or in 

 a vulnerable condition. Various applications have been tried to save 

 plants, such as potatoes and vines, from attack, and though partially 

 successful, they involve much trouble. The best security is the preven- 

 tion of the emission of large numbers of the disease spores into the air 

 from decaying or affected plants, and to cultivate only those varieties 

 of plants which are most immune from infection. The extent to which 

 plant diseases are transmissible through the air has never been ascer- 

 tained. It seems probable that, with the exception of wide-spread dis- 

 ease in exceptional seasons, the diffusive action of the wind would, in 

 general, so disperse the germs as to render them harmless to healthy 

 plants not too near together. If this be so, then the careful destruc- 

 tion of centers of infection as early as possible would very greatly 

 reduce the prevalence and damage of the diseases of plants. The 

 preservation of fruits, such as apples, is only successful where care is 

 taken that they are not too near together, and that those attacked are 

 speedily removed. But in damp, warm places the spread is too rapid 

 for such measures to be effectual. Dry, sterilized air might be found 

 a valuable means of preserving fruits, vegetables, and provisions gen- 

 erally. 



INFLUENCE OF WEATHER ON INSECT PESTS. 



The effect of a particular kind of season on insect pests is worthy of 

 more attention than it has hitherto received. The importance of attack- 

 ing in time and as far as possible destroying the insect life which, 

 if neglected, inflicts incalculable damage on crops and gardens, has 

 scarcely been realized, owing to the blight being generally regarded as 

 a necessary evil, not to be foreseen or prevented. The development of 

 insect pests is generally favored by dry weather. Stunting of the 

 growth, and overmaturation of the sap of plants induce early changes 

 in the maturing and structure of aphides; the insects multiply without 

 the interference of the ordinary destructive influences of bad weather, 

 and delicate maggots, etc., which are generally drowned in very large 

 numbers by storms of rain, emerge unharmed. At the same time it 

 may happen that corn and other crops may be enabled by earlier hard- 

 ening of the case, stalks, etc., to protect themselves against attacks 

 which in wet years would bring serious damage. In some countries, 

 and in respect to some crops, it is customary to arrange the date of 

 maturity with special regard to the protective power of the plant and 

 the period of expected attacks from insects. The whole subject is at 

 present too little under scientific observation, and great benefit might 

 result if the following branches of inquiry were systematically investi- 

 gated: (1) The influence of different kinds of weather in developing 

 insect pests; (2) the time of appearance of crop insects in different sea- 

 sons in relation to the weather, and the time at which crops are most 



