io6 DISEASE IN PLANTS. 



No less remarkable are the diseases primarily 

 due to insufficient or improper exposure to light, 

 which affects the chlorophyll-apparatus and the 

 process of carbon-assimilation and through these 

 the whole well-being of the plant. Every plant 

 is adapted to certain ranges of light inteo,.sity7and 

 most cultivators know how impossible it is to grow 

 shade plants in fully exposed situations, and how 

 easily plants which live in open sunny situations 

 are " drawn " and killed by shade. It is equally 

 important to have the right kind of light, as 

 disastrous experiences with greenhouses glazed 

 with glass which cut off certain rays of light have 

 taught. Here, again, it is important to notice that 

 the optimum intensity or quality of light may differ 

 for different functions and organs of the plant, as 

 is shown by many adaptations on the part of species 

 growing in natural situations e.g. bud protection, 

 orientation of leaves, etc. and it may be taken as 

 a rule that etiolated plants are peculiarly suscept- 

 ible to other diseases. 



As regards other factors of the inorganic en- 

 vironment, disasters which come within the scope 

 of our subject may be brought about by many 

 agencies, the mechanical effects of snow and 

 hail, wind, avalanches, etc., the effects of lightning, 

 and so forth, being a few of them. 



Notes to Chapter XI. 



For other detailed classifications of the causes of disease 

 the reader is referred to the works of Sorauer and of Frank 



