As a predisposing factor to disease abnormal temperature effects play a great part. Many wound-fungi gain their entrance through frost-cracks, bruises due to hailstones, or into tissues chilled below the normal. 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 intensity, and 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 susceptible to other diseases. As regards other factors of the inorganic environment, 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 referred to in the last chapter. Also Kirchner, _Pflanzen Krankheiten_, Stuttgart, 1890. Of more historical importance are the older classifications of Berkeley, _Gardeners' Chronicle_, 1854, and Re, _Gardeners' Chronicle_, 1849-50. These latter are interesting as showing the very different views held by the earlier workers, and comparison of these with the modern views helps to mark the progress of physiology during the half century which has intervened. CHAPTER XII. CAUSES OF DISEASE. THE LIVING ENVIRONMENT. _Causes due to animals--Vertebrata--Wounds, etc.--Invertebrata --Insects, etc.--Plants as causes of disease--Phanerogams, weeds, etc.--Cryptogams, fungi--Epidemics, etc._ Passing now to those causes of disease which are connected with the living environment, we may obviously divide them into two groups of agents, animals and plants.