146 EXPEEIMENT STATION. [Jan. 



his plants. The relation of external factors to plant diseases 

 can be most satisfactorily studied in the greenhouse, because 

 it is possible to modify and eliminate those which have a direct 

 bearing upon disease, and in this way their true significance 

 mav be determined. When the conditions surroundinoj the 

 plant are far from the optimum, injury and even death may 

 follow. A stimulus which may prove beneficial under certain 

 conditions may injure or cause the death of the organism under 

 others ; and it is only by possessing a knowledge of the opti- 

 mum conditions for stimulation and by meeting the normal 

 requirements of the plant that we can expect to obtain a perfect 

 organism. Everything which has a bearing upon the develop- 

 ment of the plant must be carefully considered if the perfect 

 type is to be realized. These factors not only aifect develop- 

 ment, but have a fundamental bearing upon immunity ; and if 

 the environment can be controlled, disease can be controlled to 

 a large extent. Even when it is not possible to modify the 

 heat, light and moisture, as is the case out of doors, infection 

 can be largely eliminated l)y making use of certain cultural 

 practices ; in fact, cultivation constitutes one of the most im- 

 portant factors in the control of disease. 



Light afi'ords a good illustration of the role a single factor 

 may play in the configuration of plants. The physiological 

 effect of light is to inhi])it growth and to induce the formation 

 of a firm texture of the tissue, On the other hand, lack of 

 light stimulates gro'wth, l)ut plants grown in darkness are etio- 

 lated and lack firmness of tissue. There are many instances of 

 the absence of light being responsible for serious troubles, and 

 in others light undoul)tedIy exerts a detrimental influence. 

 The tonic influence of the Bordeaux mixture in favoring the 

 formation of chlorophyll and carbon assimilation in many plants 

 would appear to be due to the screening or lessening of the 

 light intensity. Sun scald, which occurs on various trees, is 

 brought about by excessive light, as in the case with apple 

 trees, which, when defoliated by the gypsy moth, usually die 

 from the effects of sun scald. On the other hand, shading 

 often causes sun scald by preventing the ripening of the wood. 



There are apparently some cases, at least in greenhouses, of 

 too intense light, or the conditions resulting from it, causing 



