13 



the plant in many other ways, by modifying its structure and 

 functions. At the present time it is impossible to determine 

 quantitatively all of the factors which influence plant develop- 

 ment, but in the future methods will undoubtedly be developed 

 which will enable us to determine how many heat or light and 

 moisture units are required to develop a plant to a certain stage, 

 and to know more about the exact relationship of these factors 

 to tissue formation and blights. 



Owing to the lack of practical methods of determining quan- 

 titatively the conditions in a greenhouse, only general directions 

 can be given in regard to the control of certain factors. We 

 know, for example, that a certain amount of atmospheric mois- 

 ture maintained in a house for a certain length of time will cause 

 mildew, and that a lack of soil moisture will induce the blossom 

 end rot. We also know a great deal about the effects of cold 

 air, light, heat, etc., on plants as regards infection, and in some 

 cases more or less specific directions can be given as regards the 

 management of the house. It is necessary, however, that the 

 gardener acquire skill and judgment in handling a crop, and the 

 more he acquires the less he will be troubled with diseases. 



The effects of too much sunshine on the plant^ are obviated 

 to a considerable extent by certain methods of planting. When 

 the plants are set 12 or 15 inches apart and the rows are more 

 or less close to one another (2 feet), there is some shading, and 

 transpiration is checked. On the other hand, too close planting, 

 especially in the fall, when the light is deficient, may induce leaf 

 spots. Concrete walks in a house materially affect the moisture 

 in the air, and if the soil is dry the air is likely to become deficient 

 in moisture. 



We have found in some of our experiments that too much 

 water is disastrous to the crop. In some cases the foliage turned 

 yellow quite early and the plants died prematurely. The soil in 

 such cases was very soggy and not suited to root development. 

 This condition was present when an excess of water was ap- 

 plied, particularly in the zinc-lined benches where there was no 

 drainage. 



Too high night temperatures always result in the formation 

 of a more tender tissue, which loses water rapidly, and as a 

 consequence the plants wilt more easily. On general principles, 

 therefore, lower night temperatures should be maintained during 

 periods of cloudy weather, and too high day temperatures should 

 not be run immediately following cloudy weather. 



Since the blossom end rot, in consequence of the poor light 

 in the fall, is not so common in the greenhouse then as in the 

 spring, less attention has to be given to its control during that 

 season, since transpiration is less active. As the amount and 



