FACTORS AFFECTING SEASONAL ACTIVITIES. 5 



conditions will be useful for a better understanding of the char- 

 acter of the problems involved. The principal factors affecting 

 vegetation are undoubtedly light, temperature, moisture, food- 

 material and chemical composition and physical consistency of 

 the soil. It is obvious to the veriest novice in gardening that 

 certain intensities or concentrations of these agencies are neces- 

 sary for the welfare of the plant, and that the combinations suit- 

 able for one are not for another. 



It will be impossible to give even a brief consideration of 

 the special relations of each of these factors to the plant, but we 

 may gain an insight into their general character by a considera- 

 tion of the more important details with respect to temperature, 

 which is one of the most widely interlocking elements of climate. 

 The conclusions derived from its consideration may be held to 

 apply to the other agencies as well. 



Living matter is an extremely complex substance and we 

 must be prepared therefore to find that its relation to its en- 

 vironment is not simple ; this is especially marked with regard 

 to temperature. 



CARDINAL POINTS IN TEMPERATURE. 



All of us know by every-day experience that there is a cer- 

 tain general degree of heat at which any given species grows best, 

 and a discrimination as to the application and regulation of tem- 

 perature constitutes one of the most important features of the 

 practice of greenhouse gardening. This temperature, which is 

 customarily termed the optimum, may be ascertained to within a 

 degree or two very easily. If the heat is increased in a green- 

 house in which a plant is happily growing at the optimum, it 

 will soon be found that such increase lessens the rate and amount 

 of growth, and a continued increase will soon bring the thermom- 

 eter to a point where a supra-optimum will be reached at which 

 growth ceases. This may simply bring the plant to rest as might 

 the cold of autumn, and with but slight damage. But if the 

 heat be increased still further a third point will be found at 

 which the plant is killed and by such a test we will have ascer- 

 tained the point of fatal heat. 



Starting again with a plant at tli optimum it will be found 



