SEASONAL ACTIVITIES OF PLANTS. 219 



With SO much to our credit iu the way of advance made in 

 knowledge of the nature of the plant and its behavior, the other 

 important task which confronts us is that of making a similarly 

 exact study of climate and of all of the factors which constitute 

 the complex set of conditions which we term environment. 



A simple analysis of the relations of a plant to external 

 conditions will be useful for a l)etter 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, l)ut we 

 may gain an insight into their general character by n 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 Avell. 



Living matter is an extremely complex substance ;ind 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 be increased in a green- 

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



