Ways in Wliich Plants Increase in Size 333 



These three cardinal points vary much with different plants, 

 ranging lower in those of cold regions and higher in those of 

 the tropics; and plants can thrive only in climates where the 

 range of usual temperature corresponds somewhat closely with 

 their cardinal points. This will explain why the Orange will not 

 grow if planted in Canada, or Barley and Rye if taken to Florida. 

 In plants of our own climates these points approximate on 

 the average to 5°-30°-40° Centigrade respectively (or 40°-85°- 

 100° Fahrenheit), which means that most of our plants do not 

 grow appreciably below 40°; they grow best at about 85°; and 



Fig. 124. — A graphic illustration of the relation of growth to temperature. The copper 

 trough is heated from one end (the left), and chilled from the other, with the result 

 that the temperatures grade evenly between. 



hardly grow at all above 100°. This will explain why it is that 

 when the temperature of our fields rises higher than 100° in the 

 sun, the extra heat is no aid to plant growth, being rather a 

 hindrance thereto. The same thing would happen also in green- 

 houses in summer were it not for the shading, which is added to 

 reflect both the heat and the light. 



The reason why heat has this effect upon growth is fairly well 

 known. Growth depends upon a number of chemical and physical 

 processes which are kept in orderly cooperation by the protoplasm. 

 All of these processes, in general, are promoted by higher tem- 

 perature, which fact explains the more rapid growth up to the 

 optimum point; l3ut, as the temperature rises higher, to degrees 

 beyond those to which the plant is accustomed, the processes get 

 beyond control of the protoplasm, or run away, so to speak, thus 

 injuring and finally destroying the coordination and stopping the 



