HOW THE PLANT LIVES 119 



times known as carbon assimilation). It is in 

 its results the reverse of respiration, in which 

 oxygen is taken in and carbon dioxid given off. 



199. During the day a much greater amount 

 of oxygen is set free as a result of photosynthesis 

 than that used in respiration, so that a surplus 

 of oxygen actually diffuses into the air, and plants 

 are said to purify the air. At night, no photo- 

 synthesis goes on, and the chief end-product of 

 respiration, carbon dioxid, is given off, and may 

 be demonstrated by experiment. 



2e. Heat, or a definite temperature 



200. Heat increases the absorptive activity of 

 the roots, the rate of transpiration, the amount of 

 respiration, and the products of photosynthesis. 



201. A more or less definite degree of heat 

 is necessary for all living processes. As a rule, 

 seeds will not germinate at the freezing point, 

 and all growth is suspended at that temperature. 

 Plants grow best within a very small range of 

 temperature, known as the optimum tempera- 

 ture. As a rule, other conditions being equal, 

 plants of moist tropical regions are succulent, and 

 green tissues preponderate. In the frigid regions 

 the softer green parts are greatly reduced, and, 

 while the woody part is of less extent than in 

 the temperate regions, relatively it preponderates. 



202. Different plants are injured by different 



