HOW THE PLANT LIVES 110 



tiint»s known as carbon assimilation). It is in 

 its results the reverse of respiration, in which 

 oxy.ii:iMi is taken in and carl)on <lioxid given ulY. 

 ]\M). During the day a nui«*h 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 })urify the air. At night, no photo- 

 synthesis goes on, and the chief end-])roduct of 

 respiration, carbon dioxid, is given off, and may 

 be deinonstrated by experiment. 



2'. II' lit, !>}• (I (h finite trmperature 



'JOO. II. 'at increases the absori)tive activity of 

 thf roots, the rat(^ of transpiration, the amount of 

 res]>iration, and the pro<lucts of photosynthesis. 



201. A more or less definite degree of heat 

 is necessary for all living jjrocesses. As a rule, 

 sc.'ds will not germinate at the freezing point, 

 and all growth is susj)ended at that temperatuif. 

 Plants grow l)est within a very small range of 

 temperature, known as the optimum temi)er:i- 

 ture. As a rule, otiier conditions being equal, 

 plants of moist tropical regions are succulent, and 

 green tissues preponderate. In th«' frigid regions 

 th« softer green parts arc greatly reduce*!, and, 

 while the woody |>art is of less extent than in 

 tlie temperate region'?, relatively it preponderates. 



'JO'J. Different j)lants are injured by different 



