132 . BIOLOGY 



Temperature. Photosynthesis is dependent upon tempera- 

 ture. Even though the sunlight be brilliant, if the temperature 

 be below freezing photosynthesis cannot go on. It can, how- 

 ever, take place in temperatures very slightly above freezing, 

 and will continue from this point up to moderately high tem- 

 peratures. At higher temperatures, 120 to 130 F., the process 

 stops. The temperature at which photosynthesis goes on most 

 rapidly, the optimum temperature, varies with different plants, 

 depending upon the structure of the plant itself. Some plants 

 are so constructed that they can grow only at moderately low 

 temperatures, and others only at high temperatures. In some 

 of the arctic plants, photosynthesis, as well as all the other 

 functions of the plant, goes on very readily when the tempera- 

 ture is not much above freezing, whereas in tropical plants 

 photosynthesis does not occur unless the temperature is high. 



METASTASIS 



Photosynthesis may be spoken of as food manufacture, for 

 the starch thus made is later utilized for the life processes of 

 the plant. The use of this starch as food is generally spoken of 

 under the term metastasis (Gr. meta = beyond + histanai = 

 to place). This is too complicated a process to be described 

 here in detail, and only a few of the main features will be briefly 

 explained. 



As already stated, the plants take in through their root hairs 

 not only water but a number of ingredients dissolved in it 

 Among these are nitrates, phosphates, potash, and various other 

 substances in smaller quantities. All of these substances are 

 carried up through the plant and distributed so that each living 

 cell may receive some of this dissolved material. The starch, 

 formed chiefly in the leaves, as we have seen is converted into 

 sugar, chiefly in the night, and then transported through the 

 plant in the sieve cells of the bark. The living cells in the various 

 parts then take the water and minerals brought with the ascend- 

 ing sap, and the sugars brought from the leaves, and by changes 



