158 A TEXT-BOOK OF BOTANY. 



6CO 2 + 5H 2 O = C 6 H 10 O 5 + 6O 2 . 



(Starch) 



6C0 2 + 6H 2 = C 6 H 12 6 + 60 2 . 



(Glucose) 



(4) Certain mineral substances must be present, although, appar- 

 ently, they take no part in the photosynthetic reaction. Bokorny 

 has shown that compounds of potassium are essential to bring 

 about the reactions above given. 



Some form of iron has always been considered necessary for 

 the development of the green pigment or chlorophyll in the chloro- 

 plastid. While this element may seem to be necessary in water 

 culture, it is not always essential, particularly if plants are grown 

 under control conditions in sand. The development of chlorophyll 

 also requires the presence of oxygen. The activity of the chloro- 

 phyll apparatus is further influenced by other factors, viz., the 

 maintenance of a proper temperature. It is self-evident that there 

 is a minimum and maximum temperature at which photosynthesis 

 is scarcely perceptible, and that there is an optimum temperature 

 during which the activity of the chloroplastid is at its height. 

 The latter varies with different plants, depending on the climate 

 to which they are either indigenous or naturalized. In the Tropics 

 the optimum temperature is somewhat higher, while in the Arctic 

 regions it is much lower. In temperate climates the optimum 

 varies between 20 C. (68 F.) to 30 C. (86 F.). 



From the facts just given it would appear that considerable 

 is known in regard to the conditions and the substances which are 

 concerned in the formation of photosynthetic products. On the 

 other hand, we know practically nothing of the successive steps in 

 the formation of either starch or glucose in the plant. Numerous 

 experiments have been conducted and a number of hypotheses 

 have been advanced. According to von Baeyer, the first step 

 in the process of photosynthesis is a reduction in the CO 2 , formalde- 

 hyde being formed, and this is then polymerized into a carbohy- 

 drate, which is finally changed into dextrose. This may be repre- 

 sented by the following equations : 



C0 2 + H 2 >-HCHO + 2 

 xHCHO=('CH 2 O)x 

 6(CH a O)=C 6 H 12 6 



