150 VEGETABLE PHYSIOLOGY 



The activity of the chlorophyll apparatus is also con- 

 siderably influenced by variations of temperature. The 

 lower limit beyond which no carbohydrates are constructed 

 lies probably a little below the freezing point of water, at 

 which point, however, activity is not long maintained, and 

 then only by alpine forms. Jumelle has stated that in cer- 

 tain plants of hardy type it can proceed at as low a 

 temperature as 40 C. Plants which normally live in hot 

 climates cannot manifest any power of action below about 

 4 C. The optimum temperature for the plants of tempe- 

 rate climates is from 15 C. to 25 C., above which activity 

 diminishes, though not very rapidly, ceasing when about 

 45 C. is reached. These high temperatures affect the 

 living substance of the chloroplasts very injuriously. 



The activity of the chlorophyll apparatus is dependent 

 also to some extent upon certain of the mineral salts present 

 in the cells. According to Bokorny, it cannot be called 

 into play in the absence of compounds of potassium. 



As the activity of the chlorophyll apparatus is so essen- 

 tially dependent upon light, the process of construction of 

 carbohydrate substances from carbon dioxide and water, 

 which is its primary object, may appropriately be called 

 photosynthesis. This term has certain advantages over 

 the older expression, the assimilation of carbon dioxide, as 

 the term ' assimilation ' may preferably be reserved for the 

 process of the incorporation of food materials into the sub- 

 stance of the protoplasm. 



Photosynthesis consists, then, in the formation of some 

 form of carbohydrate from the carbon dioxide which is 

 absorbed from the air, and the water which is present in 

 the cells. When these simple bodies are exposed to the 

 action of the chloroplast in presence of light and mode- 

 rate warmth, the carbon dioxide disappears, and a volume 

 of oxygen equal to that of the carbon dioxide is exhaled. 



The apparatus shown in fig. 88 will enable this inter- 

 change of gases to be seen. Into a glass jar is poured 

 ' some water containing carbon dioxide in solution. Some 



