164 VEGETABLE PHYSIOLOGY 



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 evolved. 



The apparatus shown in fig. 90 will enable this inter- 

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

 some water containing carbon dioxide in solution. Some 

 watercress is put into the water and a funnel inserted above 

 it, the end of which rises into a burette filled with water 

 and closed by a stopcock. The whole apparatus being 



