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 
