120 BOTANY 
dioxide that has entered from the atmosphere into 
its elements carbon and oxygen, of which the oxygen 
alone is restored to the air. The second stage is the 
building up (synthesis) of the carbon with the 
elements of water to form a carbohydrate. The 
carbohydrate first formed is doubtless some variety 
of sugar. Now, since sugar is soluble, it is quite clear 
that any considerable quantity of that substance would 
reduce the cell sap to a viscid treacly mass, quite 
unsuited for circulation. Therefore, during the day, 
when the leaf is most active, the elements necessary 
to form a certain amount of water are taken away 
from the sugar, by which means that substance is 
converted into starch. The insoluble starch is tem- 
porarily stored in little granules which do not impede 
the flow of the sap. The sugar, however, that the 
plant can use immediately, retains its original soluble 
form. So much for the production of carbohydrates. 
The formation of proteins must now be considered. 
It would appear (though this is by no means certain) 
that some of the sugar in the ecells of the mesophyll 
acts on the nitrate of potash brought from the soil in 
such a way as to throw out the potash and form amides, 
soluble nitrogenous compounds consisting of carbon, 
oxygen, hydrogen, and nitrogen. Since experiment 
has shown that both chlorophyll and sunlight are 
necessary for the formation of amides, it would seem 
that such formation is a branch of photosynthesis. 
At all times, the sugar and amides make their way 
to the parts that require them, and are dealt with 
there by the protoplasm in such a manner as to form 
the living tissue of the plant or add to the accumulation 
of reserves. The sugar may form celluose, starch or 
inulin, while the amides, with the addition of phos- 
phorus and sulphur brought from the soil, are 
converted into proteins as well as the living protoplasm 
that lines the cells. During the day, there is formed 
