118 GENERAL BOTANY 



it manufactures within its green mesophyll cells and the nitrog- 

 enous salts which come up to the leaf from the roots in the 

 water stream. The leaves, through their green cells, are there- 

 fore the manufacturing organs for the other members of the plant 

 body, which are dependent upon them for nourishment. 



The term photosynthesis means literally the uniting, or com- 

 pounding, of substances by means of light. When applied to 

 the work of a green leaf it signifies the making of sugar from 

 the simple raw materials, carbon dioxide (CO 2 ) and water (H. 2 O), 

 by means of energy supplied to the leaf by sunlight. These raw 

 materials for photosynthesis are supplied from the soil through 

 the roots and stem and from the air through the stomata. The 

 carbon dioxide is absorbed by diffusion into the internal atmos- 

 phere of the leaf from the external air, and is then taken up by 

 the chloroplasts located in the mesophyll cells surrounding the 

 intercellular spaces of the leaf. The water which combines with 

 the carbon dioxide, although ultimately supplied by the roots, 

 is immediately absorbed for sugar-building from the cell sap of 

 the mesophyll cells. Since carbon dioxide, like other gases, 

 tends to move from a point where it is abundant, or concen- 

 trated, to a point where it is less abundant, we must picture it 

 as constantly flowing through the stomata into the leaf during 

 the day, to take the place of that which is absorbed from the in- 

 ternal atmosphere of the leaf in the intercellular spaces by the 

 mesophyll cells for the making of starch and sugar. The excess 

 of oxygen which is liberated during photosynthesis likewise dif- 

 fuses out of the leaf or is partly used in the process of respira- 

 tion, which goes on both day and night in the leaf as in other 

 living parts of plants. 



One of the first products of photosynthesis is undoubtedly 

 sugar, but the excess of sugar produced, which is not used by 

 the living cells for growth and repair, is usually transformed 

 into starch within the plastids of the leaf cells themselves. The 

 excess of sugar and starch formed in the leaf is later transported 

 in the form of sugar into the special storage tissues of the stem, 

 roots, fruits, and seeds. During the day this excess of starch 

 accumulates in the leaf cells, as can be demonstrated by testing 



