MINUTE STRUCTURE OF LEAVES 171 



foliage must depend on the kind of leaves, the tempera- 

 ture of the air, the intensity of the sunlight, and some 

 other circumstances. Sunflower leaves and pumpkin or 

 squash leaves have been found to manufacture starch at 

 about the same rate. In a summer day fifteen hours long 

 they can make nearly three-quarters of an ounce of starch 

 for each square yard of leaf-surface. A full-grown squash 

 leaf has an area of about one and one-eighth square feet, 

 and a plant may bear as many as 100 leaves. What would 

 be the daily starch-making capacity of such a plant ? l 



183. Assimilation. From the starch in the leaf, grape- 

 sugar or malt-sugar is readily formed, and some of this in 

 turn is apparently combined on the spot with nitrogen, 

 sulphur, and phosphorus. These elements are derived 

 from nitrates, sulphates, and phosphates, taken up in a 

 dissolved condition by the roots of the plant and trans- 

 ported to the leaves. The details of the process are not 

 understood, but the result of the combination of the 

 sugars or similar substances with suitable (very minute) 

 proportions of nitrogen, sulphur, and phosphorus is to 

 form complex nitrogen compounds. These are not pre- 

 cisely of the same composition as the living protoplasm 

 of plant-cells or as the reserve proteids stored in seeds 

 (Sects. 14, 17), stems (Sect. 127), and other parts of 

 plants, but are readily changed into protoplasm or proteid 

 foods as necessity may demand. 



Assimilation is by no means confined to leaves ; indeed, 

 most of it, as above suggested, must take place in other 

 parts of the plant. For instance, the manufacture of the 

 immense amounts of cellulose, of cork, and of the com- 



1 See Pfeffer's Physiology of Plants, translated by Ewart, Vol. I, p. 324. 



