72 THE LEAF 



the seedling must feed itself. In the soil there is no 

 supply of starch, oil, sugar, or the like, or, if there is a 

 small proportion of these matters present through the 

 decay of former vegetation, yet these would not be enough 

 to furnish material for all the new plants that grow. If 

 there is none at all, — if, for example, we grow the seedling 

 in clear sand watered with distilled water, with the addi- 

 tion merely of a few mineral salts in very small quantity, — 

 the young plant grows perfectly well. In other words, it 

 is able to form its own food. This food it makes largely 

 through the agency of its leaves. 



106. Soil and air furnish the raw materials. These are 

 water, sucked up by the root, and carbonic acid gas (car- 

 bon dioxide), absorbed by the leaf from the atmosphere. 

 These two meet in the soft green tissue of the leaf. By 

 the power of sunlight, in the presence of chlorophyll (the 

 green coloring matter), the water and the gas are decom- 

 posed, and their elements recombined in such a manner 

 that a solid makes its appearance ; namely, starch. Starch 

 is in its nature very like the living substance itself, and 

 may be used in growth. It is then food, in the most 

 appropriate sense of the word. Water, carbon dioxide, 

 and small quantities of other substances, since they can be 

 added only indirectly to the living substance, are not food 

 in the same sense as starch. 



107. The formation of organic substance (as starch) 

 from these raAV materials is called carbofi assimilation; 

 when brought about through the agency of light, as in 

 all ordinary cases, it is called photo syjithesis. 



FORM AND QUALITIES OF THE LEAF 



108. The form of the leaf results from its use. Thin- 

 ness gives full exposure to light and good aeration. The 

 leaf is translucent as well as thin, so that all parts of the 

 tissue are reached by the energizing rays. It is compara- 

 tively strong and elastic, — qualities given by the woody 

 framework of ribs and veins. The strengthening ele- 



