278 
THE LEAF 
recognize any combination of them as belonging to a par¬ 
ticular leaf. For instance, the apple leaf is ovate-oblong, 
rounded or cordate at the base, with a serrate margin. A 
leaf with a blade as large as the combined surfaces of a com¬ 
pound leaf would be much more easily torn by the wind, 
and would cut off more light from the leaves below. Some 
leaves are very finely divided, being many times compound, 
like the carrot and the yarrow. 
PHOTOSYNTHESIS 
241. Definition of Photosynthesis. — The word photo¬ 
synthesis (Greek, phos, light; synthesis , composition) means 
putting together by means of light. It is the process of 
manufacturing carbohydrates from raw materials, a vital 
process performed only by green plants. 
242. The Process. — Many facts about this process are 
not well understood, but we can state a few with certainty: 
(1) that in the manufacture of carbohydrates by photosyn¬ 
thesis, carbon dioxide is used as one of the raw materials, 
the source of this gas being the air, in which it exists 3 or 4 
parts in 10,000, and the waste of the plant’s own respiration; 
(2) that another raw material used is water, which is taken 
up by roots; (3) that some oxygen is left over as a by¬ 
product and given off as a waste of the process of photosyn¬ 
thesis; (4) that chlorophyll, the green coloring matter of 
plants, is essential; (5) that light is necessary not only to 
develop the chlorophyll but also to enable it to manufacture 
carbohydrates; (6) that another necessary factor is a moder¬ 
ate degree of warmth. 
243. The Products. — Carbohydrates are the first visible 
product of photosynthesis, but proteins and oils result from 
later processes, probably very similar to photosynthesis, 
which take place in the green leaf. 
244. The Use Made of the Products. — The carbohy¬ 
drates, proteins, and oils thus made are used in three ways: 
