FOLIAGE LEAVES. 387 



substance from the strong light at that season of the year until the chloro- 

 phyll screen, which is weak in young leaves, becomes darker in color and 

 more effective, when the red color often disappears. 



753. Function of foliage leaves. — In general the function of 

 the foliage leaf as an organ of the plant is fivefold (see Chapters 

 I\', VII, A'lII, XI), (i) that of carljon-dioxide assimilation or 

 photosynthesis, (2) that of transpiration, (3) that of the synthesis 

 of other organic compounds, (4) that of respiration, and (5) that 

 of assimilation proper, or the making of new living substance. 

 While none of these functions are solely carried on in the leaf, 

 it is the chief seat of the first three of these processes, its form, 

 position, and structure being especially adapted to the purpose. 

 Assimilation proper, as well as respiration, probably take place 

 equally in all growing or active parts. 



754. Parts of the leaf. — All fohage leaves possess a hlade or 

 lamina, so called because of its expanded and thin character. 

 The blade is the essential part. ^lany lea\'es, however, are 

 provided with a stalk or petiole bv which the blade is held out 

 at a greater or lesser distance from the stem. Leaves with no 

 petiole are sessile, the blade is attached by one end directly on 

 the stem. In some cases the base of the blade is wrapped partly 

 around the stem, or in others it extends entirely around the 

 stem and is perjoliate. Besides, man}' leaves have short append- 

 ages, termed stipules, attached irsually on opposite sides of the 

 petiole at its junction with the stem. In some species of magnolia 

 the stip)ules arc so large that each one envelops the entire portion 

 of the bud which has not yet opened. Man}- leaves possess out- 

 growths in the form of hairs, scales, etc. (See leaf protection.) 



755. Simple leaves. — Simple leaves are those in which the 

 blade is plane along the edge, not divided. The edge may be 

 entire or indented (serrate) to a slight extent as in the elm. The 

 form of the simjile leaf varies greatly but is usually constant 

 for a given species, or it may vary in shape in the same species 

 on different parts of the plant. Some of the terms applied to 

 the outline of the leaf are o^■ate, o\-al, elliiitical, lanceolate, 

 linear, needle-like, etc., but it is idle for one to waste time on 



