EXPOSURE TO LIGHT 



237 



the notches may be very deep and divide the blade into lobes, as 

 the leaves of the Gooseberry, Cotton, Dandelion, some Oaks, 

 Maples, and many other plants illustrate. In some cases the 

 blade is so divided that it is made up of independent portions 

 united to a common stalk, each independent portion being called 

 a leaflet. Many familiar plants, such as Clover, Alfalfa, Vetches, 



* c 



FIG. 219. A, Margins of leaves, a, serrate; b, dentate; c, crenate; 

 d, undulate; e, sinuate. B, lobed leaf of Grape. C, pinnately compound 

 leaf of Black Locust. A, after Gray. 



the Walnut, Ash, Locust, and Sumach, have leaves divided into 

 leaflets. The number of leaflets into which the leaves of different 

 plants are divided varies widely. In the leaves of Clover and 

 Alfalfa three leaflets are common, while leaves of the Black 

 Walnut often have twenty or more leaflets. (Fig. 220.} Leaves 

 divided into leaflets are said to be compound, while those less 

 divided are called simple. 



Leaflets resemble simple leaves and in case of some compound 

 leaves it is possible for one to mistake the axis to which the leaf- 

 lets are attached for a branch of the stem and the leaflets for 

 leaves. However, since buds occur only in the axils of leaves, 

 one can tell whether the leaf-like structure is a leaf or a leaflet 

 by the presence or absence of a bud in its axil. 



Exposure to Light. Unless the leaf is properly exposed to 

 light, it can not be an efficient food-maker. It is not always a 

 problem of securing enough light, but often one of escaping light 

 that is too intense; for too intense light often injures leaves and 

 consequently checks them in their work. The adjustment to 



