CHAPTEK II. 



FOLIAGE LEAVES: THE LIGHT-RELATION. 



8. Definition. — A foliage leaf is the ordinary green leaf, 

 and is a very important organ in connection with the work 

 of nutrition. It must not be thought that the work done by 

 such a leaf cannot be done by green plants which have no 

 leaves, as the algae, for example. A leaf is simply an or- 

 gan set apart to do such work better. In studying the 

 work of a leaf, therefore, we have certain kinds of work 

 set apart more distinctly than if they were confused with 

 other kinds. For this reason the leaf is selected as an in- 

 troduction to some of the important work carried on by 

 plants, but it must not be forgotten that a plant does not 

 need leaves to do this work ; they simply enable it to work 

 more effectively. 



9. Position. — It is easily observed that foliage leaves 

 grow only upon stems, and that the stems which bear them 

 always expose them to light ; that is, such leaves are aerial 

 rather than subterranean (see Figs. 1, 75, 174). Many 

 stems grow underground, and such stems either bear no 

 foliage leaves, or are so placed that the foliage leaves are 

 sent above the surface, as in most ferns and many plants of 

 the early spring (see Figs. 45, 46, 144). 



10. Color. — Another fact to be observed is that foliage 

 leaves have a characteristic green color, a color so universal 

 that it has come to be associated with plants, and espe- 

 cially with leaves. It is also evident that this green color 

 holds some necessary relation to light, for the leaves of 

 plants grown in the dark, as potatoes sprouting in a cellar, 



