CHAPTER II 



FOLIAGE LEAVES: THE LIGHT-RELATION 



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

 and is averj' important organ in connection with the work 

 of nutrition. It must not ])e thought that the work done hy 

 such a leaf cannot tie done hy green ])lants which have no 

 leaves,, as the alg;e, for example. A leaf is simply an or- 

 gan set apart to do such work bottei'. In studying the 

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

 set a])a.rt more distinctly than if they were confused with 

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

 trodui^tion to some of the important work carrieil on by 

 plants, l>ut it must not Ije forgotten that a plant <loes not 

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

 more etFectivelv- 



!i. Position. — It is easily oljserved that foliage leaves 

 grow only Ujion stems, a-nd that the stems which bear them 

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

 rathei' than subterranean (see Figs. 1, 75, ITi). Many 

 stems grow underground, and such stems either liear no 

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

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

 the early spring (see Figs, -to, 4(1, l-il). 



1(1. Color. — Another fact t(j l)e observed is that foliage 

 leaves have a characteristic green color, a color so universal 

 that it has come to lie associated wilh jihiuts, and espe- 

 <'ially with leaves. It is also I'vident that this green color 

 holds sonu' necessary I'elation to light, for the leaNcs of 

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



