FOLIAGE LEAVES: THE LIGHT-RELATION 



23 



stems against a support, as the ivies. It is only necessary 

 to notice how the leaves are adjusted to light on an erect 

 stem, and then to bend the 

 stem into a horizontal posi- 

 tion or against a support, to 

 realize how unfavorable the 

 same arrangement would 

 be, and how many new ad- 

 justments must be made. 

 The leaf blades must all be 

 brought to the light side of 

 the stem, so far as possible, 

 and those that belong to 

 the lower side of the stem 

 must be fitted into the 

 spaces left by the leaves 

 which belong to the upper 

 side. This may be brought 

 about by the twisting of 

 the stem, the twisting of 

 the petioles, the bending of 

 the blade on the petiole, 

 the lengthening of petioles, 

 or in some other way. 

 Every horizontal stem has 

 its own special problems of 

 leaf adjustment which may 

 be observed (see Figs. 18, 

 50). 



Sometimes there is not 

 space enough for the full 

 development of every blade, 

 and smaller ones are fitted 

 into the spaces left by the larger ones (see Fig. 21). This 

 sometimes results in what are called unequally paired leaves, 

 where opposite leaves develop one large blade and one small 



Fig. 17. A chrysanthemum, showing 

 lobed leaves, the rising of the petioles 

 to adjust the blades to light, and the 

 general cylindrical habit. 



