Leaves in Relation to Light 



67 



way in which the Hght falls on the blade during the period of 

 development. 



Strong light retards growth in length of the petiole. If the 

 blade is shaded, the petiole elongates more than usual ; if shaded 

 on one side, the petiole grows unequally on its two sides until 

 the blade is about equally illuminated. The position of the leaf, 

 when it has stopped growing, is usually fixed, and shading will 

 no longer affect the growth of the petiole. When a leaf that has 

 attained its full growth is overshadowed, it loses its chlorophyll, 

 turns yellow, and dies. You can find examples of such leaves 

 under the green leaves of rosettes, or on the lower branches of 

 trees that form mosaics. 



Vertical leaves. In a number of common plants, including the 

 iris, cat-tail, calamus, and many grasses, the leaves are vertical 

 because they are held in this position by their sheathing bases 

 rather than because of a response to hght. These plants usually 

 occur in dense growths (Fig. $S), and the vertical position of 

 the leaves permits the light to penetrate to their bases. This 

 has the advantage of allowing photosynthesis to go on through- 

 out the entire length of the leaves. 



Differences in vertical and horizontal 

 leaves. The structure of vertical leaves 

 differs from that of horizontal leaves in 

 several particulars : 



In vertical leaves the mesophyll may be 

 composed of spongy tissue, or it may be 

 composed entirely of palisade cells. Mdre 

 rarely there are pahsade layers on both 

 sides, with a spongy layer between. In 

 contrast, a horizontal leaf usually has a 

 pahsade layer beneath the upper epider- ^ , 



mis, and the lower portion of the meso- ♦./O'^y^iv-^-^-' 

 phyll is composed of loosely arranged fr/^.lirjrp::;^^ 



cells. In vertical leaves stoma ta usually by their sheathing bases. 



