76 



INTRODUCTION TO BOTANY 



74. Excessive illumination. While the leaves of plants 

 growing in the shade often suffer from lack of sunlight and 

 are usually so arranged as to utilize most fully what light 

 there is, it is possible for leaves in exposed situations to have 

 too much light. It seems certain that the most powerful sunlight 



may injure the chloroplasts 

 and therefore cripple the 

 power of the leaf to do its 

 work of photosynthesis. 



Compass plants, such as the 

 common prairie rosinweed 

 (Silphium) and the prickly 

 lettuce, have leaves some- 

 what erect, with edges di- 

 rected nearly north and south, 

 so that they secure good il- 

 lumination during the cooler 

 morning and evening hours 

 but present the leaves nearly 

 edgeways to the sun at noon. 

 Many other plants maintain 

 some or all of their leaves in a 

 nearly vertical position, but 

 with the edges not directed 

 north and south. In the olive 

 FIG. 59. Virginia creeper (Psedera), a many leaves point almost ver- 

 tendril climber : 



tically upward, while in the 



The tendrils are modified shoots. At a they . ,. ,_. 



are seen fastened to a twig and at 6 adher- commonest species ot Jbuca- 



ing strongly by means of their expanded, lyptuS (fig. 60) the leaves 



disk-like tips to the surface of a wall , 



hang vertically downward. 



In a great number of trees the young leaves from recently 

 opened buds stand erect or hang straight down. In one 

 tropical species l the illumination received by these young 

 drooping leaves is not more than one five hundredth as intense 

 as that received by the most exposed of the mature leaves. 

 1 Amherstia nobilis, from Burma. 



