racy 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, suchas 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 
TMS aleeiber many leaves point almost ver- 
The tendrils are modified shoots. At a they tically ipwand, while in the 
are seen fastened to a twig and at b adher- commonest species of Euea- 
wick ke eats thesurtacncta wal PO Cg G0) the lenyes 
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! 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. 
Fie. 59. Virginia creeper (Psedera), a 
1 Amherstia nobilis, from Burma. 
