72 BRITISH PLANTS 
absent, but the. internodes become very long and thin, 
and the leaves produced are small. The diminution in 
size of the leaves is brought about by defective nutrition, 
but the abnormal lengthening of the internodes is directly 
due to the withdrawal of light. Lilies and grasses, under 
similar conditions, produce elongated leaves. On the 
other hand, intense illumination has a retarding effect 
upon growth. It is partly for this reason, and partly 
also from malnutrition, that arctic and alpine plants 
are dwarfed in stature. In the shade of hedges and woods 
Fic. 25.—TRANSVERSE SrEcTION oF SuN-LeaF oF WHORTLEBERRY (Vac 
cinium Myrtillus). (Hiaauy MAGnirtep.) 
a, cuticle ; b, epidermis ; c, chlorophyll-tissue ; d, air-space ; e, stoma. 
there is enough light to keep the plants green, but it is 
sufficiently weak to produce long internodes. Such plants 
have a long, weedy appearance, approximating to that 
observed in plants grown in darkness (p. 119). In sun- 
plants the leaves are generally small and thick, with 
many layers of chlorophyll-containing tissue. In the 
shade, on the other hand, the leaves are thinner, because 
the light is too weak to penetrate more than one or two 
layers (Figs. 25 and 26). To some extent, however, this 
diminution in thickness is compensated by the larger 
size of the shade-leaf. In many trees and bushes there 
is a marked difference in size between the leaves: on the 
sunny side and those on the shady. The latter are much 
larger than the former. 
