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 



FIG. 25. TRANSVERSE SECTION OF SUN-LEAF OF WHORTLEBERRY (Vac 

 cinium Myrtittus). (HIGHLY MAGNIFIED.) 



a, cuticle ; 6, 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. 



