HOW PLANTS ABU INFLifENGED 



345 



as often happens, some of them are directly exposed 

 to the sun while others are continuously shaded. Figs. 

 128 and 130 show two branches from the same plant : 

 the one shown in Fig. 128 was well illuminated, while 

 the one shown in 

 Fig. 130 was shaded: 

 there is . a corre- 

 sponding difference 

 in the size of the 

 leaves. Fig. 199 

 shows the appear- 

 ance of sections of 

 sun- and shade - 

 leaves of the Prickly 

 Lettuce and Fig. 

 200 of sun- and shade -leaves of the Beech. Especially 

 noticeable in the "sun-leaf'' are the longer palisade 

 cells, the smaller air-spaces, the greater thickness of 

 the leaf, and thicker cuticle or outer wall of the 

 epidermal cells ; the number of stomata is also 

 smaller. These effects are due partly to the liglit 

 itself and partly to the dryness (or excessive evapo- 

 ration) caused by the light, heat and wind. 



Leaves arrange themselves with reference to the 

 direction of the light, and many follow the sun during 

 the day (see page 217). Some plants, inhabitants of 

 dry countries, place their leaves in a vertical position, 

 thus avoiding the full effect of the light (see page 



200. Beech leaves seen in cross-section: 

 (a) sun leaf, (&) shade leaf. 



