SUN AND SHADE LEAVES 



153 



the depth of its cells, the relative proportions of palisade 

 and spongy parenchyma — is characteristic for a given 

 species. It is, however, subject to modification in different 

 environments. This is well seen by comparing leaves 

 growing in shaded positions with those exposed to direct 

 insolation. In many plants there is a sharp distinction 

 between " sun " and " shade " leaves. A familiar example 

 is the common harebell. In 

 shaded positions, in crevices of 

 walls or in deep grass, we may 

 see it bearing large rounded 

 thin leaves ; in sunny stations, 

 where it sends up flowering 

 shoots, the leaves on these are 

 narrow, elongated and thick. 

 Less obvious, but just as dis- 

 tinctive, differences are to be 

 found in the leaves of many 

 trees. The sun leaves of the 

 maple possess two layers of 

 palisade, and the upper layer of 

 palisade is nearly twice as deep 

 as the single palisade layer of 

 the shade leaf ; the ratio of the 

 thickness of palisade to the 

 thickness of spongy paren- 

 chyma, is, in the sun leaf, 

 3'9 : 1 ; in the shade leaf, i"i : i. 

 The total volume of the inter- 

 cellular spaces is greater in 

 the shade leaves by about 50 



per cent. The beech (Fig. 14) shows similar differences. 

 Willstatter and Stoll (1913) found that the shade leaves of 

 the beech contain more, those of the elder less, chlorophyll 

 than the respective sun leaves. In both trees the ratio of 

 green pigments to yellow pigments is greater, in the beech 

 much greater, in the shade leaf than in the sun leaf. The 

 figures relate the chlorophyll to weight of fresh leaf substance, 



Fig. 14. — Sun and shade leaves 

 of beech: i, sun, 2, shade 

 leaf. The sun leaf has two 

 layers of palisade tissue at the 

 upper surface, and one layer 

 at the lower surface. X 240. 

 (After Nordhausen.) 



