SUN- AND SHADE-LEAVES 



169 



fresher green, owing to little decomposition of the chlorophyll 

 (through the absence of strong light) and the greater translu- 

 cency of the leaf ; moreover, chloroplasts are not infrequent in 

 the epidermal cells (e.g. Bracken). In the case of hairy leaves 

 (e.g. Yellow Deadnettle, Dog's Mercury) the production of hairs 

 is usually much reduced in the shade-form. A similar difference, 

 between the exposed and sheltered parts of the same plant, is 

 exemplified by the more numerous hairs on the stem-leaves as 

 compared with the radical leaves of species having rosettes. In 

 the Meadowsweet (Spircea ulmaria) the lower leaves, protected 



FIG. 88. Transverse sections of the sun- (A) and shade- (B) leaves of 

 the Yellow Deadnettle (Lamium Galeobdolori), on the same scale, i.s., 

 intercellular spaces ; p., palisade tissue ; s., stoma (in A, cut longitu- 

 dinally) . 



among the herbage, are smooth and green on the underside, 

 whilst the corresponding surface in the exposed upper leaves is 

 of a greyish tinge owing to the downy felt of hairs. 



The cuticle and outer epidermal walls of the shade-forms 

 are thinner than in the sun-forms, a feature which can be related 

 to the greater humidity of the air. A comparison of strips of 

 epidermis from the two kinds of leaves shows that the vertical 

 walls of the epidermal cells tend to be straighter in the sun-form 

 where the leaves are thicker (cf. p. 93). The palisade cells 

 (Fig. 88, p.) are shorter and the number of palisade layers, 

 as compared with leaves growing in bright light, exhibits reduc- 



