i68 



SUN- AND SHADE-LEAVES 



of environment upon structure is, however, best illustrated by 

 the changes exhibited in one and the same organ when growing 

 under diverse conditions, and leaves are particularly plastic in 

 this respect. A comparison of leaves of the same species growing 

 under different conditions as regards light-intensity and degree 

 of humidity, factors which often go hand in hand, helps to 

 emphasise the significance of some of the features above con- 

 sidered. Thus, the structure of the leaves of the Ground Ivy 

 (Nepeta glechoma), growing in an exposed sunny situation, is 

 quite distinct from that of the leaves of the same species, de- 



B 



A 



FIG. 87. Diagram of transverse section (A), and detail of small portion (B), 

 of the assimilating stem of the Whortleberry (V actinium myrtillus). 

 aq., aqueous tissue ; as., assimilatory tissue ; /., fibres ; p., pith ; 

 ph., phloem ; xy., xylem. 



veloped in the shade of a wood, and a similar contrast is found 

 between the sun- and shade-forms of many other plants 1 (e.g. 

 Enchanter's Nightshade, Circcea lutetiana ; Dog's Mercury, Mer- 

 curialis perennis ; Yellow Deadnettle, Lamium Galeobdolon, 

 Fig. 88). Comparable anatomical differences are encountered, 

 in trees, between the leaves on the outside of the canopy and 

 those growing in the shade of the interior. 



In general shade-leaves are larger, thinner (cf. Fig. 88), and 

 very commonly, if the leaf be lobed or compound, not so deeply 

 cut as the corresponding sun-forms. The colour is usually a 

 i Cf. F. & S., p. 197. 



