SECT. II 



PHYSIOLOGY 



235 



white in colour. The leaf-blades are small and of a yellow colour, 

 and remain for a long time folded in the bud (Fig. 196, E). A 

 plant grown under such conditions is spoken of as " etiolated." 



This diminution in the size of the leaf-blades and the elongation of the stem 

 (and leaf-stalks) are not manifested by all plants, nor under all circumstances. The 

 stems, for instance, of certain Cacti are much shorter when grown in the dark than 

 in the light. Similarly, the leaves of varieties of the Beet (Seta) grow as large, 

 or even larger, in the dark than in the light ; this is 

 also true, under conditions favourable to nutrition, of 

 the leaves of other plants (Oucurbita). In the shade of 

 a forest leaves often become larger than in full daylight. 

 They are then proportionally thinner, and the palisade 

 cells which, in leaves fully exposed to the light, are in 

 close contact, become pointed below, and thus produce 

 intercellular spaces between them. In this way the 

 modifying influence of light of diminished intensity is 

 apparent in the internal structure of such scotophilous 

 leaves. Flowers, however, if sufficient constructive 

 material be provided by the assimilating leaves, develop, 

 according to Sachs' observations; as well in the dark as 

 in the sunlight, except that they are sometimes paler in 

 colour. If, however, the assimilatory activity of the 

 green leaves be reduced or destroyed by depriving them 

 of light, many plants, as Vochting found, form only 

 inconspicuous or cleistogamous flowers. 



The tissues of etiolated stems and leaf-stalks 

 are fuller of water and thinner-walled than in 

 normally growing plants. Even the roots of 

 such plants are often found to be less strongly 

 developed. The supply of reserve material at 

 the disposal of plants growing in the dark is 

 utilised, together with the help of an unusual 

 amount of inflation water, in the elongation of 

 the axis. This elongation of the shoot axis, 

 resulting from growth in darkness, is of especial 

 value in the development of young plants from 

 underground tubers, rhizomes, and seeds; for 

 in this way the light is quickly reached, and 

 they are the sooner capable of independent 

 nutrition. The advantage derived from a rapid 

 elongation is especially apparent when the leaves 

 must themselves reach the light by their own elongation. This is 

 often necessary, particularly for the leaves of Monocotyledons, which 

 sprint from bulbs and rhizomes. They act just as the stems of 

 Dicotyledons, and attain an abnormal length in the dark. 



From what has already been said it would seem that plants must 

 grow more rapidly during the night than day, and this is actually the 



Pig. 196.— Two seedlings of 

 Sinapis alba, of equal age : 

 E, Grown in the darki 

 etiolated ; N, grown in or- 

 dinary daylight, normal. 



