1 86 Chapters in Modern Botany CHAP. 



since Goethe's day (see chap. x. p. 199), the flower itself 

 consists of four whorls of " modified leaves " the sepals pro- 

 tective, the petals attractive, the stamens producing pollen, 

 the carpels bearing the ovules which when fertilised be- 

 come seeds. 



Substitutes for Leaves. The functions of the blade of 

 the leaf are shared to some extent by the petiole or stalk 

 and by the green skin of the stem and branches, and often 

 by the sepals and ovaries ; in short, every part of the plant 

 exposed to light tends to utilise it by producing chlorophyll, 

 excepting only those parts of the flower where, in current 

 phrase, more conspicuous colouring matters are required 

 for the attraction of insects. Thus the green stems of 

 Cactuses and some Euphorbias, which are practically leaf- 

 less, do in a measure discharge the ordinary work of leaves. 

 The green thorns of the gorse or whin are by no means 

 too hardened to be of some use as leaves, and there is no 

 doubt as to the use of the leaf-like shoots of Butcher's 

 Broom (Ruscus), or of the reduced thread-like filaments 

 (really abortive flower-stalks) of the Asparagus. 



Vitality of the Leaf. We saw that in the Indian Tele- 

 graph Plant (Desmodium gyrans) the lateral leaflets are in 

 constant motion, but this is only a specially marked example 

 of the movement which Darwin observed in all young and 

 vigorous leaves. When we touch the sensitive plant, the 

 petiole sinks downwards and the leaflets fold together, but 

 this after all differs but little from the daily sleep-movements 

 of clover, wood-sorrel, and hundreds of common plants, ex- 

 cept in its being induced by a sudden shock. Other move- 

 ments of leaves in relation to the light, as they bend towards 

 it, or arrange themselves transversely or sideways to its rays, 

 are of constant occurrence. We saw too that the modified 

 leaves of insectivorous plants seemed to exude a digestive 

 juice, but it is more important to recognise that there is, 



