128 HOUSE, GARDEN, AND FIELD 



the chlorophyll-corpuscles are widely spaced, and the 

 colour is rendered pale by the numerous air-spaces. Here 

 are found most or all of the stomates and air-spaces, and 

 it is by this under surface chiefly that water is exhaled 

 and carbonic acid absorbed ; let us call it the pore-bearing 

 surface. When a two-sided leaf takes a special sleep- 

 position, the rule is that the assimilating surface is screened 

 as completely as possible. Clover, like most other plants, 

 conceals its assimilating, and exposes its pore-bearing 

 surface ; wood sorrel conceals its pore-bearing, and exposes 

 its assimilating surface. 



The upper surface of a clover leaflet is un wet table, 

 being protected by a waxy bloom ; the lower surface is 

 easily wetted ; in wood sorrel both surfaces, but especially 

 the under surface, wet with difficulty. In each case the 

 surfaces which are pressed together are unwettable ; it 

 might be hard to separate surfaces which had been pressed 

 together when wet, and they would not soon dry. 



Make a model of a clover-leaf, cutting the leaflets out 

 of card, and joining them by tapes. Paint the upper 

 surface bright green, and the lower surface pale green. 

 This model will enable you to represent to yourself the 

 different sleep-positions of clover and wood sorrel. 



Clover and wood sorrel are strongly contrasted in almost 

 every way. Clover seeks the sun ; it does not shrink from 

 competition with close-growing grasses, and it can even 

 endure trampling. So long as daylight lasts, it keeps its 

 leaflets open, and then, when there is nothing more to be 

 got, closes them tightly. Its most conspicuous defence 

 against the weather is one which hinders excessive heat- 

 radiation under a clear sky. Wood sorrel, on the con- 

 trary, loves shade and a damp, still air ; a bank, sheltered 

 from wind and sun, or the edge of a wood, are among its 

 favourite sites. The light that it prefers is chequered 

 light the gleams that enter through waving boughs, or 

 shine in horizontally when the sun is low. Strong sun- 

 light, wind, and drought are all harmful to it. The very 



