MOVEMENTS AND ATTITUDES OF PLANTS 149 



as to expose a relatively small area to the cold and drying 

 winds. As they expand and grow larger and stronger 

 they assume a favourable position with regard to air and 

 sunlight and form a mosaic. 



Motile organs of hedgerow plants. Examine the trees and 

 shrubs in a wood or hedgerow and note how frequently the 

 blades face the light. What part has moved to bring them 

 into this favourable position? Privet, Yew (Fig. 97), Ivy 

 (Fig. 222, i), White Bryony (Fig. 91), 

 Convolvulus (Fig. 89), Elm, Syca- 

 more, and many other examples 

 will be found. The bases of such 

 leaves are swollen, and it is this 

 cushion which is usually the organ 

 of movement. Sometimes there is 

 a motile organ at the upper end 

 of the leaf-stalk near the blade. 

 In other cases the cushions become 

 highly specialized organs, and the 

 leaves, and even the leaflets, are 

 able to execute periodic, and some- 

 times rapid, movements. Obser- 

 vations should be made on a few 



common plants, such as the White Clover and Wood 

 Sorrel. 



Sleep-movements in Clover, Wood Sorrel, and False Acacia. 

 The Clover leaf (Fig. 98) is borne on a long stalk with 

 a cushion at the base, covered by a pair of stipules. During 

 the day the blade at the end is horizontal and divided into 

 three leaflets (trifoliate). A slight variation in the form of 

 the cushion will cause much movement of the blade at the 

 end of its long lever. Similar cushions are found at the 

 base of each leaflet, and at dusk, by their aid, the two side 

 leaflets move into a vertical position, exposing their inner 

 edges to the sky and their outer edges to the ground. The 



Fig. 97. Branch of 

 Yew. Leaves arise spi- 

 rally on the axis, but the 

 blades turn to the light. 



