VARIOUS MOfEMEtfTS OF PLANTS 



107 



FIG. 109. 

 Day position of oxalis leaflets. 



leaves of purslane, and even of Indian corn and 



grasses, seem to wilt or to roll up when the 



weather is hot, and 



loss of moisture is 



thereby prevented. 

 140. The flower of 



the California poppy, 



or eschscholtzia, which 



is common in gardens, 



opens at day and closes 



at night. Observe, 



also, the flower of 



" pussley", the garden 



portulaca or rose-moss, oxalis, and some of the 



mallows. Other flowers open at night and close 



at day. This diurnal move- 

 ment of the parts of plants 

 is known as the "sleep of 

 plants." 



140a. It is not a -sleep, however, in 

 the sense of being a rest or period of 

 recuperation for the plant. How these 

 movements are produced is not definitely 

 known, but they are associated intimately 

 with the stimuli exerted by light and 

 darkness, heat and cold. The utility of 

 the movement is also in dispute. Darwin 

 found that the position of sleeping leaves 

 at night is such as to conserve the vital 

 heat of the plant, and it is possible that 

 some of this leaf-movement has arisen as 



FIG. 110. 



Night position of oxalis 

 leaflets. 



