VARIOUS MOVEMENTS OF PLANTS 



403 



Fig. 422. 

 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. 

 509. 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." 



509a. 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 



Fio. 423. at night is such as to conserve the vital 



Night position of oxalis heat of the plant, and it is possible that 



leaflets. some of this leaf-movement has arisen as 



