84 Chapters in Modern Botany CHAP. 



ciated with altered water-tension, etc., in the cells expresses 

 another step, and so on. The problem is to discover all the 

 observable conditions of the movement before finally con- 

 fessing that what remains unexplained is due to some 

 still unknown property or power of the living matter or 

 protoplasm. 



This above all the student should recognise that the plant 

 is in all its actions no mere mechanical system, completely 

 explicable according to the facts of mechanics, hydrostatics, 

 and the like, but a living organism. And just as no one 

 will pretend to give a "mechanical explanation" of how a 

 Serpula worm makes its calcareous tube towards the light, 

 nor pretend to be content with calling the animal " helio- 

 tropic," so we must avoid both extremes in regard to 

 plants. 



The Sleep of Plants. Every one has noticed how the 

 three leaflets of the clover and the wood-sorrel change their 

 position as the light of day grows and wanes ; they are 

 expanded during the day, and fold downwards in the even- 

 ing . Many may have observed similar movements in Lupine 

 and Melilot, Mimosa and Acacia. Indeed these movements 

 are very common, and were noticed by early naturalists such 

 as Pliny. Since Linnaeus wrote his famous essay called 

 Somnns Plantarum they have been spoken of as the sleep 

 of plants. 



The movements are sometimes very marked, changing 

 the whole appearance of the plants ; thus " a bush of Acacia 

 farnesiana appears at night as if covered with little dangling 

 bits of string instead of leaves." 



Although sleep - movements are very general, their 

 amount and their nature are greatly diversified. Thus 

 Darwin enumerates 37 genera in which the leaves or 

 leaflets rise, and 32 genera in which they sink at night. 

 In some species the leaves sleep, but not the cotyledons ; 



