SLEEP OF PLANTS. 241 



plants growing out of doors) and looked at them early 

 on several successive mornings after they had assumed 

 their diurnal positions. The difference in the amount 

 of dew on the pinned-open leaflets, and on those 

 which had gone to sleep was generally conspicuous ; 

 the latter being sometimes absolutely dry, whilst the 

 leaflets which had been horizontal were coated with 

 large beads of dew. This shows how much cooler 

 the leaflets fully exposed must have become, than 

 those which stood almost vertically, either upwards or 

 downwards, during the night." x 



When a seed germinates in the ground, the first 

 pair of leaf-like organs which appear above the 

 surface are usually different in form and texture from 

 the true leaves which are afterwards developed. 



These primary organs are the cotyledons, and 

 between them the delicate little bud, or plumule, of 

 the succeeding leaves nestles. This remark applies 

 generally to the large number of plants which have a 

 pair of cotyledons. Up to the period when Dr. 

 Darwin commenced his experiments, from the record 

 of which we have already quoted, sleep movements 

 had only been observed in the cotyledons of two 

 plants. This observer has determined, however, that 

 these movements take place ■ in a large number of 

 plants. In some instances the cotyledons sleep, 



1 Darwin, " Movements of Plants," p. 293. 

 R 



