THE SLEEP OF PLANTS 



299 



Indeed, at the time when the plant 

 is in its most active state of growth, 

 certain processes, such as the elongation 

 of stalk, for instance, take place more 



WOOD SORREL : DAY POSITION. 



vigorously in the darkness than during 

 the day-time. 



Despite the fact that we cannot say 

 that plants commence a period of repose 

 with the coming of evening, the state 

 upon which they enter at this time will 

 almost justify us in saying that they go 

 to sleep. We must remember that it 

 is only during the hours of bright day- 

 light that the elaboration of the 

 wonderful chlorophyll — the vital 

 green part of the plant — can 

 take place. It is likely, also, 

 that the sap movements are 

 much less vigorous at night 

 than when the exhalation of 

 moisture is being stimulated by 

 the action of the hot sun on 

 the leaves. Moreover, as with 

 the animal, so it is with the 

 plant, the night is a time of 

 danger ; two great enemies will 

 be abroad — cold and damp — 

 and it is to guard against the 

 damage arising from these con- 

 ditions that all the j^recautions 

 of the plant are taken. To re- 

 strain ■ a plant in such a way 

 that its flowers and foliage are kept 

 expanded throughout the night, is in 

 many cases enough to bring about the 

 death of the poor su Inject. 



It must have fallen to the lot of most 



people to walk in a field of clover about 

 an hour before the sun actually dips 

 below the horizon. A very great change 

 has just come over the Uttle ])lants, for 

 instead of the trifohate leaves 

 iK'ing fully expanded to the 

 light and air. the leaflets are 

 gathered together, whilst even 

 the stalks bend shghtly towards 

 the ground. The whole appear- 

 ance of the plant suggests that 

 it has huddled itself up in 

 order that it may keep warm ; 

 and this is actually the case. 

 Experiments have shown that 

 if the leaflets of a clover plant 

 are fixed in such a way that 

 they cannot close, and are then 

 exposed to the low temperature 

 of a clear night, the delicate 

 foliage will be badly damaged. 

 On this account a wise pro- 

 vision of Nature has ordained 

 that the clover leaves shall be extra- 

 ordinarily sensitive to light ; this is so 

 much the case that even on a dull day 

 the foHage will be found to be partly 

 closed. These sleep movements are to 

 be observed not only in the clovers, but 

 as well in almost all the other members 

 of the great leguminous tribe. The pink 

 Rest Harrow, the dainty Ladies' Fingers. 



WOOD SOKKEL: NIGHT POSITION. 



the Meadow \'etchling. to menti.)n only 

 three common pod-bearing species, each 

 and all show a slight change in tlie bear- 

 ing of their foliage during the night- 

 time. 



