154 ON THE POWER OF MOVEMENT IN PLANTS. 



the petioles of course very greatly assists to this end^ while the 

 vertical position of the leaves themselves acts in a like direction. 

 In many plants exposed to a clear sky on a frosty night, this 

 alteration of position means simply life or death. Darwin by many 

 beautiful experiments has conclusively proved this. 



Many other instances of the periodic movements of leaves 

 might be here adduced, had I space. From the age of Pliny, who 

 first noticed it, down to the time when Linnaeus wrote his Soimius 

 plafitarum, the list of known cases has been increasing, and is 

 destined to increase. If som^e few enterprising seekers after fact 

 and truth could arrange a series of all-night watches between them 

 on different trees and plants, I have no doubt many would be 

 added to the list. We find one rule pretty generally operating ; 

 it is this : that plants that sleep do not get a good night's rest unless 

 they have been exposed to a proper degree of temperature the day 

 before, the degree varying of course in nearly every case. The 

 leaves of French Bean sleep better in full summer than in 

 early summer. A violent wind-shaking will sometimes keep species 

 oiMaranta (Arrow-root) awake for two nights in succession. Among 

 a perfect host of plants whose leaves sleep, we may just name the 

 following : Stitchwort, Mallow, Flax, Wood-Sorrel, Balsam, Ti'opoz- 

 olwn^ Lupine, Clovers, Loftis, Acacias, Wistaria, Milk-vetch, and 

 many Leguiimioscn ; Evening Primrose {CEnothera), Passion-flower, 

 Tobacco-plant, Polygonum, Goosefoot, Spurge, Arrowroot, and 

 only one among Cryptogams, namely Marsilea or Pepperwort. 



II. — Sleep of Flowers. The ' Floral Clock ' of good old Linnaeus 

 was a happy idea, but it must be taken cum gra?io, because a dull 

 day or a bright one, a dry morning or a moist one will often 

 modify the accuracy of the statements. Still, there is no doubt 

 that certain flowers have a tendency, in many cases very pronounced, 

 to open and close at specific times, or within a few minutes of 

 those times. 



A sufficiently accurate idea of these times may be gained from 

 the following list, made out by Linnaeus, but abbreviated by De 

 Candolle, who confirmed each instance : — 



Purple Convolvulus ... ... opens at 2 a.m. 



Great Bindweed „ „ 3 — 4 a.m. 



Chicory , „ 5 „ 



