FUNCTION.] AND OXALTDS. 147 



are very irregular ; the leaflets rise or fall more or less quickly, 

 and retain their position for uncertain periods. Cold water 

 poured upon it stops the motion, but it is immediately re- 

 newed by warm vapour. Of a similar nature is the singular 

 phenomenon called by Linnaeus, in his figurative language, the 

 sleep of plants. In plants with compound leaves, the leaflets 

 fold together, while the petiole is recurved, at the approach 

 of night ; and the leaflets again expand and raise themselves 

 at the return of day. In others the leaves converge over the 

 flowers, as if to shelter those most delicate organs from the 

 chill air of night. Similar phenomena have been remarked 

 by Brignoli and Morren among the Wood-sorrels of Europe. 

 The remarks of the latter botanist deserve to be quoted : 



" During the great heats of the month of June, when the 

 thermometer was at + 35 (E.) in the sun, the excitability 

 and movement of the leaves were very evident in our three 

 indigenous species of Oxalis : Oxalis Acetosella, Oxalis 

 stricta, and Oxalis corniculata. When the sun darts his 

 rays in the middle of the day directly on the leaves 

 of these plants, their three obcordate leaflets are level, hori- 

 zontal, and so placed that the margins which are directed 

 towards the point of the heart, or towards the very short 

 partial petiole, nearly touch one another ; so that then there 

 is, so to say, no space between the leaflets. This is the posi- 

 tion of repose. Now, if we strike the common petiole with 

 light but repeated blows, or if we agitate by the same means 

 the entire plant, we see, after the space of a minute, less if 

 it be very hot, more if it be cool, three phenomena take 

 place. 1. The leaflets fold themselves up along their midrib 

 just like the moveable limb of the Dionsea muscipula, in such 

 a manner that their two halves approach each other by their 

 upper surface ; the movement, therefore, in this case is from 

 below upwards, and it is a folding together. 2. Each lobe of 

 the leaflet bends inwards, so that outwardly and on its lower 

 surface it presents a convexity more or less decided. This 

 is a movement of incurvation. 3. Each partial petiole, 

 although very short, bends itself from above downwards, so as 

 to cause the leaflets to hang downwards, which then nearly 

 touch each other by their lower surface around the common 



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