CHAP. VIII. PAKAHELIOTROPISM. 447 



or twist laterally, so as to place their edges in the direction of the 

 sun's light; but Cohn long ago observed that the leaflets of 

 Oxalis bend downwards when fully exposed to the sun. We 

 witnessed a striking instance of this movement in the very 

 large leaflets of 0. Orteyesii. A similar movement may fre- 

 quently be observed with the leaflets of Averrhoa Ulimbi (a 

 member of the Oxalidse) ; and a leaf is here represented (Fig. 

 180) on which the sun had shone. A diagram (Fig. 134) was 

 given in the last chapter, representing the oscillations by which 

 a leaflet rapidly descended under these circumstances ; and the 

 movement may be seen closely to resemble that (Fig. 133) by 



Fig. 180. 



Averrhoa bilimbi : leaf with leaflets depressed after exposure to sunshine ; 

 but the leaflets are sometimes more depressed than is here shown. 

 Figure much reduced. 



which it assumed its nocturnal position. It is an interesting 

 fact in relation to our present subject that, as Prof. Batalin 

 informs us in a letter, dated February, 1879, the leaflets of 

 Oxalis acctoselia may be daily exposed to the sun during many 

 weeks, and they do not suffer if they are allowed to depress 

 themselves ; but if this be prevented, they lose their colour and 

 wither in two or three days. Yet the duration of a leaf is about 

 two months, when subjected only to diffused light ; and in this 

 case the leaflets never sink downwards during the day 



As the upward movements of the leaflets of Kobinia, 

 and the downward movements of those of Oxalis, have 

 been proved to be highly beneficial to these plants 

 when subjected to bright sunshine, it seems probable 

 that they have been acquired for the special purpose 

 of avoiding too intense an illumination. As it would 

 have been very troublesome in all the above cases to 

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