114 NUTATIONS, ETC. [CH. 



night with the accession to the cells of such osmotic 

 substances as sugars, acids, &c., as the sun goes down, or 

 whether the light acts more directly on the protoplasm 

 controlling the cell-vacuoles, are points not yet definitely 

 settled by experiment. 



Even more obscure, though no less certainly existing, 

 are other movements of leaves known as spontaneous, 

 and which are also doubtless to be traced to changes in 

 the turgescence of the cells of the pulvinus. 



The one feature they have in common is that they are 

 apparently due to internal causes and occur independently 

 of variations in the physical environment, so far as can be 

 discovered. 



At present, then, and usefully for our purposes, we 

 may divide these movements into two chief categories, 

 inasmuch as some of them are brought about by differ- 

 ences in the rate of growth of two sides of the whole leaf, 

 or of parts of it, and cease altogether when the leaf is fully 

 developed and has ceased to grow; whereas the others the 

 true spontaneous movements occur by periodic changes 

 going on in the fully-grown leaf, and are traceable to tran- 

 sient changes in the turgidity of the cells of the pulvinus. 



The first of these categories results in nutations, or 

 nodding movements of the leaf, owing to waves of 

 enhanced growth affecting successively different sides of 

 the petiole, or of the lamina, or of both together. Since 

 these local accessions of growth usually prevail alternately 

 above and below more than at the opposite sides, the 

 course described by the tip of the whole leaf is usually a 

 very narrow ellipse, with the long axis vertical, and ap- 

 proaches closely to a mere up and down movement in 

 the vertical plane, but in some cases e.g. Camellia, 

 Eucalyptus, and especially Cissus, &c. the ellipses are 

 much wider, and in the last case, as also in many other 

 twining plants, approach the circular and spiral form. 



