294 MODIFIED CIECUMNUTATIOy. CHAP. VL 



them early on several successive mornings, after they 

 had assumed their diurnal positions. The difference 

 in the amount of dew on the pinned-open leaflets 

 and on those which had gone to sleep was generally 

 conspicuous ; the latter being sometimes absolutely 

 dry, whilst the leaflets which had been horizontal 

 were coated with large beads of dew. This shows how 

 much cooler the leaflets fully exposed to the zenith 

 must have become, than those which stood almost 

 vertically, either upwards or downwards, during the 

 night. 



From the several cases above given, there can be no 

 doubt that the position of the leaves at night affects 

 their temperature through radiation to such a degree, 

 that when exposed to a clear sky during a frost, it is a 

 question of life and death. We may therefore admit 

 as highly probable, seeing that their nocturnal posi- 

 tion is so well adapted to lessen radiation, that the 

 object gained by their often complicated sleep move- 

 ments, is to lessen the degree to which they are 

 chilled at night. It should be kept in mind that 

 it is especially the upper surface which is thus pro- 

 tected, as it is/never directed towards the zenith, and 

 is often brotfght into close contact with the upper 

 surface of an opposite leaf or leaflet. 



We failed to obtain sufficient evidence, whether 

 the better protection of the upper surface has been 

 gained from its being more easily injured than the 

 lower surface, or from its injury being a greater evil 

 to the plant. That there is some difference in consti- 

 tution between the two surfaces is shown by the foil >w 

 ing cases. Cassia floribunda was exposed to a clear sky 

 on a sharp frosty night, and several leaflets which 

 had assumed their nocturnal dependent position with 

 their lower surfaces turned outwards so as to be 



