CHAP. VL USE OF SLEEP MOVEMENTS. 295 



exposed obliquely to the zenith, nevertheless had these 

 lower surfaces less blackened than the upper surfaces 

 which were turned inwards and were in close contact 

 with those of the opposite leaflets. Again, a pot 

 full of plants of Trifolium resupinatum, which had 

 been kept in a warm room for three days, was turned 

 out of doors (Sept. 21st) on a clear and almost frosty 

 night. Next morning ten of the terminal leaflets were 

 examined as opaque objects under the microscope. 

 These leaflets, in going to sleep, either turn vertically 

 upwards, or more commonly bend a little over the 

 lateral leaflets, so that their lower surfaces are more 

 exposed to the zenith than their upper surfaces. 

 Nevertheless, six of these ten leaflets were distinctly 

 yellower on the upper than on the lower and more 

 exposed surface. In the remaining four, the result 

 was not so plain, but certainly whatever difference 

 there was leaned to the side of the upper surface 

 having suffered most. 



It has been stated that some of the leaflets experi- 

 mented on were fastened close to the cork, and others 

 at a height of from | to f of an inch above it ; and 

 that whenever, after exposure to a frost, any difference 

 could be detected in their states, the closely pinned 

 ones had suffered most. We attributed this difference 

 to the air, not cooled by radiation, having been pre- 

 vented from circulating freely beneath the closely 

 pinned leaflets. That there was really a difference in 

 the temperature of leaves treated in these two dif- 

 ferent methods, was plainly shown on one occasion ; 

 for after the exposure of a pot with plants of Melilotus 

 dentaia for 2 h. to a clear sky (the temperature on the 

 surrounding grass being - 2 C.), it was manifest that 

 more dew had congealed into hoar-frost on the closely 

 pinned leaflets, than on those which stood horizontally 



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