CuAP. VI. USE OF SLEEP MOVEMENTS. 289 



purpurea) were exposed for 2 li. and 3 h. to a clear sky with the 

 temp, on grass — 2° C, and none of the leaves, whether free or 

 pinned open, were at all injured. 



Arachls Jtypogcea. — Some plants in a pot were exposed at night 

 for 30 m. to a clear sky, the temperature on the surrounding 

 grass being — 2° C, and on two nights afterwards they were again 

 exposed to the same temperature, biit this time during 1 h. 30 m. 

 On neither occasion was a single leaf, whether pinned open or 

 free, injured ; and this surprised us much, considering its native 

 tropical African home. Two plants were next exposed (March 

 16th) for 30 m. to a clear sky, the temperature of the surrounding 

 grass being now lower, viz., between — 3° and — 4° C, and all 

 four pinned-open leaves were killed and blackened. These two 

 plants bore 22 other and free leaves (excluding some very young 

 bud-like ones) and only two of these were killed and three some- 

 what injured; that is, 23 per cent, were either killed or injured, 

 whereas all four pinned open leaves were utterly killed. 



On another night two pots with several plants were exposed 

 for between 35 m. and 40 m. to a clear sky, and perhaps to a 

 rather lower temperature, for a thermometer on a dial, 3 feet 

 high, close by stood at — 3'3° to — 3"8° C. In one pot three 

 leaves were pinned open, and all were badly injured ; of the 

 44 free leave§, 26 were inj^ired, that is, 59 per cent. In the 

 other pot 3 leaves were pinned open and all were killed ; four 

 other leaves were prevented from sleeping by narrow strips of 

 stiff paper gummed across them, and all were killed ; of 24 free 

 leaves, 10 were killed, 2 much injured, and 12 iinhurt; that is, 

 50 per cent, of the free leaves were either killed or much in- 

 jured. Taking the two pots together, we may say that rather 

 more than half of the free leaves, which were asleep, were either 

 killed or injured, whilst all the ten horizontally extended leaves, 

 which had been prevented from going to sleep, were either killed 

 or much injured. 



Cassia floribunda. — A bush was exposed at night for 40 m. to 

 a clear sky, the temperature on the surrounding grass being 

 — 2° C, and not a leaf was injured.* It was again exposed on 



• Cassia Ixvigaia was exposed injured. But when 0. Ixvigata 



to ft cleiir sky fur 35 m., and C- was exposed fur 1 li., the temp. 



cctUiantlut (a Guiana species) for on the suriounding gi-ass being 



00 m., the temperature on the bi-tween — 3° and — i° C, everj 



Burrounding grass being — 2° C, leaf was killed. 

 and neither were in the least 



