290 MODIFIED CIKCUMNUTATION. Chap. VI. 



another night for 1 h., when the temperature of the grass was 

 - 4° C. ; and now all the leaves on a large bush, whether pinned 

 fiat open or free, were killed, blackened, and shrivelled, with 

 the exception of those on one small branch, low down, which 

 was very slightly protected by the leaves on the branches 

 above. Another tall bush, with four of its large compound 

 leaves pinned out horizontally, was afterwards exposed (temp, 

 of surrounding grass exactly the same, viz., - 4° C), but only 

 for 30 m. On the fo'Iowing morning every single leaflet on 

 these four leaves was dead, with both their ujjper and lower 

 surfaces completely blackened. Of the many free leaves on the 

 bush, only seven were blackened, and of these only a single one 

 (which was a younger and more tender leaf than any of the 

 pinned ones) had both surfaces of the leaflets blackened. The 

 contrast in this latter respect was well shown by a free leaf, which 

 stood between two pinned-open ones; for these latter had the 

 lower surfaces of their leaflets as black as ink, whilst the inter- 

 mediate free leaf, though badly injured, still retained a plain 

 tinge of green on the lower surface of the leaflets. This bush 

 exhibited in a striking manner the evil effects of the leaves not 

 being allowed to assume at night their normal dependent posi- 

 tion; for had they all been prevented from doing so, assuredly 

 every single leaf on the bush would have been utterly killed by 

 this exposure of only 30 m. The leaves whilst sinking down- 

 wards in the evening twist round, so that the upper surface is 

 turued inwards, and is thus better protected than the outwardly 

 turned lower surface. Nevertheless, it was always the upper 

 surface which was more blackened than the lower, whenever 

 any difference could be perceived between them ; but whether this 

 was due to the cells near the upper surface being more tender, 

 or merely to their containing more chlorophyll, we do not know. 

 MeHIutus officinalis. — A large pot with many plants, which 

 had been kept during the winter in the greenhouse, was exposed 

 during 5 h. at night to a slight frost and clear sky. Four 

 leaves had been pinned out, and these died after a few days ; 

 but so did many of the free leaves. Therefore nothing certain 

 could be inferred from this trial, though it indicated that the 

 norizontally extended leaves suffered most. Another large pot 

 with many plants was next exposed for 1 h., the temperature on 

 the surrounding grass being lower, viz., — 3° to — 4° C. Ten 

 leaves had been pinned out, and the result was striking, for 

 on the following morning all these were found much injured or 



