AS PROTECTIVE AGAINST RADIATION. 625 



He further adds*: — "We exposed on two occasions during the 

 summer to a clear sky several pinned-open leaflets of Trifolium 

 pratense, which naturally rise at night, and of Oxalis purpurea, 

 which naturally sink at night (the plants growing out of doors), 

 and looked at 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 were 

 sometimes absolutely dry, whilst the leaflets which had been hori- 

 zontal were coated with large beads of dew. This shows how 

 much cooler the leaflets freely 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." 



It is the object of the present communication to show that 

 many, perhaps the majority of, leaves as they develop on the 

 expansion of buds in spring assume the same positions as leaves 

 of those particular plants which are remarkable for sleeping at 

 night, and appear to exhibit a similar care, but in a more perfect 

 way, in protecting the upper surfaces ; and the inference to be 

 drawn from all the phenomena presented by young growing leaves, 

 as it seems to me, is the same, namely, to shield their upper sur- 

 faces especially from injury caused by radiation, viz. chill and 

 desiccation. Moreover, it is not an unusual thing for the young 

 leaves only to be subject to hypnotism, the older ones ceasing to 

 rise and fall by day and night : thus Mr. Darwin mentions Me- 

 lilotus as an instance ; and I find it is the same with the French 

 Bean. Lastly, experiments made to test this theory are decidedly 

 corroborative. 



Vernation, as explained and illustrated in textbooks on 

 Botany, is solely concerned with the various methods by which 

 the rudimentary leaves are folded up in the buds of different 

 plants ; and the internal wooliness, as well as external resinous 

 matters &c, are usually regarded as being safeguards against 

 injury to the buds within from the severity of winter weather. 

 Beyond that surmise, I am not aware that any attempt has 

 ever been made to offer any rationale of the processes of verua- 

 * Pp. 293, 294. 



