Nov. 1 6, 1888.] 



SCIENTIFIC NEWS. 



505 



ON VERNATION AND THE METHODS 

 OF DEVELOPMENT OF FOLIAGE, 

 AS PROTECTIVE AGAINST RADIA 

 TION. 



A Paper read by the Rev. George Henslow, M.A., 



F.L.S., F.G.S., before the Linnean Society, and 



Reprinted from the Journal by permission of 



the Society. 



TN his work on "The Movements of Plants," Mr. Dar- 



■*■ win says : — " The fact that the leaves of many plants 



place themselves at night in widely different positions 



from what they hold during the day, but with the one 



point in common, that their upper surfaces avoid facing 



the zenith, often with the additional fact that they come 



into close contact with opposite leaves or leaflets, clearly 



indicates, as it seems to us, that the object gained is the 



protection of the upper surfaces from being chilled at 



night by radiation."* 



He furthers addsf : — " 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 dif- 

 erence 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 horizontal were 

 coated with large beads of dew. This shows 

 how much cooler the leaflets freely ex- 

 posed 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 tempera- 

 ture, 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 grow- 

 ing leaves, as it seems to me, is the same, namely, to 

 shield their upper surfaces especially from injury 

 caused by radiation — viz., chill and desiccation. More- 

 over, 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 

 Melilotus 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 text-books 

 on botany, and on page 33 of this volume of Scientific 

 News, is solely concerned with the various methods 

 by which the rudimentary leaves are folded up in the 



* P. 284. 



t Pp. 293. 294- 



Fig. 1. — 



Portugal 



Laurel. 



buds of different plants ; and the internal wooliness, as 

 well as external resinous matters, etc., 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 

 vernation ; nor has any one noticed the special 

 behaviour of developing leaves and stipules, as well as 

 of the axes and petioles which support them. If, how- 

 ever, we keep in view the two assumed objects — first, 

 the securing an erect or dependent position of the blades 

 so as to place their surfaces in a plane at right angles to 

 the surface of the earth ; and secondly, the protection of 

 the upper surfaces, — it will be seen how various are the 

 efforts of nature to secure these two ends during the 

 period of development, and while the young leaves and 

 shoots are succulent and delicate, such being only too 

 readily, and often severely, injured by the cool nights, 

 sharp frosts, and cutting winds of spring, at the very 

 time when the buds are expanding. The various 

 methods of protection are more perfect than in sleep- 

 ing plants, inasmuch as the young leaves are more deli- 

 cate than when adult. Testing the effects of desiccation 

 by the heat of the sun, I found that when young clover- 

 leaves, naturally conduplicate, were forcibly spread out 

 with the upper surfaces exposed to the sun for twenty 

 minutes, the edges soon curled inwards, and they lost 

 37 - 2 per cent, of weight. Those spread out with their 



Fig. 2. Fig. 3. Fig. 4. 



The Lime in Different Stages of Development. 



under surfaces exposed remained flat, but lost 43-2 per 

 cent, of weight in the same time, i.e., rather more than 

 the preceding. In all experiments with clover-leaves 

 exposed at night, I found the differences were not so pro- 

 nounced as in many other plants. 



In observing the effects of frost upon delicate leaves 

 one sees that they are especially injured along the mar- 

 gins ; and, moreover, where so affected, they are more 

 or less dried up. The first fact gives a significance to 

 the revolute and involute kinds of vernation, in which 

 the margins are rolled outwards and inwards re- 

 spectively. This led me to suspect that whenever leaves 

 suffered from radiation not only was there a reduction of 

 temperature, but a loss of moisture would seem to 

 accompany the loss of heat. A similar loss of moisture 

 would occur by desiccation, in consequence of cool dry 

 winds, and must be equally guarded against. If this 

 were so, then the balance would indicate the loss. With 

 this object in view, I weighed a number of undeveloped 

 leaves together of several plants, and made two groups 

 of each, selecting leaves as nearly like as possible for 

 each group of the same plant. I, then, following Mr. 

 Darwin's method, fixed all of one group with their upper 

 surfaces exposed upwards with card-clips, such as are 

 use by entomologists for spreading out the wings of 

 butterflies ; the other group of leaves were laid on cork, 



