Dec. 4, 1S84] 



NA TURE 



»C5 



deserves more than a passing notice in our columns. To 

 name one only of his many claims to scientific recognition, 

 he commenced meteorological observations in Stromness 

 in the year 1S22, and continued them, either there or in 

 the adjacent parish of Sandwick, to within a fortnight of 

 his death in 1884. 



He belonged to the old Norse stock in Orkney, coming 

 from the township of Clouston in Stennis. Two families 

 of this name now live in the township, having succeeded 

 to their farms, by direct descent, for over 400 years. He 

 studied in Edinburgh University, and had at first been 

 destined for the medical profession. He became a 

 Licentiate of the R.C.S. in Edinburgh in 1819, and at his 

 death was probably the father of the College. When in 

 1826 he entered on his duties as assistant and successor 

 to his father, in the combined parishes of Stromness and 

 Sandwick, there was no medical man in the latter place. 

 For nearly fifty years he acted as the local doctor, in 

 addition to his clerical duties, giving advice and medicines 

 gratis. His father had been minister of Stromness for 

 over sixty years, so that father and son had kept up a 

 continuous ministry for 120 years. He received the 

 degree of LL.D. from the University of St. Andrew's 

 many years ago. 



In the year 1862 Dr. Clouston's reputation as a careful 

 meteorological observer was so well established that 

 Admiral FitzRoy intrusted to his charge an anemometer, 

 which has been kept in constant operation for the space 

 of twenty-two years. The original instrument was re- 

 placed by a new one in 1869. A discussion of the results 

 of the first rive years' records (1863-6S) appeared in the 

 Quarterly Weather Report for 1871. In addition to his 

 regular observations and deductions therefrom, which he 

 occasionally published, he wrote an essay, " An Explana- 

 tion of the Popular Weather Prognostics of Scotland, on 

 Scientific Principles," which gained the prize allotted by 

 the Marquis of Tweeddale in 1867. His observations for 

 the last thirty years, at least, have been regularly published 

 by the Registrar-General for Scotland. 



Ur. Clouston was not only a meteorologist, but an 

 ardent follower of every branch of science which came in 

 his way. In his " Guide to the Orkney Islands," a reprint 

 of a portion of " Anderson's Guide," he modestly says, 

 " Taking the Orkney Flora, as Dr. Neill left it, to include 

 462 specimens, and adding our own contribution of 156, 

 it now contains 618 species." In archaeology he took an 

 active part in the exploration of Maes How, and the 

 House of Skaill, both of them within a walk of his home. 



Dr. Clouston leaves a widow, two sons, and two 

 daughters, but more than one member of his family 

 passed away before him. In conclusion, «-e can only say- 

 that a visit to Sandwick was ever a rare treat ; the warm 

 hospitality of the manse, and the interest of the conver- 

 sation carried on round the table, could not fail to leave 

 an impression which will not easily wear away. 



".V THE AUTUMXAL TIXTS OF FOLIAGE 



AFTER the fine display of autumnal tints which we 

 have lately seen it may, I trust, be of interest to 

 some of the readers of Nature if I give an account of 

 the chief conclusions to which I have been led by care- 

 fully studying the subject for many years. 



As a general rule the colour of leaves in their normal 

 condition depends on a variable mixture of two perfectly 

 distinct green pigments and of at least four perfectly 

 distinct yellow substances. The development of the 

 autumnal tints is mainly due to the disappearance or 

 change of the green constituents and to the production 

 of highly-coloured pigments by the oxidisation of pre- 

 viously existing very pale or colourless substances. It is, 

 in fact, due to a more or less complete loss of the vitality 

 which previously counteracted these chemical changes, 

 and the order in which the tints are developed can be 



easily explained, if we assume that the death of the leaves 

 takes place somewhat gradually. The first visible effect 

 of the reduced vitality is the change in the green pig- 

 ments. In many cases they appear to be converted into 

 colourless products, since the resulting bright yellow 

 leaves differ from the normal green in the absence of 

 chlorophyll, and merely contain the usual previously- 

 existing yellow pigments. At the same time it is quite 

 possible that an increased quantity of some of these 

 yellow substances may be formed as a product during the 

 change, but of this there is no positive proof. In the 

 case of such trees as the alder, the chlorophyll does not 

 thus disappear, but is changed by the presence of a weak 

 acid into a very stable brownish-green product which 

 resists further change. The production of bright yellows 

 or dull browns thus clearly depends on whether the chloro- 

 phyll does or does not disappear before being modified 

 by the action of acids, as may be verified experimentally 

 by exposing suitable solutions to sunlight. It is, however, 

 very clear that the manner in which it changes depends 

 very much on the conditions of the case. Thus, if chloro- 

 phyll is exposed to sunlight dissolved in bisulphide of car- 

 bon, a reddish-coloured product is formed, and though 

 this differs very greatly from the red pigment met with in 

 many autumnal leaves, it seems probable that under some 

 conditions the chlorophyll in leaves is changed by the 

 action of light into a red substance. By taking green 

 sorrel leaves and keeping them somewhat fresh by stick- 

 ing the stalks into moist ground, I found that those exposed 

 to the sun with the under side upwards turned to a bright 

 red, whereas those kept in the shade did not develop any 

 fine colouring. We may often see that partially broken 

 leaves or twigs undergo this change when all other parts 

 of the tree remain green, and this and various other facts 

 lead me to conclude that the change of chlorophyll into a 

 red product depends on a certain amount of reduced 

 vitality as well as on little-understood conditions varying 

 in different kinds of plants. Though I fully admit that 

 there are some facts not easy to understand, yet on the 

 whole it seems to me that these principles fairly well 

 explain why certain leaves turn red in autumn. Slight 

 frosts reduce their vitality in such a manner that the 

 chlorophyll is changed by the action of the light into a 

 red product. Thus, according to the character of the 

 season and the nature of the plants, the first effect of the 

 reduced vitality in the leaves is that the chlorophyll is 

 removed so as to show their normal yellow colour, or is 

 changed into a red pigment, or is altered into a compara- 

 tively stable dull brown green product. These are the 

 three extreme changes, but in many cases intermediate 

 mixed results give rise to such less perfect and well- 

 marked tints as dirty yellows and reds. 



The next series of changes is best studied in the case of 

 those leaves which in the first instance turn to a bright 

 yellow, and it appears to me that they depend mainly, if 

 not entirely, on the production of deeply-coloured pig- 

 ments by the oxidisation of tannic acid and other more or 

 less colourless substances. The difference in the resulting 

 tint seems to depend on the nature of these substances. 

 Thus, for example, the tannic acid in the yellow oak 

 leaves changes into a brown substance, whereas the quino- 

 tannic acid in yellow beech leaves changes into the fine 

 orange-brown colour which makes those trees so orna- 

 mental in autumn. On the contrary, the bright yellow 

 poplar leaves rapidly pass to a dark dirty brown by the 

 alteration of another constituent. Other kinds of leaves 

 give rise to tints of an intermediate and less well-marked 

 character. In many cases it is almost impossible to 

 draw the line between the colour of this stage in the 

 change and the final dark and dirty browns of dead and 

 decaying leaves. For fine effect very much depends upon 

 the production of each special tint in a fairly pure state, 

 so as to show bright yellows, reds, and browns. This 

 seems to be influenced by the character of the weather. 



