March 



88r] 



NA TURE 



409 



original and startling ; but it involves a deliberate renun- 

 ciation of the exercise of reason. 



The translation of Prof. Semper' s highly entertaining 

 and really valuable and suggestive book has been remark- 

 ably well executed. Throughout great care has been 

 taken to give the correct English equivalents for the 

 German names of many obscure animals, and to preserve 

 the sense of the original. At the same time there is not 

 from beginning to end any trace of that awkward diction 

 which sometimes infects a translation from the German. 

 It is not too much to say that it is the best e.xecuted 

 translation of a foreign work on science which has 

 appeared for twenty years. E. Ray Lankester 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, or 

 to correspond with the writers of rejected mamtscripts. No 

 notice is taken of anonymous communications. 

 The Editor urgently requests correspondents to keep their letters as 

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 is impossible otherwise to ensitre the appearance even op com- 

 munications containing inteir^ting and ncrjel pacts.} 



Movements of Plants 



Fritz Muller, in a letter from St. Catharini, Brazil, dated 

 January 9, has given me some remarkable facts about the move- 

 ments of plants. He has observed striking instances of allied 

 plants, which place their leaves vertically at night, by widely 

 different movements ; and this is of interest as supporting the 

 conclusion at v\ hich my son Francis and I arrived, namely, that 

 leaves go to sleep in order to escape the full effect of radiation. 

 In the great family of the Graminere the species in one genus 

 alone, namely Strephium, are known to sleep, and this they do 

 by the leaves moving vertically upwards ; but Fritz Muller finds 

 in a species of Olyra, a genus which in Enlicber's "Genera Plan- 

 tarum" immedi.itely precedes Strephium, that the leaves bend 

 vertically down at night. 



Two species of Phyllanthus (Euphorbiacere) grow as weeds 

 near Fritz Muller's house ; in one of them with erect branches 

 the leaves bend so as to stand vertically up at night. In die 

 other species with horizontal branches, the leaves move vertically 

 down at night, rotating on their axes, in the same manner as do 

 those of the Leguminous genus Cassia. Owing to this rotation, 

 combined with the sinking movement, the upper surfaces of the 

 opposite leaflets are brought into contact in a dependent posi- 

 tion beneath the main petiole ; and they are thus excellently 

 protected from radiation, in the manner described by us. On the 

 following morning the leaflets rotate in an opposite direction, 

 whilst rising so as to resume the diurnal horizontal po^ition with 

 their upper surface exposed to the light. Now in some rare 

 cases Fritz Muller has observed the extraordinary fact that three 

 or four, or even almost all the leaflets on one side of a leaf of 

 this Phyllanthus rise in the morning from their nocturnal verti- 

 cally dependent position into a horizontal one, without rotating, 

 and on the wrong side of the main petiole. These leaflets thus 

 project horizontally with their upper surfaces directed towards 

 the sky, but partly shaded by the leaflets proper to this side. 

 I have never before heard of a plant appearing to make a 

 mistake in its movements ; and the mistake in this instance is a 

 great one, for the leaflets move 90° in a direction opposite to the 

 proper one. Fritz Muller adds that the tips of the horizontal 

 branches of this Phyllanthus curl downwards at night, and thus 

 the youngest leaves are still better protected from radiation. 



The leaves of some plants, when brightly illuminated, direct 

 their edges towards the light ; and this remarkable movement I 

 have called paraheliotropism. Fritz Muller infonns me that the 

 leaflets of the Phyllanthus just referred to, as well as those of 

 some Brazilian Cassife, " take an almost perfectly vertical posi- 

 tion, when at noon, on a summer day, the sun is nearly in the 

 zenith. To-day the leaflets, though continuing to be fully ex- 

 posed to the sun, now at 3 p.m. have already returned to a nearly 

 horizontal position." F. Muller doubts whether so strongly 

 marked a case of paraheliotropism would ever be observed 

 under the duller skies of England ; and this doubt is probably 

 correct, for the leaflets of Cassia neglecta, on plants raised from 



seed formerly sent me by him, moved in this manner, hut so 

 slightly that I thought it prudent not to give the case. With 

 several species of Hedychium, a widely diftVrent j^taraheliotropic 

 movement occurs, which may be compared with that of the leaf- 

 lets of Oxalis and Averrhoa ; for " the lateral halves of the leaves, 

 when exposed to bright sunshine, bend downwards, so that they 

 meet beneath the leaf." Charles Darwin 



Down, Beckenham, February 22 



Barometric and Solar Cycles 



Regarding one of the conclusions drawn by Mr. F. 

 Chambers in his paper on "Abnormal Variations of the 

 Barometer in the Tropics," and Dr. Balfour Stewart's remarks 

 concerning the same in the first article of Nature (vol. xxiii. 

 p. 237), I and other meteorologists would like very much to 

 know which side of the earth is to be considered the east, and 

 which the west. 



In other words, if waves of high barometer travel slowly from 

 west to east, on what meridian do they commence, and is there 

 any reason why they should commence on one meridian more 

 than on another ? The only reason that I can think of is that 

 some meridians embrace more land than others ; but in this 

 respect the meridians passing through the centres of America, 

 Europe-Africa, and East Asia-Australia are very much alike. 

 Again, if barr)metric changes originate, say at St. Helena, and 

 travel slowly east-^ards, as Mr. Chambers supposes, they 1 Ui^ht 

 after several months to reappear on the meridian from which 

 they started, but Mr. Chambers's paper gives no evidence of this 

 whatever. 



Dr. Balfour Stewart says it is unmistakably indicated by all 

 the elements that the connection between the state of the sun's 

 surface and terrestrial meteorology is of such a nature as to 

 imply that the sun is most powerful when there are most sjiots 

 on his surface. The barometric evidence, however, is all the 

 other way. 



Mr. Blanfird, following up a suggestion originally made by 

 the present writer, has shown clearly enough that the decemual 

 variation of the Iieight of the barometer has nearly opposite 

 phases in the Indo-Malayan region and in Western Siberia, 

 especially it the winter season, when the pressure is higher 

 over Siberia than in South-Eastern Asia, he considered alone 

 (Nature, vol. xxi. p. 480). From Mr. Planford's paper it is 

 clear that the barcimetrical differences, on which the strength of 

 the winds defends, are greater when the sun-spot area is small 

 than when it is large. 



The true relation between the variations of sun-spot area, solar 

 radiation, and barometric pressure will, I feel confident, be 

 soon discovered through the agency of the United States 

 Weather Maps in the manner pointed out by you at page 567, 

 vol. xxi., in discus ing the United States Weather Map for July, 

 1878. It is there shown that in the middle of summei' in tie 

 last year of minimum sun-spot, the pressure of the air was below 

 the average over all the great continents, and above it over the 

 neighbouring oceans. In India, it is true, the pressure was 

 above the average ; but then India is not Asia, but merely a 

 narrow triangular peninsula surrounded on two sides by the 

 ocean, and on the third by a broad zone of snow-covered 

 mountains w hich may be likened to an oceanic area as far as 

 constancy of temperature is concerned. 



Meteorologists will aU agree with Dr. Balfour Stewart that 

 "unexceptionable observati' ns of the sun's intrinsic heat-giving 

 power, if these could be obtained, would furni-h a more trust- 

 worthy instrument of prevision than the sun-spot record." We 

 may soon hope for a nearly continuous series of such observa- 

 tions, for, according to the hst published Administration Report 

 of the Indian Meteorological Department, a trustworthy form of 

 actinometer is being sent to Leh, 11,500 feet above the sea, in 

 the dry region of Tibet, v here observations will be taken with it 

 under the superintendence of Mr. Ney Elias. 



Meantime Ave may perhaps adopt what is considered by Mr. 

 Blanford the best criterion of the sun's heating power which can 

 be obtained fronr ordinary meteorological observations, viz. the 

 highest excess of the vacuum black-bulb thermometer above the 

 maximum in shade for each month. At ten stations in India 

 where comparable thermometers have been used since 1875, the 

 mean maximum solar excess has been : — 



1875 

 67°-o 



6f-2 



1877 

 68'-S 



68°-i 



