586 



NA TV RE 



[April 19, 1894 



luminous beam has traversed a thickness of 60 metres of the 

 gas under the pressure of two atmospheres. The dark band 

 situated near D requires for its production a thickness of 60 

 metres at a pressure of six atmospheres. It is extremely diffi- 

 cult to raise such long columns of gas to high temperatures, and 

 the better plan is to reduce the length and increase the pressure. 

 By means of an electrical method, Dr. Janssen has been able to 

 heat a column of oxygen to incandescence without sensibly 

 heating the tube containing the gas. He has used a steel tube 

 2 "2 metres long and about 6 centimetres in external diameter, 

 with aa internal diameter of 3 centimetres. This tube is able 

 to resist pressures of over 1000 atmospheres. The temperature 

 of the gas is raised by means of a platinum spiral traversing the 

 length of the tube, and insulated from it by means of a layer of 

 asbestos. Dr. Janssen will shortly give an account of the 

 results obtained when oxygen was introduced into the tube, 

 heated, and spectroscopically observed. 



Melting of the Polar Caps of Mars — In the April 

 number of Astronomy and Astro- Physics, Prof W. H. Pickering 

 calls the attention of astronomers to the fact that on May 30 

 Mars will reach the same part of its orbit with regard to the sun 

 that it did on July 12, 1892, when a series of conspicuous 

 changes were observed upon the planet's surface (see Nature, 

 vol. xlvi. p. 179). It is therefore presumable that a similar 

 series of changes will occur about the end of next month, and 

 though the planet will not be so favourably situated for ob- 

 servation, the phenomena will probably be observable with any 

 telescope of moderate size. Mars is a morning star at the 

 present time, rising an hour or two before the sun. At the end 

 of May it will be in Aquarius, and will then rise shortly after 

 midnight, and be on the meridian at about 6.30 a.m. Prof. 

 Pickering points out that the centre of the Northern Sea, around 

 which a series of striking changes of shape and colour occurred, 

 is central on May 30, at lyh. 511). Eastern Standard Time. There 

 is no reason, however, for expecting the meteorological pheno- 

 mena to occur on precisely the same date in two different years, 

 and observers would do well to take every opportunity of watch- 

 ing the planet, as it is possible that the southern ice-cap may 

 begin to melt earlier than usual. 



Ephemeris for DenninC's Comet (a 1894). — The follow- 

 ing ephemeris is given for Denning's comet in Aslronomische 

 Nachrichten, No. 3223. 



Ephemeris for Berlin Midttighl. 

 R.A. 



Decl. 



April 19 



May 



23 



25 



27 



29 



I 



3 



5 



7 



9 



II 



13 

 15 



II 4 o 

 8 19 

 12 29 

 16 30 

 20 25 

 24 14 

 27 55 

 31 32 

 35 4 

 38 32 



41 55 

 45 14 

 48 30 



51 43 



The comet's brightness on April 22 is 0'3i, that at the time 

 of discovery (March 26) being taken as unity. It is fading, and 

 on the last date given in the above ephemeris it will only be 

 about one-tenth of the original brightness, and therefore ex- 

 tremely difficult to see. 



The Spectrum of Nova Norm.'E. — Prof W. W. Campbell 

 made some visual observations of the spectrum of Nova Normae 

 during February (^j/r. TVrtc/i. 3223). On February 13 the star 

 exhibited an exceedingly faint continuous spectrum in the yellow 

 and green, and four bright lines apparently identical in position 

 and relative intensity with the bright lines at wave-lengths 575, 

 501, 496, and 486 in the spectrum of Nova Aurigse in August 

 1892. Rough measures of the two brightest lines gave the 

 positions 5013 and 4953. Prof. Campbell says that there can be 

 no doubt that the star has a nebular spectrum. 



A New Southern Comet. — Mr. Gale, of Sydney, dis- 

 covered a comet in R.A. 37° 42', Decl. 55° 35' S., on April 3. 

 This was the second comet of this year, and will therefore be 

 known as comet d. 



NO. 1277. VOL. 49I 



IRRITABILITY OF PLANTS. 



A T the last meeting of the Versammlung, or meeting of 

 ■'"^ German naturalists and physicians, Prof. Pfeffer gave an 

 address on the above subject — one which his own work has 

 done so much to elucidate. Irritability, he points out, is not an 

 exceptional characteristic found in special plants : it is a funda- 

 mental quality existing in all plants, from the highest to the 

 lowest, although its manifestations in great measure escape 

 superficial observation. The sensitiveness of a Mimosa, the 

 curling up of tendrils when touched, or the curvatures of grow- 

 ing internodes in response to light and gravitation, are well known 

 and easily observed instances of irritability. But the less 

 obvious reactions are of equal interest. Pfeffer instances the 

 remarkable researches of Hegler on the effect of mechanical 

 traction on growth stems, which when stretched by a weight, gain 

 mechanical strength through the developmentof the mechanical 

 tissues, which follows as a response to the pull to which they 

 are subjected. Pfeffer has recently shown that resistance put in 

 the way of growing roots increases enormously the energy with 

 which they grow. Other instances of adaptive stimulation 

 escape ordinary observation because of the microscopic character 

 of the reaction. For instance, the extraordinary directive in- 

 fluence of malic acid on the movement of the antherozoids of 

 ferns, or of potash salts on the movement of 'oacteria. In the 

 same way the irritability of the higher plants is commonly ex- 

 hibited by movements so slow as to be imperceptible to the 

 naked eye. It is no wonder indeed that the layman does not 

 realise that plants have the same power of reaction to stimula- 

 tion as animals. Pfeffer remarks, in a striking passage, that — 

 " Man would not have inherited such a belief, if the world of 

 plants had been visible to him from childhood as it appears 

 under the higher powers of the microscope. Then he would 

 have had constantly before his eyes the innumerable host of 

 free swimming plants and other low organisms; and the hurrying 

 bacterium turning and rushing towards its food, would have been 

 as familiar as the beast of prey springing on its victim. To 

 such eyes the growing stems and roots of the higher plants 

 would have appeared circling with a search-like movement, and 

 many other rapid reactions to stimulus would have been 

 apparent. Under the influence of a multitude of such images, 

 irritability would, without a doubt, have seemed to be a self- 

 evident and universal property of plants. " 



He goes on to point out how necessary it is to clear our judg- 

 ments in regard to reactions in which movement is the observed 

 factor. A bacterium rushing across the field of the microscope 

 moves nothing like so fast as a snail, yet it moves rapidly in 

 reference to its own minute dimensions, since it will traverse 

 three to five times its own length in a second, while man at a 

 walk only gets over half his height per second. 



The one thing common to all the varied stimulus-reactions is 

 that in each of them we recognise a phenomenon of release 

 (Auslosung-), or, to put it in familiar language, a trigger-action. 

 Stimulation is therefore release-action in living matter. 



In classing irritability among trigger-effects, we express the 

 fact that the stimulus is only the releasing agent : the nature 

 of the effect depending on the specific qualities of the organism. 

 Just as the touch of a finger may, in the case of human machines, 

 either blow up a powder magazine, start a steam-engine, or set 

 a musical-box a-playing, so in the case of plants, the same 

 stimulus produces different or even opposite effects on different 

 species. 



In machines, as in the living organism, every degree of 

 disproportion between the releasing agent and the amount of 

 energy released occurs. The latent period again is not peculiar 

 to the manner of reaction of organisms, but finds a place in 

 machines of human manufacture. In a clock set in action, a 

 period elapses before the striking of the hour (part of released 

 action) comes into play. 



It is again no peculiarity of the organism that reaction to 

 stimulus is usually adaptive, since machines are adaptive and 

 self- regulating. The adaptive character of most reactions is as 

 comprehensible as the failure of an organism to adapt itself to 

 conditions not met with in nature. A bacterium being lured 

 to certain death in a mixture of corrosive sublimate and extract 

 of meat, is an example of what is meant. 



Doubts may arise whether or no certain processes are to be 

 called release-actions. Take the case of enzymes, from the point 

 of view of the plant they serve to bring about a wide change at 

 the cost of a relatively small amount of energy. Or take a 



