Sept. 14, 1882] 



NA TURE 



483 



(CAus fatuellns) from Guiana, a Red-billed Toucan (Ramphastcs 

 erythoryhnchus) from Cayenne, purchased ; a Smooth Snake 

 (Corondla /avis), European, received in exchange. 



OUR ASTRONOMICAL COLUMN 



Definitive Comet-oriuts. — I. The fourth comet of 1874 

 (Coggia, April 17). Dr. Hepperger, of Vienna, has investi- 

 gated the orbit of this comet from the whole extent of observa- 

 tion, founding his work upon 17 normals from 638 observed 

 positions. He finds the orbit an ellipse with period of 13,708 

 years, and considers that his results exclude equally a parabola 

 and any ellipse with a revolution shorter than Sooo years. The 

 aphelion distance is 1144-9 (the earth's mean distance from the 

 sun being taken as unity), at the descending node the radius- 

 vector is 0717, near the orbit of Venus, ami at ascending node 

 it is 11 734. Loggia's comet became visible to the naked eye 

 at the beginning of June, and so continued unt:l it was lost in 

 thee latitudes in the middle of July, when the tail had gradually 

 increased to 23°. 



2. Definitive elements have also been determined for the 

 second comet 1 f 1847, by M. Folke Engstrom of Lund. The 

 co net was discovered by Colla at Parma, on May 7, and was last 

 observed by the late Mr. I.assell at Starfield, Liverpool, on 

 December 30, or over a period of nearly eight months. The 

 orbit is chiefly remarkable for the large perihelion distance, 2 - l 15, 

 which has been exceeded in very few cases. The resulting 

 elements are hyperbolic c — 10006549 So far as we know 

 this i- the only instance where the latest observations for posi- 

 tion have been obtained with a reflector, the statement that has 

 been more than once made that Halley's comet in 1836 was last 

 obsfrved by Sir John Herschel with his 20-feet reflector at 

 Fel 'hansen, Cape Colony, being a mistake ; the last observation 

 wa< made by Lamont with the 11 -inch refractor at Munich on 

 May 17. 



The Vakiaule Stak Algol.— The following are the 

 Greenwich times of minima of Alcaol, occurring before 15I1., 

 during the last quarter of the present year, taking Prof. 

 Winnecke's ephemeris as authority : — 



Oct. 14, 13 o 

 17, 9 49 

 20, 6 38 



Nov. 3, 14 42 

 6, 11 31 



Nov. 9, 8 20 



26, 13 13 



29, 10 2 



Dec. 2, 6 51 



Dec. 16, 14 55 



19, II 44 



22, 8 33 



25, 5 22 



The Motion of 61 Cl'GNI. — The following formulae appear 

 to represent the observations of this remarkable system up to 

 the present epech within about their probable errors ; P is the 

 angle of p >sition, D the distance : — 



D sin P = + i6 : 4657 + [8-63013] (/ - 1850-0) 

 I> c. s P = - 3-6892 - [9-27178] (/ - 1850-0). 

 Hence v e find — 



Diff. R A. Diff. Deck 



The Comet of 1763. — The comet ob.erved by Dunbp at 

 Paramatta in 1833 has been referred to as affnding an instance 



of near approach to the earth's orbit at both nodes ; according 

 to Dr. Hartw ig's elements the distance at ascending node is 

 0-092, and at descending node 0'l86. But a much more notice- 

 able case is offered by the comet of 1763. In Burckhardt's 

 ellipse we find the distance at ascending node 0-0315, and at 

 descending node 0-0252, the time occupied in passing from 

 node to node is 77-2 days. 



THE EXCITABILITY OF PLANTS 1 

 II. 

 ""THE complete knowledge we have gained from our study of 

 -*■ the anther filaments of Centaurea of the mechanism of 

 the excitable plant cell, can he applied to every other known 

 example of irrito-contractility in the organs of plants, and par- 

 ticularly to that most remarkable of all such structures, the leaf 

 of Dionaa muicipula. Although I described the structu e of 

 the leaf just eight years ago in this room, I w il occupy a moment 

 in repeating the descripti in. The blade of the leaf is united on 

 to the stalk by a Utile cylindrical joint. Here are two models, 

 in one of which the leaf is represented in its closed state, in the 

 other in which it is in its unexcited or open state. The leaf is 

 everywhere contractile — that is, excitable by transmission, but 

 not everywhere susceptible of direct excitation — or, in common 

 language, sensitive. It is provided villi special organs, of which 

 we do not find the counterpart in any of the plants to which 

 reference has been made, for the reception of external impres- 

 sions — organs which, from their structure and position, can have 

 no other function. 



The ac ion of the leaf, to which the p'ant owes its name, and 

 by which it seizes its prey, is, in its general character, too well 



Fig. 6.— Transverse section ofl ,be of leaf of D.o 



comprising the root --f 



known to require description.' In the shortest possible terms, it 

 is the sudden change of the outer surface of each bbe of the 

 leaf from convex to concave, and at the same time the cro sing 

 of the two series of marginal hairs, as the fingers cross when 

 the hands are clasped. What I de-i e to show with respect to 

 it is, that here also the agents are individual cells — that is, that 

 the individual elements out of which the whole structure is 

 built are the immediate agents in the production of the 

 movement. 



A cross-section of the leaf shows the following facts: If the sec- 

 tion be made in the direction of the parallel fibro-vascular 

 bundles which run out from the mid-rib nearly at right angles, 

 and happen to include one of these bundles, it i- seen that it 

 con-ists of three parts, viz. the fibro-vascular bundle in the 

 middle and equidi-tant from both borders ; of the cylindrical 

 cells of the parenchyma on either \ ide, and of an external and 

 internal epidermis. The external epidermis is smooth and 

 glistening, and its cells possess thicker walls than those on the 

 opposite surface. 



1 Lecture delivered at the Royal Institution June 9. 1882, by Prof. Eurd n 

 Sanderson, F.R.S. Continued from p. 356. 



