340 



NATURE 



[August 9, 1900 



plant — is readily induced to m-\ke its appearance from 

 the cut ends of the stems and leaves of these plants. 

 Prof. Correns has done a useful service in bringing 

 together, in a classified manner, the numerous methods 

 employed by mosses to ensure their propagation and 

 dispersal by means less expensive than by the production 

 of spores. The readily friable stems of some species of 

 Ani&eaea, the easily detached branchlets of Dicranum, 

 are instances, well known to muscologists, of a large 

 class of propagative bodies. These simpler forms of re- 

 production are also widely spread amongst plants other 

 than mosses, and in some cases — e.g. Lycopodium Selago 

 — the superficial resemblance is rather striking. Less 

 obvious are the subterranean bulbils or buds, such as 

 are met with in Dicranella, Batbula, or Funaria, in 

 which special tuberous bodies are formed. Dicranella 

 heieromalla affords a pretty example of a form transi- 

 tional from the simple to the more complex types, inas- 

 much as the subterraneous bulbils of this moss are little 

 more than rows of swollen rhizoid-cells arranged some- 

 what like a string of beads. Many of these bulbils are 

 regarded by Correns rather as of the nature of food 

 reservoirs than as brood bodies ; but it is at least certain 

 that they are in most cases able to function in the latter 

 capacity as well as in that of mere storehouses of food- 

 reserves. 



Other and very common cases of brood bodies are 

 afforded by the so-called ''^ folia yrrto^/Zza "—leaves which 

 readily become detached from the parent plant, and with 

 greater or less intervention of protonematal filaments 

 give birth to new individuals. Oftentimes the leaves 

 destined to this end undergo considerable contraction in 

 size, and, indeed, may assume a totally rudimentary 

 appearance. 



Again, as in some species of Orthotrichum, cells grow 

 out from the ends of leaves, and the sausage-shaped pro- 

 liferations, after detachment from the parent plant, grow 

 out to filaments, on which new plants arise. 



The above are only a few of the many forms cited by 

 Correns of gametophytic reproductions in the mosses by 

 vegetative means. But as Pringsheim long ago pointed 

 out, it is also possible to reproduce these plants from the 

 sporophyte generation, especially from cut fragments of 

 the seta or stalk of the moss-capsule. These are far more 

 interesting, as they resemble the curious aposporic de- 

 velopment met with in a number of ferns. Indeed, these 

 latter offer, perhaps, a means of attacking the details of 

 the phenomena of apospory with a greater chance of 

 success than in the case of the ferns, since they seem 

 more easily induced by simpler experimental devices than 

 is the case with the higher plants. 



A general synopsis of the various types and forms of 

 brood-bodies forms a useful adjunct to the main de- 

 scriptive part of.a book on which the author has evidently 

 expended much labour, and which should earn for him the 

 gratitude of all those muscologists who are not merely de- 

 scribers of species, as well as of botanists who seem too 

 often rather to be disposed to ignore an important section 

 of the vegietable kingdom. 



Village Notes, and Some Other Papers. By Pamela 

 Te.nnant. ■ Pp. xii -1- 204;' 13 plates. (London : 

 William Heinemann, 1900.) 



These notes revealsome of the humour and pathos of 

 rural life in South Wilts, and here and there they lightly 

 touch natural scenes and objects other than human. 

 The plates, which are reproductions from original photo- 

 graphs of Wiltshire views, are excellent, and the book 

 itself is. a dainty volume suitable for a drawing-room 

 table. Reference is made to the "pernicious habit of 

 'underlining' in their letters " which some people adopt, 

 yet we notice an abundance of italicised words in the 

 book, and they are equivalent to the underlined words 

 so severely condemned. 



NO. 1606, VOL. 62 J 



LETTERS TO THE EDITOR. 



{The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manv scripts intended for this or any other paj-t of Nature. 

 No notice is taken of anonymous covununications.'] 



The Conductivity produced in Gases by the Motion of 

 Negatively-charged Ions. 



Recent researches have shown that gases are rendered con- 

 ductors of electricity when negatively-charged ions move through 

 them with a high velocity. Thus the kathode rays and the 

 Lenard rays possess the property of ionising gases through 

 which they pass (J. J. Thomson, " The Discharge of Electricity 

 through Gases"). Becquerel (Comptes rendus, March 26, 1900) 

 also has recently shown that the conductivity produced by 

 radium is due to small negatively-charged particles given off by 

 the radio-active substance. In these cases the charged particles 

 which ionise the gas move with velocities nearly equal to the 

 velocity of light. 



Some experiments which I have recently made show that ions 

 which are produced in air by the action of Rontgen rays will 

 produce other ions when they move through the gas with a 

 velocity which is small compared with the velocity of light. 



When Rontgen rays are sent through a gas, at atmospheric 

 pressure, the current between two electrodes immersed in the 

 gas increases in proportion to the electric force, when the force 

 is small. For large forces the current attains a value which is 

 practically constant. 



When the pressure of the gas is reduced, the connection 

 between conductivity and electromotive force is more compli- 

 cated. The accompanying tables show the connection between 

 current and electric force for air at 2 and "8 mm. pressure. 

 At these pressures the current is practically constant for forces 

 of about 10 volts per centimetre, and when forces of this order 

 are acting, all the ions are produced directly by the rays. When 

 the electric force is increased these ions produce others, so that 

 the current again increases. 



It appears from the following investigation that the new ions 

 are produced by the collisions between negatively-charged ions 

 and the molecules of the gas. 



Let us suppose that n negative ions are moving in a gas 

 between two parallel plates at a distance t/ apart. Let X be 



the electric force between the plates 



Vi 



and / the 



pressure of the gas. In going a distance dx the n ions produce 

 a. X « X dx others, where a is a constant depending on X, /, 

 and the temperature, which is constant in these experiments. 

 (The coefficient a is practically zero for small values of X, 

 unless/ is also small). 



dn = andx 

 and Ji — WflE"^ 



Hence «(, ions starting at a distance x from one of the plates 

 will give rise to «q(E°'^-i) others. When the ions arrive at the 

 plate, the formation of new ions ceases and the current stops, 

 although the electromotive force is kept on. Let «„ be the 

 number per unit volume produced by the rays. The total number 

 of ions produced will therefore be 



n^E-^dx =,-»(£" 



I) 



per unit area, ft^d being the number produced by the rays. 

 Hence 



c I 



ad 



(E«d-i) 



where c is the current for a large force X, and C(, the current 

 composed of ions produced by the rays. 



The following experiments were made in order to test the 

 accuracy of this formula for currents produced between two 

 parallel plates whose distance apart could be varied. 



The rays fell normally on one of the plates, which was made 

 of thin aluminium, and after passing through the air between 

 the plates, the rays were completely stopped by the second 

 plate, which was of brass. The plates were 10 centimetres in 

 diameter, and the rays were allowed to fall on a circular area at 

 the centre 4 centimetres in diameter. The conductivity was 

 thus confined to a region where the force was constant. A 

 large part of the conductivity (cq) arises from the secondary 

 radiation from the brass disc. At high pressures the secondary 



