May 2, 1889] 



NATURE 



II 



of halls in sinjjle file, the sloping faces all showing the sph;res 

 in triangular orJer. 



Suppose a bag, impermeable to water, i> filled with le id shot, 

 placed in an hydraulic press, and subjecled to great pressure. 

 The lead spheres will ba flattened ajainst each o'.har in regular 

 •cell structure into a solid ma>s, each sphere being chxnged into 

 a rhombic dodoahcdron ; and in this manner the form of tha 

 •cell of the bee has been considered as arising in a natural manner 

 by Mrs. Bryant, D. Sc, in a paper read before the Lond)n 

 Mathematical Society, vol. xvi.. " O 1 the Ideal Geometrical 

 Form of Natural Cell- Structure." The plane surfaces of separa- 

 tion al-o form a possible arrangement of the films of a mass of 

 soap-bubbles; but the instability of the corners where six 

 edges meet modifies the soap bubble arrangement to the form 

 investigated by Sir W. Thomson in the Ac^a Mdthemalica. 



April 27. A. G. Greenhill. 



Name for Unit of Self-induction. 



A NAME for the unit ciefificient of self-induction is much 

 Avanted. No one is satisfied with secohm, and yet it seems 

 ■making its way ; by reason, no doubt, of Ayrton and Perry's 

 ingenious commutating arrangement for helping to measure it. 

 It is an unpleasing name, and it is too big a unit. The name 

 ■quad, which I formerly suggested, is on further consideration still 

 less satisfactory for permanent use, becau-e it emphasizes unduly 

 the accid*nt that in electro- mxgmtic measure self-ind action 

 liappens to be a length. One 1 )oks forward to the time when 

 till distinction between electrostatic and electro-magnetic measures 

 shall vanish by both ceasing to be ; and at that not far-distant 

 time, names e nphasizing the present arbitrary state of things 

 will be anachronism-;, as well as s'.u nbling-blocks to beginners. 

 I beg to suggest that a milli secohm shall be called a vo. It is a 

 short and harmless unmeaning syllable not yet appropriated. It 

 should be its own plural. The unit of conductivity is already 

 style! a W(»;and 8 vo will look well alongside 12 mo. " Vo- 

 meter" is short and satisfactory. A unit of magnetic induction 

 Avill then be tlie vo-ampere ; and this, bein j of a size convenient 

 for dynamo makers, may be hoped to replace their abominable 

 fnongrel unit " Kapp-lines." 



The vo in electro-magnetic measure is 10 kilometres, and 

 hence a vo ampere per square decimetre is a magnetic field of 

 a thousand C.G. S. units, and might be called a "Gauss." For 

 Jightning-conductor work the natural unit of self-induction will 

 t)e a n:;illi-vo, or 10 metres of electro-magnetic measure. 



Grasmere, April 16. Oliver J. Lodge, 



Hertz's Equations. 



Permit me to add a line of explanation of my letter on this 

 subject, printed in Nature, vjI. xxxix. p. 558. I intended no 

 •criticism of Hertz's general result, bat merely to draw attention 

 to the neces-ity of reje :ting all solutions of the equation in n 

 which made the fore (Z) infinite for points on the vibrator. 



Berkswell, April 24. H. W. Watson. 



A NEW PEST OF FARM CROPS. 



DURING the past three or four years, in the exaini- 

 nation of plants affected by various injurious 

 vorms and Arthropods, and of the soils in which such 

 plants grew, I have from time to time been led to suspect 

 that certain small species of Oligochceta were concerned 

 in damaging, if not ultimately destroying, several species 

 of cultivated plants. With a view to converting sus- 

 picion into proof, experiments on isolated growing pot- 

 plants have been carried on. 



Within the past few weeks I have received, through 

 the kindness of Miss E. A. Ormerod, additional evidence 

 •of a striking character, which induces me to place the 

 main facts on record. 



In the spring of 1885, Miss Ormerod forwarded to me 

 for inspection two small white Oligochce/a, i\ inch lon^--, 

 received by her in soils from the roots of plants. In 

 reporting on them I replied that it did not seem very 

 probable that they could seriously injure the plants. 



In April 1888, an inquiry reached mc as to the natur-? 

 and means of prevention of a serious attack of "small 

 white worms" destructive to pot and green-house plants. 

 On being placed in communication with the observer, th:; 

 Rev. William Lockett, Rector of Littlede.in, I receiveii 

 from him a box of soil taken from his affected flower- 

 pots, and much valuable information in answer to a series 

 of questions put by me. The soil itself contained some 

 hundreds of the white worms described ; and the detailed 

 information all pointed to these worms as the cause of 

 many serious losses which had been sustained. 



Th'j worms were Enchytraeidae, of the genus 

 Ettchytrcstis, apparently near to E. Buchholzii, Vejd 

 I took three plants, a sunflower, a geranium, and a 

 tradescantia, and had them re-potted in carefully examine 1 

 sifted earth ; when they were well established, I put fifteen 

 of the worms into each pot, and left them to be tended by 

 the gardener. I kept a number of the worms in soil 

 which was alternately wetted and dried at regular 

 intervals. They all kept alive and vigorous ; when wet 

 to complete immersion they were most active, when 

 dried they remained quiescent, apparently dried up, and 

 difficult to discover. 



After two months, the sunflower drooped and bent over, 

 and examination showed the roots and rootlets dead and 

 the stem rotting. Within the decaying stem some of the 

 Enchytrtfidiie were found alive and active. The other two 

 plants are still living, but it will be shown that the 

 number of worms supplied them was too small. Mr. 

 Lockett lost spiraeas, vegetable marrows, fuchsias, 

 gloxinias, and many other plants, and the dead roots often 

 contained in and around them many hundreds of worms 

 to each plant. Both in his garden and a neighbouring 

 ash-heap he found an abundance of them. 



I was on the point of repeating my experiments this 

 spring with various seedlings, when I received by the kind- 

 ness of Dr. Gilbert, of Rothamsted (at the suggestion of 

 Miss Ormerod), a quill with two or three specimens of 

 worms of the same genus. Mr. John J. Willis, the 

 superintendent of the field experiments at Rothamsted, 

 in sending them described them as obtained from a field 

 of clover " with a good plant except across one portion of 

 the field, where all the plants were dying off," the small 

 worms occurring at the roots of the clover along with 

 larvae of Sitones z.x\A wire-worm. "There is scarcely a 

 plant that has not one or more of these creatures 

 attached." Mr. Willis has been good enough to send me 

 several communications on the subject, and a supply of 

 the worms, living and in spirit. Much of his informa- 

 tion is interesting, as that the more decayed the root, the 

 larger the number of worms ; that even healthy plants 

 harbour a few specimens ; that the worms seem some- 

 times to enwrap the rootlets with their coiled body. 

 He hears of other fields of clover in a similar condition 

 apparently to those at Rothamsted. I have a quantity of 

 detailed information, but to sumnarize it, there appears 

 to be but little room for doubt that these small OlIgochcBta 

 are one cause of the decay of the clover at Rothamsted, 

 as they were of the many varieties of garden plants at 

 Littledean. 



The Enchytraeidas have not hitherto, so far as I can 

 learn, been accused of causing serious injury to plants. 

 Vejdovsky, in his " Monographie der Enchytraeiden," says, 

 " Die Enchytrasiden bewohnen trockene und feuchte 

 Erde, susses und salziges Wasser, Siimpfe und morsches 

 Holz." In what manner they directly injure the plant 

 remains to be observed— probably by sucking the fine 

 root-hairs. Under observation the pharynx is rapidly 

 everted and withdrawn in the act of feeding. 1 have so 

 far recognized two species. If, as seems not improbable, 

 further corroboration should be forthcoming, we may find 

 that we have to add to the list of enemies of the clover 

 plant from which it so mysteriously suffers, these un- 

 suspected Oligocheetes. The discovery, though fraught 



