i So 



NATURE 



[August io, 191 i 



LETTERS TO THE EDITOR. 



[Tlie Editor docs not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

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

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The " Stomatograph." 



The recent controversy regarding the causes which had 

 depreciated the yield of Egypt's cotton crop, terminating 

 in the Egyptian Government's Cotton Commission of 19 10, 

 showed very clearly how little accurate knowledge of the 

 physiology of the cotton plant was available, more especi- 

 ally in regard to field conditions. This ignorance was 

 most striking in respect to the water relationships, which 

 — Egypt being dependent on the Nile for its water — are 

 very largely within human control. 



One point of interest in this respect is the action of the 

 stomata, especially since I showed that growth is com- 

 pletely arrested by direct sun, from the first emergence of 

 the seedling until late in the summer, the heavy water 

 loss involved by the high sun temperature (160° C. in May 

 to August] and the low day humidity being the limiting 

 factor. 



Investigation of plant physiology in the field, however, 

 usually requires special and automatic apparatus, and no 

 suitable appliances were available for investigation of the 

 stomata. After several trials I succeeded in constructing 

 a self-recording form of Mr. Francis Darwin's " poro- 

 meter," which will work without appreciable errors under 

 any variation of wind, sun, temperature, or barometric 

 pressure, in rain or in dust-storms. 



Briefly described, the apparatus consists of an electrical 

 air-pump expelling 3 c.c. of air at each stroke under a 

 constant pressure of 1 millimetre of mercury. The out- 

 flow tube is closed by a definite area of leaf, so that the 

 air in escaping has to pass through the leaf tissues. The 

 resistance to this escape depends almost entirely on the 

 aperture of the stomata, of which there are some seventy 

 on the upper leaf surface and two hundred on the lower 

 in adult leaves of Egyptian cotton. The rate of escape is 

 a direct measure of the porosity of the leaf, as in the 

 " porometer." A relay circuit is operated when one stroke 

 of x the pump has been completed, and this telegraphs the 

 signal to an electromagnet carrying a pen on its arma- 

 ture ; this pen writes on a spiral drum which revolves 

 once every hour. 



The complete appliance is very convenient in use, being 

 composed of a box like a microscope case, which is placed 

 on the ground under the plant to be examined ; from this 

 box issues the air-tube leading to the leaf and the tele- 

 graph wire to the recorder. The number of adjustments 

 which have to be made in the field is thus reduced to a 

 minimum, which is no unimportant consideration under 

 the field conditions obtaining during an Egyptian summer. 



The trace of five consecutive days recorded from the 

 same leaf without adjustment shows the stomata slowly 

 closing as the hottest part of the day approaches, then 

 closing when the sun goes behind the trees at 1.40 p.m., 

 and remaining closed all night, opening slowlv after sun- 

 risi , more rapidly when the direct sun strikes tlie leaf 

 abonl 7 a.m., attaining a maximum at q a.m., and thence- 

 forward closing steadily, in spite of the brilliant illumina- 

 tion, since the soil around the roots is being dried up by 

 leavy transpiration. 



\ Mde-issue from such records as this, which promises 

 to give further unexpected results, deals with the effecl ol 

 such stomatal closure on assimilation of CO.. I have 

 already mentioned that the stem of an Egyptian cotton 

 plant does not grow in sunshine, and it appears that the 

 growth of the root is also checked; it now seems not un- 

 likely that photosynthesis may also be arrested or reduced 

 after a certain hour of the day through stomatal closure, 

 this closure being dependent on the development of the root 

 and on the water content of the soil. 



The beneficial effects of Egyptian sunshine seem to be 

 rather indirect. YV. Lawrence Balls. 



: 1 louse, Cairo, Julv 10. 

 NO. 2l8o, VOL. 87] 



Photograph of Multiple Lightning Flash. 



1 beg to enclose a copy of a remarkable flash of lightning; 

 which 1 took some years ago in Uruguay, South America, 

 and which, so far as I know, has never been published — 

 with my knowledge or permission, at any rate. 



This copy is an enlargement from a small neg 

 about 1 J inches square, and shows the curious curl-like 

 form in some of the streaks. One noteworthy point about 

 this photograph is that 1 " snapshotted " it. I had wi 

 many plates in the usual way, and obtained nothing ot 

 any value, but noticed that before every big flash there 

 was a slight flicker in the clouds where the flash came 

 from; I waited for this, let the shutter go just as I saw 

 the flicker, and obtained the enclosed result. 



Uruguay is noted for its severe thunderstorms, and 

 animals, fences, trees, and houses are constantly struck. 

 Some years before this photo was taken a powder- 

 house was blown up by lightning not far from when 

 this flash occurred, and in the sand a few miles off 



1 found a fulgurite — a glass tube about 23 inches in 

 diameter made by the lightning striking and fusing the 

 sand. These fulgurites are mentioned by Darwin in " Thi 

 Cruise of the 1 found some near Monte Video,, 



also in Uruguay. A. E. WALBY. 



67 Lansdowne Street, Hove, Brighton, July 27, 



The Rearing of Sea Urchins. 

 I have read with much interest in Nature of July 27 

 a short letter from Prof. Stanley Gardiner in which he 

 communicates the fact that a hybrid of the species Echinus 

 esculentus and E. miliaris was successfully reared from 

 the egg through all the larval period until after the com- 

 pletion of metamorphosis in the zoological laboratory at 

 Cambridge. It may interest your readers to learn that by 

 means of salt-water aquaria established last year in the 

 zoological laboratory of the Imperial College of Science ir. 



