S EPTEMB ER"' t^ ,' ' i§ll ] 



"NATURE 



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found that Java indigo can tolerate wide changes in 

 reaction, growing in Bihar in soil at />H87 and in 

 Assam at pUs-^- It, however, does much better in 

 the latter for various reasons. 



In the British Isles it has been found that the 

 yellow stonecrop, Sedum acre, flourishes in soil from 

 pYi-j to 8 or over, whereas the white S: angliciim 

 may be growing at from pH6S to 5-1, possibly 

 slightly outside these limits. Plants which are by 

 some regarded as typical species of limestone districts 

 may be found elsewhere provided the soil reaction is 

 suitable; thus Saiwa verbenaca grows between pHy 

 and 8, Crithmum maritimum around *H8, and Coch- 

 learia danica from pHy^ to 8, possibly over it, much 

 the same range being occupied by Linaria cymbalaria; 

 for Centranthus ruber values from pli-j.\ to 8-8 have 

 been obtained. The common gorse, L'lex europaeus 

 is usuallv found on acid soil ; in seven cases where it 

 was observed growing in abundance the reaction was 

 pH6S to 54, but one plant was found at pHSi, one at 

 8-2, and three or four at 8-6. In other cases the soil 

 was probably alkaline, but was not tested. 



The sea pink, Armeria maritima, may be found 

 between ^H6-8 and 82, but the typical sand-dune 

 plants, Ammophila arenaria, Euphorbia Paralias, Sal- 

 sola Kali, are found onlv in the neighbourhood of 

 ^H8. 



Moorland plants. Erica tetralix, Anagallis tenella, 

 Drosera rotundifolia, jasione montana, etc., are com- 

 monly found at pyi-S-) to 5, or thereabout, but the 

 limits are certainly wider. 



The accumulation of data of this type is of neces- 

 sitv a slow process, but one cannot fail to be im- 

 pressed by the fact that the presence or absence of a 

 plant in a given locality stands in close relation to 

 the hydrogen-ion concentration of the soil. Plants 

 mav survive, or even do well, in cultivation outside 

 their normal limits, but in free competition with 

 their neighbours the soil reaction is often the deciding 

 factor — always, in fact, if the divergence from the 

 normal ^H value for the species is sufficiently great. 



Considerable changes in the soil reaction may be 

 met with in quite a short distance. Thus on crossing 

 a road at Youghal, Co. Cork, one passes from a soil 

 of about ^H7-5, with Salvia verbenaca and Ononis 

 arvensis, to an acid soil, /)H6-8 to 6-4, with gorse, and 

 in the wet parts Iris pseudacorus and bog-cotton. 

 This, in turn, passes into sandy pasture and sand 

 dune, the latter giving about ^H8. Again, near Caw- 

 sand, in Cornwall, gorse is plentiful on the felsite soil 

 at /)H6-4 to 54, but absent from the adjacent, and 

 similarly situated, soil of the Staddon Grits, which 

 normally gives pHy to 78. 



It appears as if corresponding differences are shown 

 by water-plants and fresh-water algae, the upland 

 waters which are very slightly acid or almost neutral 

 favouring the desmids. There is much room for 

 further work along these lines. 



W. R. G. Atkins. 



August 30. 



" Smoky " Quartz. 



The deeply tinted varieties of quartz, such as 

 " smokv " quartz and the yellow or Madagascar 

 varietv, are generally transparent in the infra-red 

 region of the spectrum to the same extent as clear 

 rock-crvstal, as may easily be demonstrated with the 

 aid of a thermopile and galvanometer. I wish to 

 suggest that a very simple physical explanation of this 

 property may be offered. As has been emphasised in 

 a paper by Prof. R. J. Strutt (now Lord Rayleigh) 

 in the Proceedings of the Royal Society' for 19 iq, 

 these varieties of quartz are really optically turbid 

 media, the opacity arising from the scattering of the 



NO. 2707, VOL. 108] 



radiations iii their passage through the crystal by a 

 cloud of small particles present as inclusions. Since 

 scattering of this kind is effective in inverse propor- 

 tion to the fourth power of the wave-length, it can 

 easily be seen why the longer heat-waves can traverse 

 the crystal without appreciable loss. Some photo- 

 metric observations which I have made of the relative 

 transparency of the yellow and colourless varieties in 

 different parts of the spectrum support this explana- 

 tion. 



In the paper just quoted Rayleigh has described 

 the very beautiful and striking effects that arise 

 owing to optical rotatory dispersion when a strong 

 beam of polarised light is sent through a block of 

 smoky or yellow quartz in the direction of the optic 

 axis; the track of the beam, as made visible by the 

 scattering particles and observed in a transverse direc- 

 tion, shows bright and dark bands if monochromatic 

 light be used, and alternations of colour if the incident 

 beam is of white light, the effect being due to the 

 fact that the scattering particles themselves act as 

 analysers of the light incident on them. I find that 

 the phenomenon discovered by Lord Rayleigh can be 

 very prettily shown in another way which is also 

 instructive. A thin, flat sheet of tinpolarised white 

 light may be sent through the crystal in a direction 

 transverse to the optic axis, and the track of the beam 

 observed in a direction parallel to the optic axis 

 through a Nicol. In this case the scattering particles 

 act as polarisers, and the scattered light suffers a 

 rotatorv dispersion of its plane of polarisation in 

 traversing the quartz along the optic axis before 

 reaching the observer's eye. Hence the whole track 

 of the beam as seen through the observing Nicol 

 appears coloured, the tint fluctuating periodicallv with 

 the thickness traversed as the block is moved to and 

 fro in the line of sight or when the analysing Nicol 

 is rotated. 



Rayleigh has shown in his paper that the track of 

 a beam of light traversing a beam of transparent 

 colourless quartz can be successfully photographed. 

 I find that by using a concentrated beam of sunlight 

 it is possible visually to detect the Tyndall blue cone 

 even in this case. Its intensit},-, however, is exceed- 

 ingly small. ' C. V. Raxun. 



22 Oxford Road, Putney, S.\V.i5, 

 September 4. 



Brown Bast and the Rubber Plant. 



In Nature of June 16 (p. 499), in a paragraph 

 which announces the discovery by the Botanv Depart- 

 ment of the Imperial College of Science and Tech- 

 nology that ■■ ' brown bast ' (the most serious disease of 

 Hevea brasiliensis) is essentially a question of phloem 

 necrosis," it is stated that Sanderson and Sutcliffe 

 have shown that "burrs result from the inclusion of 

 areas of diseased laticiferous tissue in stone-ccU 

 ' pockets ' formed by the activities of wound cam- 

 biums." 



It should be pointed out that the presence of latex 

 vessels in the core of nodules (burrs) was first recorded 

 by Bateson (Agric. Bulletin Fe'd. Malay States, August, 

 1913, p. 24), and later corroborated by Richards and 

 Sutcliffe C' Hevea brasiliensis," 1914, Malay Penin- 

 sula Agric. Assoc.), and by myself (Bulletin 28, Dept. 

 of Agric, Ceylon, October, 1916, and Annals Rov. 

 Bof. Gdn., Peradeniva, vol. 6, p. 257, 1917). 



Workers in Java have further confirmed this inclu- 

 sion of laticiferous tissue as regards the nodules which, 

 follow brown bast, and the fact that nodules in the 

 most general case result from the inclusion of areas of 

 diseased laticiferous tissue has been common know- 

 ledge in the East for the last five years. That the 

 formation of nodules after brown bast is a secondary 



