September. 1910. 



KNOWLEDGE. 



365 



that practically all previous writers agree in attributing to 

 acids an accelerating influence on transpiration, but he points 

 out that in his experiments the acids employed were added in 

 smaller amounts than those used by Sachs and others ; while 

 his experiments were extended for twelve to fifteen days 

 instead of only two to four days. 



(6) Wiegand's recent paper [Bot. Gaz., June. 19101 on the 

 relation of hairy and cntinised coverings to transpiration is of 

 special interest from its direct bearing on Ecology, as well as 

 from the beauty and ingenuity of the author's methods. He 

 first made a series of experiments, in which blotting paper was 

 used as the evaporating surface, his plan being to expose 

 saturated square pieces, all of the same size, to the air, each 

 having been previously weighed and covered with the material 

 used to stimulate a protective covering — cotton to represent 

 hairs, beeswax to represent cuticle. The evaporation experi- 

 ments showed that porous hairy co\erings must be very thick 

 to produce any appreciable effect in retarding evaporation if 

 the surrounding air is still, but become very efficient even in 

 thin layers when the air is in motion. On the other hand, a 

 waxy covering is effective at all times, though, of course, 

 somewhat more so in wind. In simshine, also, the hairy 

 covering shows a greater increase in eificiency than does wax. 

 It would appear that a hairy covering is employed to retard 

 transpiration by plants which grow in situations where a 

 normal water supply is available, and where transpiration 

 must be reduced in excessively dry times, but not interfered 

 with when the surrounding air is damp and transpiration 

 therefore difficult. Cutin, on the other hand, is probably 

 employed where there is considerable danger of too great 

 desiccation at all times. 



CHEMISTRY. 



By C. .\iN's\voRTH Mitchell, K..A. (Oxon).. F.I.C. 



THE CHEMISTRY OF SOME T.AXXIXG .A.GEXTS.— 

 In addition to oak bark, chestnut bark and sumach, there are 

 numerous vegetable products used as the source of tannin in 

 the manufacture of leather, some of which are but little known 

 outside the circle of those engaged in the industry. The most 

 important of these tanning agents are divi-divi, myrobalatis and 

 valonia, each of which has its own effect upon the leather. 



Divi-divi, which was first introduced into Europe towards 

 the end of the eighteenth century, is the commercial name of 

 the dried pods of a large shrub, Cacsalpinia coriaria, 

 which grows in swamp\' districts in South .America. 



In the dried 

 condition the 

 pods are curious 

 looking objects, 

 which, when un- 

 curled, are an 

 inch and a half 

 to three inches 

 in length. They 

 are dark brown 

 in colour and 

 have a very as- 

 tringent taste, 

 duetothetannin, 

 which is chiefly 

 concentrated in 

 the rind beneath 

 the epidermis. 

 .As a rule they 

 \-ield from thirty 

 to fifty-two per 

 cent, of tannin, 

 and a commer- 

 cial sample ex- 

 amined by the writer contained thirty-six per cent. 



The tannin closely resembles the gallotannic acid from 

 galls, both in composition and reactions, but differs from it in 



C 



FlGVRE 1. Divi-Divi Pods. 



yielding a deposit of ellagic acid when dissolved in water and 

 heated in a sealed tube. Hence Lciwe (Zcit. anal. Chcni.. 

 14. 35), described divi-divi tannin as cUagitannic acid, and 

 ascribed to it the formula, Cu Hm Oio, which may be 

 represented as gallotannic 

 acid, Cu Hio Oa, plus one 

 atom of oxygen. 



Myrob.a.l.\ns are the 

 dried fruit derived from 

 different species of Tcr- 

 III i Italia. indigenous to 

 India and the East Indies. 



The so-called " black " or 

 '"chebulic" myrobalans, 

 which are also widely 

 employed as a black dye. 

 are the product of the 

 Indian species, T. chcbiila. 



A commercial sample ex- 

 amined by the writer con- 

 tained thirty-nine per cent, 

 of tannin, which behaved 

 towards reagents in the same 

 way as divi-divi tannin. In 

 fact Lowe doc. citl con- 

 cluded that the two tannins 

 were identical — both con- 

 sisting of ellagitannic acid. 



V.'\L0NIA is the name gi\en in connnerce to the dried acorn 

 cups of certain kinds of oak growing in Greece and .Asia 

 Minor. The best kinds are obtained from Qncrcus aegilops 

 and O. macrolcpis. and are gathered in spring before the 

 fruit matures. The valonia beaten from the trees in the 



Figure 2. Mvrobalans. 



Figure 3. \'aIonia. 



autumn contains less tannin, and consequently fetches a 

 lower price in the market. The freshly -gathered fruit is 

 partially dried on the spot, and sent by mules to Smyrna, 

 where it is allowed to stand for a partial fermentation to take 

 place, so as to detach the acorns from the cups, the latter 

 being then dried and exported. Excessive fermentation, such 

 as may take place if the acorns are exposed to rain after 

 being gathered, causes the valonia to turn black and to be 

 deficient in taimin. 



The ordinary commercial product is in the form of semi- 

 circular, prickly cups, which may contain anything between 

 twenty and forty-five per cent, of tannin, according to the 

 species of oak whence they were derived and the season of 

 gathering. 



The tannin appears to be identical with that contained in 

 divi-di\i and myrobalans, but in the impure state differs more 

 than the latter from gallotannic acid. Thus, on the addition of 

 an iron salt it forms a compound with a much bluer tinge. It 



