By way of verification of this iodine test, which is somewhat capricious, it is advisable to test the reagents with a standard substance. The best for the purpose is a linen yarn which has been _partially_ bleached. Under the action of the reagents the fibres composing this yarn, which must, of course, be suitably “teazed out” for mounting, are coloured a light blue, the centre, however, showing a yellow line, {37} marking the distribution in the interior canal of a non-cellulose fibre constituent. Should these effects not appear, it may be concluded that the acid requires to be strengthened. On the other hand, too great a concentration is equally to be avoided; it is evidenced by causing a _distortion_ of the fibre, easily recognised by comparison with the fibre mounted in a neutral medium. _Chlorine Water._—One of the most characteristic reactions of lignose, or lignified cellulose, is that of combining with chlorine. The reaction of the chlorinated derivatives with sodium sulphite solution, is an important feature in the microscopic diagnosis of lignified fibres and cells. The reagent is prepared by dissolving chlorine to saturation in water. The sodium sulphite solution is prepared by dissolving the crystallised salt in 20 parts distilled water. _Aniline Sulphate Solution._—With this reagent lignose gives a characteristic deep yellow colouration. A convenient strength is a 2 per cent. solution of the salt. The colour is more quickly developed if the reagent is acidified; a few drops of sulphuric acid should therefore be added. _Solutions of the Aniline Colours._—Some of these are of importance in enabling the microscopist to differentiate plant tissues. The “affinities” of the fibre substances for these are very various in kind and intensity. The phenomena of staining cannot be adequately treated in our histological scheme, which is necessarily very restricted. We, therefore, merely mention the more important colours which are used in staining, viz., magenta, methylene blue, eosine, diphenylamine blue. A convenient strength is a solution of 1 in 2000. (See also p. 43.) The employment of the aniline sulphate solution, as well as of the solutions of the aniline colours, presents no difficulties, and therefore needs no detailed description. The former strikes a more or less deep yellow with lignose; the aniline colours stain or dye the tissue or fibre more or less deeply, according to its composition, and, as it is a reciprocal action, {38} according also to the composition of the colouring matter. In following up this subject, the student will require to consult works on vegetable histology.