THE MOLECULAR FORCES IN PLANTS. 



109 



fibro-vascular bundles are coloured red, but also the elements of 

 the bast fibre bands between the strips of soft bast, and therefore 

 these also have lignified membranes. 



If we treat sections of twigs of Fagus sylvatica, 2 mm. thick, 

 with phloroglucin and Hydrochloric acid as above described, it 

 will be found that lignification has taken place in all the follow- 

 ing tissues : in the whole of the pith, in the medullary rays, in 

 the wood of the vascular bundles, and in the masses of bast fibres 

 which adjoin the soft bast on the outside. The elements of the 

 cambium, of the soft bast, of the cortex, and of the periderm, do 

 not stain red, and therefore are not lignified. Another very use- 

 ful reagent for woody material is aniline sulphate. We prepare 

 a concentrated aqueous solution of this substance, add to it some 

 Sulphuric acid, and treat the section on the slide with a drop of 

 the reagent. The lignified elements quickly become more or less 

 yellow in colour. I found, e.g., that the bast fibres in twigs of 

 Fagus sylvatica become bright golden-yellow in colour when 

 treated with aniline sulphate. On treatment with an aqueous 

 solution of methyl green, lignified elements take on a beautiful 

 greenish-blue colour, 



A 



while the unlignified 

 elements for the most 

 part stain blue. 



To familiarise our- 

 selves with the most 

 important forms of 

 thickening in the 

 elements of woody 

 tissue, the following 

 observations must be 

 made. The bordered 

 pits of the tracheides 

 of coniferous wood are 



3 



m 



FIG. 32, Finns sylvestris. A, a bordered pit of a tracheide 

 in surface view ; B, a bordered pit in longitudinal tangential 

 section, t. the torus; C, the transverse section of an entire 



best studied in deli- tracheide ; m, middle lamella, i, the limiting membrane. 

 Magn. 540. (After Strasburger.) 



sec- 



in 



cate transverse 



tions and radial longitudinal sections from the peripheral parts of 

 the wood of old stems of Pinus sylvestris which have been preserved 

 in alcohol (see Fig. 32). The tracheides are elongated, and their 

 tapering ends interlock with each other. Their bordered pits are 

 easily recognised on the radial walls, the walls, viz., which are 

 turned towards the medullary rays. If we prepare delicate radial 



