78 V. CELL-WALL, AND CELL CONTENTS. 



brane (i), which is specially clear in the more strongly-thickened 

 tracheides of late summer. It is still clearer after the action of 

 concentrated sulphuric acid ; the thickening sheaths swell, and 

 are finally dissolved ; the limiting membrane resists for a longer 

 time and stands out sharply ; between the swelling thickening 

 layers are seen the primitive walls of the cells, of which finally 

 only the delicate network of middle lamellae remains, stained 

 yellowish-brown. These middle lamellae owe their resistance 

 to concentrated sulphuric acid to a specially strong lignifaction 

 which characterises the middle lamella of wood. Concentrated 

 chromic acid solution acts upon a section in a similar way, and 

 after prolonged action leaves of it only a network of middle 

 lamellae, appearing bright in the dark fluid. The resistance of 

 this network indicates an especially strong lignifaction, which 

 universally distinguishes the middle lamellae of wood. With 

 slower swelling in sulphuric acid, by using diluted acid, we 

 can often determine, and especially in the strongly thickened 

 tracheides of autumn, that the thickening layer consists of very 

 numerous delicate lamellae. With chlorzinc iodine the cross- 

 section is coloured yellow-brown, the middle lamellae a purer 

 yellow ; in individual cells, also, the inner thickening layer, 

 immediately surrounding the limiting membrane, shows well 

 the violet colour of cellulose ; and if the section which has been 

 in chlorzine iodine be also treated with dilute sulphuric acid (2 

 parts acid 1 part water), in many cases the entire thickening 

 layer will take on a blue colour. 



Exceptionally thin cross-sections of the wood of this Pinus 

 serve as excellent test-objects, with the aid of which we 

 can form a judgment upon the quality of the medium and 

 higher magnification of our microscope. Sufficiently delicate 

 sections being presupposed, the figure must appear perfectly flat, 

 brightly lighted, sharp in outline, clear in structural details, and 

 free from colour. 



Reactions of Lignin. In order to become acquainted with 

 the characteristic reactions of lignified membranes (lignin) We 

 will make use of phloroglucin and sulphate of aniline. We 

 dissolve a trace of phloroglucin in alcohol, and lay some sections 

 of wood in this solution. After this we place it in a drop of 

 water on the object-slide, and allow, from under the edge of the 

 cover-glass, hydrochloric acid to act upon it. The walls of the 

 cells quickly take a beautiful violet-red colour. Other sections 



