PITTED TRACHEIDS 49 



matter which is localized in the cell walls as the only way pro- 

 vided for the disposal of waste matter. Such mineral salts are 

 important factors in imparting an increased specific gravity and 

 a greater element of hardness to the structure as a whole, while 

 they appear upon combustion in the form of an unoxidizable ash 

 residue, which varies in the North American Coniferac from 0.08 

 per cent in Libocedrus and Pseudotsuga - - in specific cases of 

 the latter falling as low as 0.02 per cent --to 1.34 per cent in 

 Torreya taxifolia (65, 333). 



The variations in the cell wall thus noted are accompanied 

 and more or less exactly indicated by the reactions of the wall 

 toward chemical reagents and stains. Thus aniline chloride or 

 sulphate imparts a brilliant yellow color to the entire lignified 

 membrane, serving at the same time to differentiate the primary 

 from the secondary walls by virtue of the greater depth of color 

 and the greater degree of translucency imparted to the former. 

 Iodine is absorbed with great energy by all parts, but rather more 

 strongly by the primary wall, which thereby acquires a deeper yel- 

 low or yellowish brown. When lignified membranes are treated 

 with strong sulphuric acid the secondary wall is attacked and 

 brought into solution, while the primary wall is left intact if the 

 action be properly limited. In this case the action is one of 

 hydrolysis. On the contrary, a strong oxidizing agent, such as 

 Schulze's maceration, acts first upon the primary wall, thereby 

 separating the secondary walls from one another ; or, in accord- 

 ance with Mangin's reaction with alcohol and hydrochloric acid, 

 whereby the pectic acid is set free from its original combinations, 

 it is possible not only to secure a more intense reaction with 

 ruthenium red but also to separate the cells from one another 

 when the pectic acid has been neutralized by ammonia. Similar 

 disintegrations may be effected in the primary wall by other 

 reagents, in which case they constitute the scientific basis for 

 such important economic processes as are involved in the manu- 

 facture of wood pulp by chemical means. 



The total thickness attained by the cell wall necessarily varies 

 as between the spring wood and the summer wood. This relation 



