36 THE VEGETABLE CELL IN GENERAL. 



addition of water, this cavity becomes more clearly defined, the 



whole mass of the cell swells, and the mucilage can then be 



made out as a distinctly 

 stratified structure belong- 

 ing apparently as much to 

 the outer as to the inner 

 face of the cell-wall. But 

 if the action of water is 

 prolonged, the stratified ap- 

 pearance vanishes, and the 

 wall becomes optically ho- 

 mogeneous, with the excep- 

 tion of its middle portion, 

 the so-called primary mem- 

 brane, which remains un- 

 changed. On the addition 

 of iodine and sulphuric 



acid, the primary membrane, but not the mucilage, becomes blue. 



Furthermore, the lateral walls of the cells are not converted 



into mucilage. 



150. The mucilaginous modification can be examined to ad- 

 vantage in the seeds of some Polemoniaceae (especially Collomia) 

 and a few Acanthaceae, e. y.< Ruellia. These seed-coats are 

 covered with hairs which break open when wet, and allow not 

 only the mucilage but also slender coiled threads to escape. 

 The achenes of some Composite of the Senecio group and the 

 nutlets of a few Labiate (the Salvia tribe) exhibit nearly the 

 same phenomenon. 



151. Ligniflcation. Induration of the cell- wall is caused 

 most commonly by the presence of an incrusting substance 

 known as lignin. Owing to the difficulty of separating it from 

 the cellulose, with which it is associated, its chemical composi- 

 tion must be regarded as uncertain. Although generally spoken 

 of as a single substance, it is probable that the lignin, or in- 

 crusting matter, is made up of several different substances, 1 



1 Payen (Mem. des savants etrangers, ix., 1846, pp. 68, 5) distinguished 

 four such incrusting matters, differing in their composition and in their l>e- 

 havior to solvents. Lignose: insoluble in water, alcohol, ether, and ammo- 

 nia; soluble in solutions of potassa and soda. Lignone : insoluble in water, 

 alcohol, and ether ; soluble in ammonia, potassa, and soda. Lignin : in- 

 soluble in water and ether ; soluble in alcohol, ammonia, potassa, and soda. 



FIG. 5. Section of the albumen of Ceratonia siliqua, showing mucilaginous modifica- 

 tion. (Sacha.) 



