24] CHAPTER I. THE CELL. 107 



innermost layer consisting frequently of pare cellulose, though it 

 is sometimes more or less lignified (tfork). This can be shown by 

 treating the ctiticularised tissue with strong chromic acid for 

 some time, or by warming it in a mixture of nitric acid and 

 chlorate of potash, when the cutin is removed, and the remaining 

 tissue gives the characteristic cellulose-reactions. 



ft. The cell-wall may undergo lignification ; that is, the cell-wall 

 becomes impregnated with a substance termed lignin, which 

 makes it hard and elastic, and though readily permeable to water 

 it cannot absorb or retain much in its substance. The character- 

 istic tests for lignin are, that a cell- wall containing it (a) turns 

 yellow when treated with aniline chloride and hydrochloric acid, 

 and (6) turns pink when treated with phloroglucin and hydro- 

 chloric acid. When a lignified cell-wall is macerated in a mixture 

 of nitric acid and chlorate of potash, or in a strong solution oi 

 chromic acid, the lignin is dissolved out and the wall ceases to 

 to give the lignin-reactions, and now gives the cellulose-reactions. 

 Lignification takes place in the sclerenchymatous and tracheal 

 tissue?, less commonly in the parenchymatous tissue, of the 

 sporophyte of the Pteridophyta (Vascular Cryptogams) and 

 Phanerogams ; it does not occur in any of the lower plants, nor 

 in any gametophyte. 



y. The cell-wall may become more or less mucilaginous ; in its 

 dry state it is then hard and horny ; when moistened, it absorbs 

 a large quantity of water, becoming greatly increased in bulk and 

 gelatinous in consistence; it usually turns blue when treated 

 with sulphuric acid and iodine, or with iodised chloride of zinc, 

 but in some cases it does not give this reaction, and in yet others 

 (e.g. asci of Lichens, bast of Lycopodium, endosperm of Peony, and 

 cotyledons of various leguminous seeds) it turns blue with iodine 

 alone. Mucilaginous cell- walls are common in the coats of seeds 

 (e.g. Flax or Linseed, Quince) ; they are very remarkable in the 

 case of the macrospores of Pilularia and Marsilia ; in tissues, they 

 are well seen in the Malvaceae : they occur in all sub-divisions of 

 the vegetable kingdom. 



In some cases the change goes so far as to result in the con- 

 version of the cell-wall into gum, soluble in water, as in some 

 species of Astragalus and in certain Rosaceous trees (Cherry, 

 Plum, Almond, Peach, etc.) 



These modifications may occur either singly or together in 

 the different layers of one cell- wall, as in corky, or suberised 



