THE VEGETABLE CELL 293 



640). In many plants copious deposits of silica are found in the 

 cell-wall, notably in the epidermal cells of Grasses and Equi- 

 setacese. In some plants of the N ettle family cvirious cellulose 

 projections occur in some of the cells of the epidermis which 

 contain irregular masses of calcium carbonate. These are known 

 as cystoUths (fig. 642). 



Besides these admixtures of foreign substances, cell-wall is 

 found to contain other bodies derived from changes in its 

 cellulose or pectose constituents. Of these, three are especially 

 prominent — lignin, cutin, and mucilage. Lignin is the material 

 which is especially characteristic of wood-cells. It is permeable 

 by water very readily, but unlike cellulose it cannot retain much 

 in its substance, nor can it swell up under the influence of 

 sulphuric acid. It can be recognised by turning pink when 

 treated with phloroglucin and hydrochloric acid. The wall of 

 a wood-cell does not consist entirely of lignin, but of a cellulose 

 basis which is largely impregnated with this substance. By the 

 action of nitric acid aird chlorate of potash the lignin can be 

 dissolved, leaving the cellulose framework. 



Cutin is found chiefly in the external layers of the thickened 

 epidermal cells. It differs from cellulose in manj' points ; it is 

 but slightly permeable by water, and it stains yellow instead of 

 violet when treated with iodine and sulphuric acid. It turns 

 yellowish-brown when acted upon by strong alkalies such 

 as caustic potash. Cutin is found also in the walls of cork cells, 

 where it is associated frequently with a certain amount of lignin. 

 Like lignin, cutin is soluble in warm nitric acid and chlorate 

 of potash. 



Mucilage is found in the cell-walls of certain layers of the 

 coats of such seeds as linseed, and in the cells of many Algae. It 

 absorbs water greedily and swells up considerably. It gives a 

 violet colour with iodine and sulphuric acid as cellulose does, 

 differing from the latter chiefly in the ease with which the ab- 

 sorption of water is brought about. In some cells the mucila- 

 ginous degeneration of the cell -wall goes so far as to lead to 

 the production of gum, which is soluble in water. Exudations 

 of gum produced in this way are very common on Cherry trees. 



A peculiar modification of the cell-wall can be easily observed 

 between contiguous thickened cells, constituting the middle 

 lamella. This occupies the position of the original cell-wall, 

 and at first sight might be taken to be the latter, after several 

 thickening layers have been deposited upon it. This is not, 

 however, the case, for the thickened wall has been the seat of 



