THE SKELETON OF THE PLANT 51 



those of the cereal grasses. The value of this deposit to 

 the plant is not very evident ; it appears at first sight to be 

 an adaptation enabling the plant to remain upright, but it 

 is found that its absence does not render the grasses more 

 liable to fall. 



Some cells of the epidermis of certain plants, especially 

 among the Nettle family, contain curious ingrowths of 

 cellulose, in which there is a very large deposition of 

 calcium carbonate. They are known as cystoliths (fig. 49). 



The cell-walls of certain regions of particular plants 

 are transformed into mucilage. This material is especially 

 prominent in the large brown seaweeds, particularly the 

 F'ucacece, where it forms the bulk of the internal tissue. It 

 occurs also in certain layers of the seed-coat of such seeds as 

 linseed, and in certain regions in the sporocarps of Marsilea. 

 It is of assistance in the dissemination of the spores of 

 this plant, and possibly has a similar value in the cases of 

 such seeds as contain it. It differs from cellulose by 

 absorbing water greedily, and swelling up considerably. 

 It gives a blue colour with iodine and sulphuric acid as 

 cellulose does, differing from the latter chiefly in the ease 

 with which the absorption of water is brought about. It 

 is not clear at present whether mucilage is derived from 

 cellulose only, or whether the pectoses take part in its com- 

 position, though the latter is probable. The gums are closely 

 related to mucilage, and seem to represent a further dis- 

 integration of the cell-wall in that direction. Many of the 

 gums yield derivatives much like those of pure pectic 

 bodies, which suggests that their affinities are rather with 

 the latter. In all probability, however, they are all mix- 

 tures of the two classes of constituents. 



We see thus that in the construction of the skeleton of 

 a complex plant, while its basis is the cell-membranes of 

 the several protoplasts, which at first form an almost 

 homogeneous tissue, not only does differentiation take 

 place in the direction indicated in the last chapter, but 

 this differentiation is accompanied by changes in chemical, 



