THE SKELETON OF THE PLANT 57 



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 

 imbibing 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 imbibition 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, 

 physical, and mechanical properties, which fit the definite 

 adult tissues to perform the functions which fall to them. 

 Temporary structures generally possess a different chemical 

 composition from permanent ones. The transitory cuticle 

 gives place to the more permanent cork, and this becomes 

 strengthened by the introduction of sclerenchymatous 

 elements as the cork formation becomes continuously more 

 deep-seated. The strengthening tissue varies similarly; 

 the walls of collenchyma, though thickened in a particular 

 way, are not chemically changed in the same manner as those 

 of sclerenchyma or woody tissue, for their cellulose under- 

 goes no conversion into, or impregnation with, lignin. 

 The fibres of the bast differ from those of the wood in the 

 same particulars. 



