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 mdicated 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. 
