158 PLANT ANATOMY, 
The thickening of the cell-wall may also, under certain con- 
ditions, confine itself to the corners, thus forming the so-called 
collenchyma,' which is met with in the barks and seeds of very 
many plants. 
The relative thickening of individual cells and forms of cells 
is, moreover, very manifold. While the parenchyma remains 
mostly thin-walled until the close of life, the wood- and bast- 
cells become provided with strong walls at a very early period. 
Bast-tubes and stone-cells when observed in thin sections 
under glycerin in polarized light, are seen to be doubly refrac- 
tive (Fig. 77). A transverse section through cinchona fibres 
— 
A B 
Fic. 77.—Thin sections through bast-fibres and stone-cells, showing double refrac- 
tion in polarized light (Dippel), p, s, s’, layers of different density. 
shows four dark arms of a cross upon a brightly shining ground 
(Fig. 77, I.). 
In the preceding pages, those morphological changes of the cell- 
wall have been considered which take place in the process of 
vegetation. It still remains for us to subject the chemico-phy- 
sical changes to closer consideration. 
’ Derived from x0AAaq, glue, since it was formerly, but incorrectly, 
believed that the collenchyma cells could become mucilaginous. 
* With regard to the chemistry of the cell-membrane, compare partic- 
ularly the more recent researches of Cross and Bevan, ‘‘ The Chemistry 
of bast-fibres,” in the Chem. News, 1882; Webster, ‘* On the analysis of 
