STRATIFICATION. 155 
of their greater capability of refracting the light. Chemical 
distinctions (various degrees of lignification) also play a part in 
this. The function of the contained water may be proved by 
complete desiccation or by more complete swelling, both of 
which equalize the differences, and often either break up the 
stratification or obliterate it toa large extent. The bast fibres 
of the Cinchona barks possess almost entirely thickened walls, 
with distinct stratification (Fig. 73). When they are softened 
by means of energetic reagents, such as caustic soda, concen 
= SSS 
SSA 
SQ 
= 
=S 
SES 
Fie. 73.—Bast-fibres from Cinchona-barks. 
trated sulphuric acid, or ammoniacal oxide of copper, and the 
tension of the particles of cellulose becomes equalized, it is dis- 
tinetly seen that the thickening is not due to a simple concen- 
tric succession of layers, but to far more complicated processes," 
In the Cinchona fibres, particularly, there is brought to view in 
1A more precise elucidation of these remarkable conditions is given 
by Nageli, «‘ Bau der vegetabilischen Zellmembran.” Sitzungsberichte 
der Miinchener Akademie, June, 1864, page 145. Also Sachs, ‘‘ Lehr- 
buch der Botanik,” 1873, p. 30 et seg. Wiggersand Husemann, Jahres- 
bericht, 1866, 89. 
