CORK, 161 
regular lignification of the membrane is also found in the walls 
of vessels. Lignified membranes refract the light to a less 
extent than those consisting of pure cellulose, and mostly 
appear light-yellow under the microscope; they are hard and 
elastic, and but little capable of swelling. 
Lignified membranes are characterized micro-chemically by 
the fact that with a solution of iodine in chloride of zine they 
become yellow (not violet). In ammoniacal oxide of copper and 
Schultze’s macerating liquid (page 160) they do not dissolve. 
With aniline sulphate and dilute sulphuric acid they become 
straw-yellow, and with phloroglucin and hydrochloric acid 
cherry-red; the aniline colors are greedily taken up by them. 
By boiling with Schultze’s mixture or with alkalies, the lignin is 
removed, and the membranes thus treated then show the cellu- 
lose reaction. Morphological alteration, however, by no means 
goes hand in hand with a change of physical and chemical 
character. While, for example, the wood-cells, even in quite a 
young condition when their walls are but very little thickened, 
are strongly lignified, and the very thin-walled cork-cells are 
always suberified, the strongly thickened collenchyma, and many 
bast-cells which are thickened so as to cause the lumen or cavity 
to disappear, remain unlignified. 
The third modification of cellulose is cork. 
This is formed by the deposition between the cellulose mole- 
cules of swberin, which latter consists for the most part of the 
glycerin (propenylic) esters of stearic acid and of phellonic’ 
acid, C,,H,,0,.2. Suberin appears to be identical with cwtin.* 
The @pidermis-cells of the more delicate organs of all land 
plants are covered by a delicate film termed the cuticle. The 
older organs, especially those of the stem, on the contrary, 
develop on their outer surface a layer consisting of tabular cork- 
1 @édXAor cork. 
* Definitely established by Kiigler at least for the cork of Quercus 
Suber (‘* Ber. d. deutsch. botan. Ges.,” I., p. xxx. , and Inaugural Disser- 
tation, Strassburg, 1884), 
’ Beside suberin, there is also found in cork a wax-like body, cerin. 
The cuticle appears to contain more of the latter than the cork. 
11 
