162 PLANT ANATOMY. 
cells. The cuticle and cork are both produced through suberi- 
fication (the respective deposition of suberin or cutin in the 
wall of cellulose). 
The cuticle may occasionally become very thick (the leaves of 
Eucalyptus, Agave, Aloé) and in the same manner many layers 
often develop cork (the Oak"). Since both the cuticle and the 
cork are but slightly penetrable by aqueous vapor, they serve as 
a protection for the organs of the plant against too strong eva- 
poration. 
Suberized membranes are mostly brown. They are just as 
little digestible as lignified membranes, but resist putrefaction 
very energetically, as does also the cuticle. 
Micro-chemically, suberized membranes are characterized by 
the fact that they dissolve neither in concentrated sulphuric acid 
nor in ammoniacal oxide of copper. The cork and cuticle there- 
fore remain behind when tissues are treated with sulphuric 
acid. 
It is, however, to be observed in this connection that the mem- 
branes of the tissues of drugs, in consequence of their strong in- 
filtration with the constituents of cells, which takes place dur- 
ing the process of drying, often resist very obstinately the action 
of reagents, even when no suberification, etc., has taken place. 
It is only after repeated boiling with alcohol, water, and ether, 
that such membranes are made accessible to reagents. 
Suberin cannot be removed from the membranes by the ordi- 
nary solvent of fats, but only through the action of alcoholic 
potassa. It is therefore very firmly (perhaps chemically ?) com- 
bined with the cellulose. 
Closely related to the lignified and suberized membranes is the 
so-called intercellular substance (middle lamella), or that 
substance which cements the cells to each other (x in Fig. 64). 
The intercellular substance ? is insoluble in concentrated sul- 
phuric acid and in ammoniacal oxide of copper; on the other 
‘Compare also the section : Epidermal Tissue. 
* Compare herewith, among others, R. F. Solla, “« Beitrage zur naheren 
Kenntniss der chemischen und physikal. Beschaffenheit der Intercellu- 
larsubstanz,” in Oesterr. botan. Zeitschr., 1879. 
