
‘ing tissue, the cracking 
THE STEM 1538 
sclerenchyma cells cannot grow and divide, and can be stretched 
but little, if at all. Therefore, as the wood increases in diame- 
ter the band of sclerenchyma is ruptured and parenchyma cells 
are pressed in between the isolated strands formed by the break- 
ing up of this sclerenchyma ring. 
The epidermis frequently increases in size by radial growth 
and division, but it seldom happens that the epidermis grows 
fast enough to prevent 
it from being ruptured 
by the increase in size 
of the tissues within it. 
If the place of the epi- 
dermis were not taken 
by some ‘other protect- 
of the epidermis would 
expose the underlying 
tissue to an excessive 
rate of evaporation and 
would also allow the 
entrance of  disease- 
producing organisms. 
These contingencies are 
prevented by the for- 
mation of cork. 
Cork. Cork consists 
of dead cells which fit close together without intercellular spaces 
(Fig. 149). The walls of cork cells are impregnated with a waxy 
substance, suberin, which makes them practically impervious to 
water. Cork is familiar in the form of ordinary cork stoppers, 
which are obtained from the cork oak of Spain, on which cork 
is exceptionally well developed. Cork not only restricts tran- 
spiration but, like the epidermis which it replaces, hinders the 
entrance of parasites and affords mechanical protection. 
Phellogen. The cork is formed from a layer of secondary 
meristem known as the phellogen. In stems the first phellogen 

Fre. 150. Early stage of cork formation in 
Ixora finlaysoniana. (x 250) 
7 
