50 
PLANT MORPHOLOGY. 
Cork cells replace the epidermal cells of roots and 
stems that persist year after year. They differ from 
the epidermal cells in that the walls are uniformly 
thickened and on surface view are polygonal in shape. 
The walls consist of suberin, a substance allied to 
cutin ; in some instances they also contain ligno- 
cellulose, forming cork -stone cells, as in asclepias. 
The young cells may contain a thin layer of proto- 
plasm and a nucleus; they usually also contain 
brownish tannin, or tannin- like compounds, and 
occasionally crystals of cerin, or calcium oxalate. 
Cork not only occurs as a secondary protective 
layer, but may also arise in other parts of the plant as 
a result of injury, as in leaves, fruits, steins and tubers. 
It also arises as a result of the disarticulation of the 
leaf in autumn. 
Lenticels may be described as biconvex fissures in 
the cork which permit of the easy passage of air through 
the intercellular spaces of the rather loosely arranged 
cells lying beneath them. They usually arise as the 
product of a meristem situated beneath the stomata of 
the epidermis, the stomata being replaced by them in 
the development of cork. Several types of lenticels 
are distinguished. They are quite characteristic and 
prominent in a number of barks, as those of species of 
Betula, Prunus, Rhamnus , etc. 
UATICIFEROUS TISSUE. 
Laticiferous or milk tissue occurs in all those plants 
which emit a milk-juice on being cut or otherwise 
wounded. The juice may be colorless, as in the oleander; 
whitish, as in the Asclepiadacese and Apocynacese; or 
yellowish or orange, as in the Papaveracese. It con- 
tains caoutchouc, oils, resins, mucilage and starch, cal- 
cium oxalate and alkaloids as well. The walls are rela- 
