140 BRITISH PLANTS 
I. External Protective Equipment of Adult Plants. 
(a) Cork.—Cork is water-proof and air-proof.' The cells 
of which it is composed are empty and filled with air. 
It resists the bites of insects, and keeps out bacteria. 
Wounds are dangerous to plants just as to animals, because 
on wounded surfaces delicate tissues are exposed to bac- 
terial and fungal infection. In plants an antiseptic 
tissue, called callus, rapidly forms over a wounded surface, 
and gradually assumes the characters of true cork. When 
leaves are shed, the scars are covered with a layer of cork, 
which seals the wounds of abscission. The lenticels, or 
breathing-holes, found on bark are more or less filled with 
a loose, powdery form of cork, which permits a limited 
interchange of gases between the plant and the external 
air, but is proof against the invasion of germs. 
(6) Cuticle-—The properties exhibited by cork are due 
to the impregnation of the cell-walls with wax. The super- 
ficial walls of the epidermal cells form a continuous mem- 
brane—the cuticle—which is thickened and impregnated 
with a somewhat similar waxy substance. It is therefore 
impermeable and antiseptic. It forms the limiting skin 
covering all the exposed parts of plants not protected by 
cork, and acts much in the same way as cork. 
(c) Thorns.—A thorn is an aborted shoot. Instead of 
ending in a bud, the shoot, through lack of nutrition, 
ceases to grow, and ends abruptly in a hard pointed 
thorn. Thorny plants are xerophytes; they dominate 
the bush and scrub vegetation of semi-deserts. British 
examples: sloe or blackthorn (Prunus spinosa), gorse 
(Ulex), hawthorn (Crategus Oxyacantha). 
(d) Spines.—This term is usually applied to an aborted 
leaf or parts of a leaf, due to disturbances in nutrition. 
The leaf-veins of the holly end in pointed spines, because 
the intervening leaf-tissue has not developed. In the 
barberry (Berberis vulgaris) every leaf on the long shoots 
is reduced to a branched spine (Fig. 49). In the false 
acacia (Robinia pseudacacia) the stipules become sharp 
and spiny (Fig. 50). In Carlina and Centaurea Calcitrapa 
the involucral bracts of the inflorescence end in spines. 
(e) Prickles.—These are outgrowths of the epidermis and 
subjacent tissue. They are, in fact, little more than large 
multicellular hairs; they contain no vascular tissue, and 
