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 fmigal infection. In plants an antiseptic 

 tissue, called caUus, rapidly forms over a woimded 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 woimds 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 Umited 

 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 ru 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 {Cratcegiis 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 (Berheris vulgaris) every leaf on the long shoots 

 is reduced to a branched spine (Fig. 49). In the false 

 acacia {Rdbinia pseudacacia) the stipules become sharp 

 and spiny (Fig. 50). In Carlina and Centaurea Galcitrapa 

 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 



