DEFENCES OF PLANTS 337 



that plants have developed both inward and outward 

 characters which are resistant to such attacks. There 

 are certain by-products of their chemistry which have 

 an important value in this respect. We do not venture 

 to assert that these by-products have been deliberately 

 produced for the purpose; on the reverse, we are dis- 

 posed to declare that the plants which yield them chance 

 to have established thereby an immunity which has 

 been an important factor in their survival. Among 

 these products are cork, latex, gums, resins, turpentines, 

 ethereal oils, etc. The resin exuded from a cut made 

 in a Pine branch is an effectual means of closing the 

 wound and warding off bacteria. Latex is a powerful 

 antiseptic, a fluid of a most complex formation, generally 

 milky in appearance ; it is developed in lengthened cells, 

 which form tubes running up and down the plant and 

 occur most freely in the cortex or bark of the stem. It 

 is from the latex of the Eubber plants that the rubber 

 of commerce is obtained. In some plants latex is 

 very freely produced; opium is from the latex of the 

 Opium Poppy. The Greater Celandine {Chelidonium 

 majus, Plate XXIII.) develops an orange-coloured 

 poisonous latex in all its parts; it exudes from a broken 

 stem or leaf-stalk and congeals over the wound, forming 

 a protection which is proof against bacteria and fungi. 

 The milky juice, really the latex, found in the Lettuce, 

 Dandelion, and Sow-Thistle, is familiar to all who have 

 even the most superficial acquaintance with these plants. 

 A potato from which a piece has been cut quickly 

 develops a layer of cork cells across the exposed sur- 

 face of the cut, thus preventing evaporation of moisture 

 and offering resistance to fungi and bacteria. A layer of 



43 



