PELTATE HAIRS. STINGING-HAIRS 129 



In many plants, finally, the protective covering is made up of 

 peltate hairs or scales : excellent instances of these are found among 

 Oleaceae and Eleagnaceae (Fig. 36). Perhaps the most remarkable 

 and effective arrangement of peltate hairs occurs, according to Bobisut 

 in the Palm Arenga saccharifera, The margins of the scales which 

 cover the lower side of the leaf, in this Palm, 

 are produced into numerous tubular branches ; 

 the latter are so closely interwoven that it 

 is quite impossible to distinguish the individual 

 scales. In addition, similar tubular branches 

 grow downwards in dense masses from the 

 lower side of each scale ; these in their turn 

 are abundantly branched, and the branches of 

 adjoining tubes interlock so that the final result 

 is an extraordinarily compact felted covering, 

 which completely masks the actual epidermal fhj. 30. 



Surface. Peltate scale from the adaxial leaf- 



1Tr . surface of Hippophae rkamnoides. 



We must next pass on to consider those 

 hairs which serve for protection against the assaults of animals ; here 

 again one meets with great variety of detail, and in many cases 

 also with remarkable specialisation. Protective hairs, other than those 

 of the glandular type, may be grouped under two headings. The first 

 group includes all hairs the action of which is purely mechanical ; 

 the second form of protective hair is in addition possessed of poisonous 

 properties, and its effectiveness is indeed largely dependent upon 

 chemical effects. 



The mechanical type of protective trichome is exemplified by the 

 short stiff bristles which are characteristic of certain families of 

 Angiosperms ; the walls of such bristles are often calcified or silicified 

 and in addition covered with protuberances in the shape of knobs or 

 bosses, which act as barbs. The Boraginaceae and many Cruciferae 

 are excellently protected by such rasping hairs to use Stahl's term 

 against snails, caterpillars and other creeping animals, and also against 

 browsing mammals. Even more effective are the fish-hook-like bristles 

 of Opuntia, with their numerous barbs. 



The stinging-hairs 75 which occur in the Urticaceae and Loasaceae, 

 in Jatropha (Euphorbiaceae) and in Wigandia (Hydroleaceae) are 

 among the most elaborate of the structures employed by plants for the 

 purpose of protecting themselves against their animal foes. A typical 

 stinging-hair consists essentially of a single large cell with abundant 

 protoplasmic contents ; the lower end of this cell is expanded to form 

 a vesicle or bulb, embedded in a cup-shaped emergence. The walls of the 

 hair are thick and brittle, except in the region of the bulb. In the 



