CHAPTER X 



THE LEAF IX RELATION TO ITS EX VIROXM EXT (Continued} 



THOSE who have never studied under the microscope the singular forms 

 of the covering hairs of leaves, have pleasures offered to them for 

 many a winter evening. Possibly a glance at the illustrations which ac- 

 company this part of our text will help to kindle interest in the subject. 

 Eig. 3606 represents some simple hairs of a species of Brassica-, i some 

 forked hairs of the Whitlow Grass (Draba verna] ; and A a stellate hair of 

 the pretty Alpine Madwort (Alyssum spinosum). In these three specimens 

 the hairs are unicellular, but multicellular hairs are met with in a large 

 number of plants. When 

 the cells grow together in a 

 line, like the beads of a 

 necklace, the hairs are said 

 to be moniliform. Of this 

 kind are the epidermal hairs 

 of the Marvel of Peru (Mir a- 



flrst shows the position of the- leaflets tiurincr the day; at night 

 they fold down close to the stalk. 



pfii fig. 360 j) and of 

 the Virginian Spider wort 

 {Tradescantia virginica, k). 

 AVhen the cells spring from 

 a common point, as in the 

 Cretan Horehound (\larru- 



bium cwticum, e\ the hairs FlG - 352. LEAVES OF LUPIN. 



are said to be tufted. 

 Branched hairs, which need 

 no describing, are found in several plants. They give the downy (toinen- 

 tose} appearance to the leaves of Xicandra anomala, a Peruvian plant of 

 medicinal value (/), and to the decurrent leaves of the Great Mullein ( Ver- 

 bascum thapsus). In a few plants of succulent habit, like the South African 

 Bocheas, excessive evaporation is prevented b}^ the development of special 

 flinty cells on the epidermis of the thick fleshy leaves (g). They are many 

 times larger than the ordinary epidermal cells, and their walls are largely 

 composed of silica. A covering of this kind is simply the salvation of its 

 possessor in time of drought. Not that fleshy leaves are ill-adapted to 

 II 1 289 



