CHAPTER X 



THE LEAF IX RELATION TO ITS ENVIRONMENT (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. 

 Pig. 3606 represents some simple hairs of a species of Brassica : i some 

 fwked hairs of the Whitlow Grass (Draba verna) ; and h 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 (Mira- 

 bilisjalapa, fig. 360j) and of 

 the Virginian Spiderwort 

 {Tradescantia virr/inica, k). 

 When the cells spring from 

 a common point, as in the 

 Cretan Horehound (Marru- 



bium creticum, ), the hairs FlG - 352. LEAVES OF LUPIN. 



are said to be tufted. The flrst shows th t e j y 1 

 Branched hairs, which need 



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

 tose) appearance to the leaves of Nicandra anomala, a Peruvian plant of 

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

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

 Eocheas, excessive evaporation is prevented by 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 



of the leaflets clurinsr the day ; at night 

 iown close to the stalk. 



