HAIRS. SCURF. 



15 



-ILL/ 



it is, in general, easily raised up (fig. 9, a and c) : we often re- 

 mark in it little openings called sto'mata (from the Greek, stoma, 

 mouth), which are not visible without the assistance of a magni- 

 fying-glass (fig. 10, 6) ; the a 



edges of these pores are 

 formed by two oval or glo- 

 bular cells filled with green 

 globules, and their opening e 

 corresponds with the inter- 

 cellular vacuities or lacuna? 

 (fig. 9, fe), the uses of which 

 appear to be very important 

 in the respiration of plants. 

 No sto'mata are found upon 

 the roots ; many cellular 

 plants, such "as mushrooms Fi ?' ^.-HORIZONTAL SECTION OF A LEAF. 



and mosses, are altogether without them, and they are also want- 

 ing in certain plants that live in water. 



. 26. The hairs of plants are external appendages formed of 

 elongated and projecting cellules ; sometimes they are simple, 

 that is, composed of a single cell ; sometimes they are partitioned, 

 that is, formed of several cells arranged in a row, end to end, and 

 at other times they are more or less branching ; sometimes they 

 lie upon glands, and serve as an excretory canal to the caustic 

 juices secreted by these organs. 



Hairs vary extremely in length, density, rigidity, ancf other particulars ; 

 on this account they have received the following names : 



Down, or pubescence,when they form a short soft layer, which only par- 

 tially covers the cuticle or epidermis. 



Hairiness (hirsutus), when they are rather longer and more rigid. 



Pilosity (pilosus}, when they are long, soft, and erect. 



Villosity (villosus), when they are very long, very soft, erect, and straight. 

 Crini (crinitus) are this variety in excess. 



Velvet (velutinus\ when they are short, very dense and soft, but rather 

 rigid, and forming a surface like velvet. 



Cilia;, eye-lashes (ciliatus\ when long, and fbiming a fringe to the 

 margin, like an eye-lash. 



Bristles (setts, sctosus), when short and stiff. 



Stings (stimuli slimulans\ when stiff and pungent, giving out an acrid 

 juice if touched, as in the nettle. 



Glandular hairs (pili capilati), when they are tipped with a glandulai 

 exudation. 

 'Hooks (hami, unci, rostelld), when curved back at the point. 



Barbs (glochis glochidatus\ if forked at the apex, both divisions of the 

 fork being hooked. 



Explanation of Fig. 10. Horizontal section of a leaf, magnified : 

 dermis; b. stomata; c. cellular tissue of the paren'chyma. 



26. What are hairs ? Mention some of their varieties. 



epi- 



