56 ELEMENTS OF PLANT ANATOMY. 
tissue. There are a few exceptions to these rules ; for example, 
the thorns of the genus Rubus are trichomes, while the hairs of 
the stinging nettle, the gland-hairs of the rose and even the 
scales of some plants are really outgrowths. According to their 
function they may be classed : first, as secretory hairs or glands, 
seconc those which protect from too low a temperature, third, 
those which prevent too great 
evaporation. ‘The first class 
includes those trichomes or 
outgrowths which secrete 
fluid or half-fluid substances 
of pecuhar character which 
are generally discharged on 
the surface of the organ. The 
second, includes many short- 
lived hairs which die and 
drop off on the maturity of 
the organ which they pro- 
tect. Such are the hairs of 
the plane tree (Platanus) 
buds, supposed to fall in such 
quantities as to produce throat 
and lung diseases. Many 
others might be cited as 
= NI 
FIG. 27. 
Hair from under epidermis of leaf of Phlomis 
Russeliana, partly diagrammatic. aa simple belonging to this category. 
pee The third class comprises 
those hairs which grow so thickly over,the surface and become 
so matted together that the German botanists have given this 
covering the name Fz, or felt. In this sense the term felé may 
be defined as a hair covering whose threads are so closely matted 
together as to leave only capillary interstices. Lastly it is 
believed that this felt covering has another function, namely to 
convey water in a liquid form to the stomata; there are also 
several other arrangements of hairs supposed to promote this 
process, such as little tufts in the angles of the leaf-veins, ete. 
