186 PLANT ANATOMY, 
The form and size of the hairs, as also the relative thickness 
of the wall compared with the lumen or cavity, afford in some 
cases good points of discrimination for the recognition of adul- 
terations in foods and alimentary substances. Thus Wittmack : 
distinguishes wheat and rye flour by the fragmentary hairs of 
the so-called coma, which always occur therein in small amount; 
and upon the form of the hairs Bell? bases the distinction of 
tea, elder, willow and black-thorn leaves (Prunus spinosa Lin.). 
. The physiological function of the hairs of organs of assimila- 
tion (leaves), which when old mostly contain air, is to diminish 
the extent of transpiration. The hairs of seeds, which are often 
Fic. 96.—Oil glands of the Labiate, e. g.,0f Rosmarinus. A, Longitudinal section of 
valarge gland. a, stem-cell; b, eight delicate-walled daughter cells which produce the 
volatile oil, by the escape of which the cuticle of the parent cell, d, becomes expanded: 
Jf, epidermis of the leaf upon which the gland is formed; g, palisade-cells; e, a small 
gland. B, Transverse section of Fig. 4. Compare also De Bary, “‘ Anatomie,” Fig. 39. 
feather-like, are means of distribution; the firm hairs of climb- 
ing plants are organs which serve to fasten them. 
Some prickles, for example in Rubus, are, like the hairs, 
also of epidermal origin, and are thus trichomes. Jn the forma- 
tion of most of the true prickles or outgrowths, however, the 
tissues beneath the epidermis, and even the vascular bundles, 
also participate (Rosa, Smilax #), 
'* Anleitung zur Erkennung organischer und anorganischer Beimen- 
gungen im Roggen- und Weizenmehl.” Leipzig, 1884. 
* Bell, “ Die Analyse der Nahrungsmittel,” i, (Berlin, 1882), 36. 
"De Bary, “ Anatomie,” p. 61, where the literature is given. 
