LEAVES. 49 



Hairs. 



Several of the officinal leaves are glabrous (orange, coca), but it 

 is more common to find hairs either dispersed over the lamina or 

 restricted to the veins. The shape of these hairs is constant for 

 the same species, and often recurs in the majority of plants 

 belonging to the same genus. 



Hairs may be divided into two classes viz., simple (or pro- 

 tective) and glandular, and these may be conveniently con- 

 sidered separately 



(a) Simple Hairs. These may be either unicellular or pluri- 

 cellular, according as they consist of a single cell or of several 

 cells. The former occur in senna leaves, the latter in belladonna 

 and foxglove leaves. Their length, shape, thickness, and appear- 

 ance may give useful indications in diagnosing a powdered leaf ; 

 thus in betony leaves they are pluricellular, thick-walled, and 

 smooth, in foxglove leaves pluricellular and thin-walled, in senna 

 leaves unicellular, thick-walled, and warty, and so on. They 

 may, however, assume different forms in the leaves of different 

 plants belonging to the same natural order. 



(b) Glandular Hairs. These are terminated by a cell or collec- 

 tion of cells tnat secrete volatile oil, oleoresin, etc., and thus- 

 often impart to leaves their characteristic odour. They are 

 frequently so delicate that the touch of the finger or of the cloth- 

 ing is sufficient to rupture them, and liberate the secretion. 



Glandular hairs may also be unicellular or pluricellular. The- 

 pedicel that supports the gland is usually very short and uni- 

 cellular, but in some leaves the secretory cells are borne upon 

 four or five superposed cells. 



The gland itself may remain unicellular or it may divide by 

 vertical or horizontal walls, or by both, and thus become pluri- 

 cellular (Solanacece, Composite). In the latter case the mode 

 of division does not appear to be very regular, but when effected 

 by vertical walls it takes place in a definite manner, and the 

 glands are called bicellular, quadrioellular, or octocellular, accord- 

 ing as they are divided by vertical walls into two, four, or eight 

 cells. 



Unicellular or bicellular glands are commonly rounded or 

 oval ; they are inserted on the epidermal cells and are irregu- 

 larly distributed over the whole of the lamina of the leaf. They 

 are rather caducous, and each, as it falls off, leaves a circular sc&r 

 upon the epidermal cells, either near the centre or close to one 

 of the lateral walls. 



Quadricellular and octocellular glands are larger, and usually 

 situated in depressions in the surface of the leaf. They are 

 borne upon very short and rather broad pedicels, which are 

 inserted between several epidermal cells, and, consequently 

 when they fall off leave a scar that differs from that left by a 

 unicellular or bicellular gland. 



