Xl] GLANDULAR HAIRS 91 



Many such hairs are known as glandular hairs, owing 

 to their excreting substances of a viscid or resinous, 

 and often odoriferous, nature, or with other peculiari- 

 ties. 



In the simplest forms of glandular hairs, the uni- 

 cellular hair undergoes changes in the layers of cell-wall 

 immediately beneath its thin cuticle, such that the 

 cellulose is altered and softened, and excreta of the 

 nature of gums, resins, ethereal oils, or mixtures of 

 these accumulate and eventually burst the distended 

 cuticle, and smear the ruptured surface ; or the excreta 

 accumulate in the cell-cavity itself and burst the walls. 

 Such viscid hairs (colleters) are very common on young 

 leaves and inflorescences in winter buds, the resinous and 

 balsamic secretions smearing the surfaces of the im- 

 mature organs and serving as non-conducting protections 

 against the effects of low temperatures and damp, e.g. in 

 the Horse-chestnut, as described on p. 15. 



Glandular hairs of this nature are also common on 

 mature flower-buds and inflorescences of Saxifragacese, 

 PrimulacegB, Caryophyllacese, Labiatae, Compositse, &c., 

 where they are often of protective use against small 

 marauding insects. In very many cases these glandular 

 hairs are capitate, i.e. the one or more cells forming the 

 stalk of the hair end in a rounded dilated head of one 

 or more cells, and the secretion is formed in the latter. 

 Such capitate hairs are well seen on young twigs of 

 Hazel, inflorescences of Saxifraga, Primula, &c. 



In the Hop some of the hairs have their heads dilated 

 in a peltate manner, and secrete the bitter Lupulin in 

 their cell-walls. 



Vesicular or thin-walled bladder-like hairs, filled with 

 water, are common on some Piper acece, Begonias, Urti- 

 cacece, Ampelidece, &c., and have to do with the temporary 



