186 THE GROWTH OF ROOTS AND STEMS. 



of spines which arise at the base of the leaf-stalk, and these 

 are really modified stipules. The thorns of the Grorse ( Ulex 

 europaeus) are either modified branches or modified leaves, 

 for some of them bear buds in their axils, and are flattish 

 and narrow-pointed leaves, whereas others are cylindrical and 

 form the hard sharp-pointed ends of branches. 



225. Prickles and Hairs may be scattered irregularly 

 over the surfaces of plants, the former mainly on the stem, 

 but the latter on the stem, leaves, and roots. Hairs may 

 assume a variety of shapes, but they are always derived from 

 the layer of cells forming the epidermis, or iirst outer skin, 

 of plants, and hence they fall off when cork is formed. The 

 Stinging Nettle has several kinds of hairs, the largest of 

 which are the stinging hairs. The point of the stinging hair 

 is formed by a sharp, thin, brittle scale of silica, which breaks 

 off in the skin, making a wound into which the acid juice 

 contained in the hair is injected by the contraction of the 

 swollen base of the hair. 



Hairs are frequently glandular and often, sticky. In the 

 latter case they are of use as a protection against obnoxious 

 creeping insects, which are frequently caught and retained 

 by the secretion. The Henbane, Tobacco Plant, etc., catch 

 large numbers of flies in this way, and the leaves of a few 

 plants, such as the Butterwort and the Sundew, feed on 

 insects which are captured in this manner (Art. 241). 



Prickles such as those of the Rose are usually classified 

 as "emergences." This term is applied to outgrowths from 

 stems or leaves, which are neither hairs, roots, branches, 

 leaves, leaflets, nor stipules, and which neither bear buds in 

 their axils nor arise in the axils of leaves. Emergences arising 

 from the stem usually contain no vascular tissue, and hence, 

 when removed, they leave only a superficial wouind (as in Roses 

 and Brambles). Each of the stalked glands on the leaf of 

 Sundew, which are also regarded as emergences, contains, 

 however, a strand of vascular tissue arising from one of the 

 veins of the leaf. 



Prickles, of course, serve to protect the plant from the 

 attacks of herbivorous animals, but they often do more, 

 especially when they curve downwards as they do in the 

 "Rose and Bramble, for in this case they are so many hooks 



