PRICKLES. 95 



prickle pretty strongly thickened narrow-cavitied cells, and, far- 

 ther in, similar but broader ones. These last fill up the whole 

 central part of the prickle. All these cells are finely pitted. 

 The epidermis of the stem is separated from the chlorophyll- 

 containing inner tissue by an often strong layer of considerably 

 thickened elongated cells joining on to one another with oblique 

 end-walls, and containing no chlorophyll. These cells without 

 chlorophyll are of like origin to those which form the inner 

 tissue of the prickle. The tissue-elements of the prickle are, 

 however, separated from the chlorophyll-containing tissue of the 

 stem by a layer of flat -celled tissue, which arises by division 

 from the undermost layer of the tissue of the prickle ; it follows 

 only for a short space the chlorophyll-containing tissue of the 

 stem, and then turns towards the epidermis, in order to limit the 

 base of the prickle laterally also towards the chlorophyll-less 

 tissue of the stem. This is a cork-layer, next to the outer 

 surface of which, by the intercalation of a layer of separation 

 (absciss-layer) the fall of the prickles will result in the older 

 parts of the stem. Even before this, it is possible to break off 

 the prickle pretty smoothly from the stem, along the surface of 

 the cork-layer. If we select a prickle from the leaf-stalk for 

 examination, its structure is found to be in no way different from 

 that on the stem, excepting that at its base the cork-layer is 

 wanting. Since the leaf as a whole will fall, separate provision 

 for the fall of the prickles is unnecessary. 



By careful examination of the cortical tissue adjoining the 

 prickles of the rose, the presence of crystals in the cells can 

 be made out. As they are not dissolved in acetic acid, nor in 

 potash, but on the other hand are dissolved in hydrochloric or 

 sulphuric acid without evolution of gas, they are crystals of oxalate 

 of lime (see p. 63). They have here the form either of mono- 

 clinic prisms or of cluster-crystals. These last consist of a 

 great number of partial crystals which are deposited on an 

 original crystal. The cluster-crystals are conspicuous from their 

 size and stellate form. 



Stinging Hairs of the Nettle. In order to get the stinging 

 hairs of the common stinging nettle (Urtica dioica) uninjured, 

 we must take them from the younger parts of the plant. They 

 are found best on the veins of young actively growing leaves. 

 The hair, which is visible with the naked eye, should be cut off 

 below its point of insertion with the razor, and examined in 



