I 



STINGING AND GLANDULAR HAIRS. 97 



results in the broken surface being sloped downwards, and thus 

 a sharp point to the hair is produced. It will also be seen that 

 though we speak of the hair as ending in a knob, this latter is 

 finer than the finest needle, and hence, in case of careless hand- 

 ling, the hair pierces the skin, and being very brittle, the knob 

 breaks off, and the strongly acid sap enters the wound and 

 causes slight inflammation. On the same piece of the epidermis, 

 near to the stinging hairs, are also small unicellular bristles 

 (cf. Fig. 36), which are distinguished by the strong thickening of 

 their walls, and their fine tapering points. Similar bristles are 

 found on the .edge of the leaf. For this purpose it suffices to 

 place a piece of the leaf in water under a cover-glass. In old 

 leaves the bristles can be thickened almost to the obliteration of 

 their cavity ; their surface is covered with small protuberances. 



Glandular Hairs in Primula. We have already found glan- 

 dular hairs on the edge of the petals of Verbascum nig rum ; they 

 can be studied under more favourable conditions in the common 

 greenhouse Primula, P. sinensis. For this purpose cross-sections 

 are taken through a leaf-stalk. The body of the hair is divided 

 from the epidermoid foot-cell by a cross wall situated out beyond 

 the epidermis, and forms a call-row, which consists of usually 

 two (sometimes more), longer and broader, and one (rarely two), 

 narrower and shorter cells. The last cell bears the globular 

 head, and upon this is borne a more or less strongly-developed 

 cap of highly refractive resinous yellowish substance. The secre- 

 tion of this takes place between the cuticle and the rest of the cell- 

 membrane. The cuticle is raised, distended, and finally ruptured, 

 whereupon the secretion overflows the upper part of the hair. 

 The addition of alcohol removes trje secretion, and then the 

 raised cuticle can be clearly seen lying in folds. The cells of the 

 hair show a beautiful network of protoplasm, with suspended 

 nucleus, in which lies a large nucleolus. Small chlorophyll 

 bodies are embedded in the peripheral protoplasm. 



Hairs of Pelargonium zonale. Instead of Primula sinensis, 

 and, like it, available all the year round, we can use the ordinary 

 zonal bedding " geranium," Pelargonium zonale, in any of its 

 innumerable varieties. The glandular* hairs have exactly the 

 same structure as those of Primula, and can be very conveniently 

 studied upon cross-sections through a full-grown but not too old 

 petiole. This plant is indeed a very favourable object for the 

 study of hairs generally, for they are present in various typical 



7 



