OUTER CELLS AND TISSUES. 33 



Secretory hairs are occasionally composed of one or, at most, 

 a few cells, the apical one of which forms the glandular portion 

 (Pelargonium). 



In the leaf of Pelargonium (see Fig. 18) the hair consists of 

 three distinct cells — viz., a basal cell and two upper ones — the 

 apical one being spheroidal in shape and possessing protoplasmic 

 contents which manufacture a sticky secretion. Such a hair is 

 known as a capitate glandular hair, and occasionally these hairs 

 serve as organs of absorption for ammonia and nitric acid 

 existing in the atmosjjhere.* 



Another type of secretory hair is seen in the stinging nettle 

 (Urtica urens) ; each hair is here an elongated cell which arises 

 from an epidermal cell of the stem or leaf, having a broad base 

 surrounded In' a cup-shaped receptacle formed by a large number 



Hg. 18. — A Cawtate Glandular Hair from the Leaf op 

 Pelargonium. 



of small cells which have been produced by the divisions, in an 

 early stage, of adjacent epidermal cells. The whole hair tapers 

 towards the apex, which is extremely delicate and surmounted 

 by a small knob ; internally are seen protoplasm and nucleus. 

 Pormic acid (strictly speaking, an excretion, and not a secretion) 

 is formed in the hair, and it is this substance which produces the 

 stinging sensation and rash when the fine broken apex of the 

 hair penetrates the skin (see Fig. 20). The hair of the nettle is 

 thus seen to be mainly protective in function. 



In Bhododendron secretory hairs arise which are composed of 

 many cells, each of these possessing protoplasm, and secreting 

 a sticky substance (see Fig. 19). 



* Kerner and Oliver, Natural History of Plants, vol. i. 



