92 EMERGENCES, PRICKLES, ETC. [CH. 



storage of water. In some cases they only occur on quite 

 young leaves. 



So far I have regarded hairs, whether unicellular or 

 multicellular eventually, as derived from a single (or 

 rarely more than one) superficial cell. The case is not 

 essentially altered when, as often happens, more than 

 one cell of the epidermis takes part in the initiation 

 of the multicellular outgrowth, and cells of the sub- 

 epidermal tissues also enter into its composition. 



Morphologists distinguish such composite outgrowths 

 as Emergences, from the true Hairs or Trichomes 

 hitherto considered : as before, the cells may lose their 

 living contents or not, have their cell-walls thickened, 

 exude excretions, and so on. 



One of the simplest forms of emergence is found in 

 the glands at the margins of the stipules of the Pansy 

 {Viola tricolor), where an axial row of thin- walled sub- 

 epidermal cells emerges and is capped by the enlarged 

 secreting epidermal cells. 



More remarkable are the Tentacles of various insecti- 

 vorous plants (e.g. Drosera) where all the tissues enter 

 into its structure. The cells at the upper end secrete 

 a viscid slime at the surface, and after an insect has 

 become entangled in this, the tentacle secretes digestive 

 juices from its cells, and eventually absorbs the peptonised 

 proteids. Similar digestive and absorptive functions 

 appertain to true hairs in the case of other plants (e.g. 

 Pinguicnla, Utricularia, Nepenthes, &c.), but since we are 

 not here concerned with this interesting biologic group of 

 plants, I pass them by with this mere reference. 



Prickles, which must be distinguished from Thorns, 

 are either emergences (Rose, Gooseberry) or multicellular 

 hairs of pointed form and with the cell-walls hardened 

 and dry. Similar structures are common on fruits or 



