04 VEGETABLE ORGANS. 



terminated by a very minute dilatation or knob. The 

 sting contains an acrid liquid,, which escapes when 

 the little knob is broken off in wounding the skin, and 

 produces the well-knoAvn irritation. By the side of 

 the figure of the sting is represented the point of a 

 fine needle (fig. 20), showing that the expression 

 ' ' sharp as a needle " has no force when microscopic 

 bodies are in question. 



Stomata (<jTo/u,a, mouth). On viewing a strip of 

 epidermis, the observer will be sure to notice certain 

 oval or roundish bodies (PL I. fig. 13 a), composed of 

 mostly two kidney-shaped cells in apposition but 

 leaving a chink between them ; these are the stomata. 

 They communicate beneath with the intercellular pas- 

 sages, of which they may be considered the mouths ; 

 and by their agency a direct communication is esta- 

 blished between these passages and the air. The two 

 cells which guard the orifice are termed the " guard 

 cells." 



Stomata are most numerous on the under surface 

 of leaves; they are entirely absent in plants growing 

 under water, and in most of the lower plants. In 

 many of the stomata, viewed in the ordinary way, the 

 air situated between the guard cells is indicated by 

 the black spot or dot present ; but after a time, or by 

 the application of a gentle heat to the slide, the air 

 becomes displaced by the water, and their structure 

 becomes very distinct. 



In certain plants, the epidermis is imbued with 

 flint or sil'ica ; so that even when burnt to an ash the 

 stomata are still quite distinct. Examples of this 

 may be found in the stalk or culm of grasses, as in 

 straw, the shining epidermis of which is siliceous ; or 

 the epidermis of canes. Among the lower plants, 

 this peculiarity is especially curious in the species of 

 Equisetum, or mares'- tails. 



The manner in which the veins of leaves are ar- 

 ranged is worthy of special attention, as it forms one 



