VEGETABLE HISTOLOGY AND PHYSIOLOGY. 



155 



apex breaks off and the contained irritating liquid is 

 discharged into the wound, producing the sting. 



394. Stoma-cells are epidermal. Stomata are 

 mouths or openings into the intercellular spaces of the 

 leaves and young bark, sometimes called breathing- 

 pores. They are guarded each by two half-moon- 

 shaped cells whose concave sides lie next each other ; 

 when filled with liquids, their concavity increases, leav- 

 ing an orifice between the guard cells. For the passage 

 of air and moisture when the atmosphere is dry and 

 the plant can not afford to part with its fluids, the 

 guard cells lose their concavity, the sides become 

 straight, the orifice closes, and evaporation is arrested. 

 The number of these openings is very great, many 

 thousands appearing in a square inch of surface (Figs. 

 498-500). 



498, Cells of epidermis with a stoma from leaf of Helleborus fcetidus. 499, Vertical section of a stoma of 

 Narcissus ; a, cuticle. 500, Epidermis cells with stomata of Tradescantia Virginica a, a, guard cells. 



395. Cork cells are cubical or tabular, and fit closely 

 together ; in the outer layers they are dead and empty, 

 and constitute the outer bark layers of old trees, prom- 

 inent in the Quercus suber (Cork Oak). 



