STOMATA. 27 



process throughout the same leaf in different stages of growth. In 

 Marchantia, Mirbel found several tiers of cells forming the stoma, and 

 he supposed that the opening was produced by the absorption of a 

 central cell, leaving the others to form the rim or border. 



56. The number of stomata varies in different parts of plants. They 

 are most abundant on the under surface of leaves exposed to the air, 

 and are often entirely wanting on the upper surface, more especially 

 when it has a dense shining cuticle. In floating leaves the stomata, 

 when present, are on the upper surface only. When plants usually 

 under water are made to grow for some time in the air, their 

 leaves exhibit stomata. When leaves grow vertically, the stomata are 

 often equal in number on both sides. The number of stomata varies 

 from a few hundreds to many thousands on a surface of one inch square. 

 The following table exhibits the number of stomata in the leaves of a 

 few plants : 



STOMATA IN ONE INCH SQUARE. 



Upper side. Under side. 



Misletoe, 200 200 



Tradescantia, 2,000 2,000 



Rheum palmatum, 1,000 40,000 



Crinum amabile, 20,000 20,000 



Aloe, 25,000 20,000 



Clove-pink, 38,500 38,500 



Yucca, 40,000 40,000 



Mezereon, None 4,000 



Paeony, None 13,000 



Vine, None 13,600 



Holly, None 63,600 



Cherry-laurel, None 90,000 



Lilac, Tew 160,000 



57. Appendages of the Epidermis, or Appendicnlar Organs. 



The epidermis frequently exhibits projections or papilke on its surface, 

 in consequence of some cells being enlarged in an outward direction 

 (fig. 73 e e). When these assume an elongated or conical form they 

 constitute hairs (pili or villi), as seen in (fig. 77 h h K). 



Hairs, then, are composed of one or more transparent delicate cells 

 proceeding from the epidermis, and covered with the cuticle (fig. 

 70). They are erect (fig. 78 a), or oblique, or they lie parallel to 

 the surface, and are adpressed. Sometimes they are formed of a 

 single cell, which is simple and undivided, (fig. 78 a), or forked (fig. 

 78 >), or branched (fig. 78 c); at other times they are composed 

 of many cells either placed end to end, as in moniliform or neck- 

 lace-like hairs (fig. 79), or united together laterally, and gradually 

 forming a cone, as in compound hairs (fig. 80), or branched (fig. 81). 

 When several hairs proceed from a common centre, they become stellate 

 (stella, a star), or radiated (fig. 82). The latter arrangement occurs in 



