30 



EPIDERMAL APPENDAGES — HAIES. 



turn or partition is formed. This septum ultimately splits, leaving a slit 

 or opening which constitutes the stoma. Mohl has traced this process 

 throughout the same leaf in different stages of growth. In Mar- 

 chantia, 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. 



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 OP SUREAOE OP THE LEAF. 



Mistleto (Viscum album) 

 Spiderwort (Tradescantia) 

 Rhubarb (Ebeum palmatum) . 

 Crinum amabile 

 Aloe ..... 

 Carnation (Diantbus Caryophyllus 

 Yucca ..... 

 Mezereon (Daphne Mezerenra) 



1 amencana . 

 Holly (Ilex Aquifolium) 

 Olive (Olea europsea) 

 Potamogeton natans 

 Victoria regia , . , 

 Vine (Vitis vinlfera) 

 Cherry-laurel (Laurocerasus communis) 

 Lilac (Syringa vulgaris) . 



Upper Side. 

 200 



2,000 



1,000 

 20,000 

 25,000 

 38,500 

 40,000 

 None. 

 None. 

 None. 

 None. 

 None. 



7,800 

 21,600 

 None. 

 None. 

 Few. 



Under Bide. 



200 



2,000 



40,000 



20,000 



20,000 



38,500 



40,000 



4,000 



13,000 



1,560 



63,600 



57,600 



None. 



None. 



' 13,600 



90,000 



160,000 



Appendages op the Epideemis, or Appendicular Organs. — 

 The epidermis frequently exhibits projections or papillae on its surface, 

 in consequence of some cells being enlarged in an outward direction 

 (fig. 76 e e). When these assume an elongated or conical form they 

 constitute hairs (pili or villi). 



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

 proceeding from the epidermis, and covered with the cuticle (fig. 73). 

 They are erect (fig. 80 «), or oblique, or they lie parallel to the sur- 

 face, and are appressed. Sometimes they are formed of a single cell, 

 which is simple and undivided (fig. 80), or forked (fig. 81) or 



