330 



MANUAL OP BOTANY 



regarded as the means of communication between the inter- 

 cellular space system and the outer air. 



Fig. 707. 



Fie. 708. 



Fuj. 707. Grrowing poiut of shoot of Equisetuni arvense. X 250. a. Apical 



cell, (i, c. Successive segments cut off from it. Fig. 708. Growing 



point of the thalhis of Pelvelta. a. Apical coll. x 260. After Kny. 



Fig. 709. 



The cells of the epidermis are often prolonged outwards in 

 the form of 7ia irs, or trichomes. These are borne alike upon the 

 sporophyte and the gametophyte, and they 

 present considerable variety of form and 

 structure. 



Sfomata. — These are orifices situated 

 between the sides of some of the epidermal 

 cells, and opening into the intercellular 

 cavities beneath {figs. 709, s, and 710, s). 

 They originate by the vertical division of 

 a mother-cell into two. The partition 

 between the latter thickens slightly, and 

 splits so as to form an opening between 

 them, leaving them still attached to each 

 other by their ends. The split constitutes 

 the stoma, and the two cells are the 

 guard-cells. They are commonly of a 

 more or less semilunar form, and contain some chlorophyll grains ; 

 when they are full of water their form causes them to become 

 curved, so that the orifice is widely open ; when they lose their 

 water they relax and then' edges come into contact, closing it. 



Fiij. 709. Vertical section 

 of a portion of the epi- 

 dermis of ii leaf, show- 

 ing a stoma lightly 

 depressed, a. The stoma. 

 e, e. Epidermis. 2'. 

 Parenchyma beneath 

 the epidermis, l. In- 

 tercellular space into 

 wliich the stoma 

 opens. 



