8 EPIDERMIS, ETC. 



caused only by a thickening of the external layer of 

 cellules composing the epidermis itself, and a partial 

 separation of them from the other layers. 



34. On the epidermis of plants exist a great number 

 of little pores or openings' presenting a peculiarity of 

 structure, and which are known by the name of stomata. 



35. Stomata are formed thus, (see fig. 8, a.) Between 

 certain of the cellules of the epidermis there is an open- 

 ing of the layer whichcommunicates with a hollow and air- 

 cells beneath ; above this opening of the epidermis, are 

 placed two little bodies of an oblong shape, which, when 

 adhering close together, cover over the opening, so that 

 no communication between the air-cell and the external 

 air exists (b.) These little bodies can, however, sepa- 

 rate from each other in the centre, remaining connected 

 together only at their extremities, leaving as it were an 

 oval space between them, and partially uncovering the 

 opening in the epidermis over which they are placed. 



36. The nature of the stomatorial bodies themselves 

 seems to be glandular, and various modifications of struc- 

 ture and form are to be seen in different plants. 



37. True stomata exist chiefly on the epidermis of 

 leaves, especially on their under surface, though they are 

 found in other parts as well. They seem disposed in 

 most cases without regard to much regularity, though 

 in others, as in Begonia, the reverse holds good. 

 Their number on a given space of epidermis varies, 

 for in Pinus halepensis, 19 were found, and in Citrus 

 aurantium, 2846. 



38. Stomata do not exist in all plants ; it is said 

 never in those constantly growing under water, or in 

 those growing in darkness : they are few and small in 

 the Pine tribe, and likewise in Grasses. On the epi- 

 dermis of many plants, other openings exist, differing in 

 their conformation from true stomata ; such are seen in 

 Nuphar : and in Nerium oleander are large cavities, from 

 whose sides spring a number of hairs, no true stomata 

 being present. In the Ferns and Moss allies, stomata- 

 like structures are also to be found. 



