68 THE MECHANICAL FUNCTIONS. 



the stem and branches, and interpos'ing a barrier to the ac- 

 tion of fluids, or other extraneous bodies, on the living or- 

 gans. The cuticle is formed originally by the condensation 

 of a layer of cellular tissue, of which the cells, being conso- 

 lidated by exposure to the air, and by compression, compose 

 a thin but impervious pellicle. Amici has distinctly shown, 

 by means of his powerful microscope, the cellular structure 

 of the cuticle, and also that the layer of cells of which it con- 

 sists is independent of the subjacent cellular tissue.* Fig. 

 20 is intended to show this circumstance, the shaded part 

 representing the cuticle with its series of cells. 



Oval orifices, or stomata, as they have been termed, are 

 discoverable on almost every part of the surface of the cuti- 

 cle, but more especially in those that have a green colour.! 

 They are placed at nearly equal distances from one another, 

 and are particularly numerous in the cuticle of the leaves, 

 where they occupy the intervals between the fibres. These 

 orifices conduct into the interior of the plant, probably into 

 the general cavity of the intercellular spaces. It is evident, 

 from the functions they perform, that they must occasional- 

 ly open and close; but the minuteness of their size precludes 

 any accurate observation as to the nature of the apparatus 

 provided for the purpose of performing these motions. Ami- 

 ci describes their margins as formed by two cells, by the 

 movements of which, combined perhaps with those of the 

 adjoining cells, he conceives these orifices are opened and 

 closed.! Great variety, however, is observable in the struc- 

 ture of the stomata in different species of plants. 



Many plants have no stomata, either on the cuticle of the 

 leaves, or on that of the stem. This is the case with such 

 aquatic plants as are habitually immersed in water. In those 

 that are only partially immersed, stomata are met with in 

 those parts exclusively which are above the water. The 



• Annales des Sciences Naturelles, II. 211. 



•j- Fig". 22 is a magnified representation of the appearance in the cuticle 

 of the Lycopodium denticulatum^ tixken in the central part of the lower sur- 

 face of the leaf, from De Candolle. Fig. 21 is a still more magnified view 

 of the stomata in the leaf of the Lilium ca?ididum, from Amici. 



t Ibid. II. 215. 



