LECT. XI.] ANATOMY OF LEAVES. 613 



slice of the cutis of Marchantia 

 under the microscope. The aper- 

 tures are oval, and placed in the 

 centre of a slight elevation, as re- 

 presented in the marginal cut (*). 



In respect of size, pores differ 

 considerably in different plants ; 

 but on the leaves of the same plant their size is 

 nearly uniform. The largest, as far as rny obser- 

 vations extend, are those found on the leaves of 

 Oleander; and the smallest on those of the genus 

 Myrteae. Sprengel says, that in the Coronariae 

 " their longitudinal diameter is from -rV to ^ 

 " part of a geometrical line, and their diameter, 

 " in the cross direction, is from -^V to ^V part;" 

 but in " the Myrteae, Rosaceae, Leguminosae, 

 " and Caryophylleae, two hundred of them, at 

 " least, might lie upon a geometrical line-f~." 



In number the foliar apertures vary, also, in dif- 

 ferent plants. The more minute they are, the more 

 numerous. On the lower surface of the leaf of 

 Gardenia latifolia, we find an aperture in almost 

 every mesh ; but in the Aloe tribe scarcely one pore 

 for twenty meshes, and on the leaf of Oleander, one 

 among sixty. With regard to position, these aper- 

 tures are in some instances arranged in lines from 

 the base to the apex of the leaf, and have the same 



t Elements of the Phil, of Plants, fyc. by A. P. DecandoUe 

 and K. Sprengel, 310. 



RR3 



