SECT. IV. tEAF-STALK AND LEAF. 273 



sfection of a circle, upon which the external form of 

 the petiole depends. If the circle is completed in 

 the distribution of the several bundles, the petiole is 

 cylindrical ; and if the circle is not completed, it is 

 then semi-cylindrical, being concave above and con- 

 vex below, and thus presenting, like the leaf, two 

 different surfaces. 



The expansion of the leaf, which is merely a pro- 

 longation of the petiole, discovers upon dissection 

 the same component parts, though differently modi- 

 fied. If a leaf is taken and torn asunder, either in 

 a transverse or longitudinal direction, fragments of 

 a fine and transparent pellicle will be seen project- 

 ing beyond the edge of the torn part. This is the 

 epidermis. It may be detached in large portions 

 from the leaf of the common Sorrel ; but enough of 

 it may be detached from the surface of almost any 

 leaf for the purpose of microscopical inspection. 



When the epidermis is stripped off, the parenchyma The fibres 

 is then laid bare, being of a green and pulpy sub- theexpln" 

 stance, constituting for the most part the mass of ^" 

 the leaf, interspersed with the prolongations of the 

 fibres of the petiole, which are now divided into a pro- 

 digious number of ramifications mutually embracing 

 arid intersecting one another, and forming a sort of 

 fabric similar to a piece of fine net-work. The princi- 

 pal fibre, extending from the base to the apex of the 

 leaf, forms what is called the midrib, and the rami- 

 fications form what are called the nerves and veins, 

 the nerves being composed of the larger bundles of 



VOL. i. T 



