1 82 PRACTICAL BOTANY 



V. Cut transverse sections of leaves : this may easily be done 

 by holding the terminal bud of a mature plant between pieces 

 of pith, or by embedding in paraffin, and then cutting transverse 

 sections of the whole bud. Mount all the sections as before, 

 and examine first with a low power. Neglecting the almost 

 circular transverse sections of the stem, recognize 



1. Those transverse sections which have passed through the 

 sheathing basal portions of the leaves : these may be readily 

 distinguished by their broad lateral wings, only one layer of 

 cells in thickness. 



2. Those which have been taken from the upper part of the 

 leaf: these may'be distinguished by their more bulky appear- 

 ance. 



Having recognized these sections, put on a high power and 

 examine them in detail : 



i. In the section of the sheathing base of the leaf observe 



a. The two lateral wings, consisting of a single layer of cells, 

 with thickened outer walls, and but little chlorophyll. 



b. The more bulky central portion consisting of 



1. An irregular layer of superficial cells with thickened outer 

 walls, covering both upper and lower surface : beneath these 

 are 



ii.- Bands of sclerenchyma, in which the lumen is almost 

 obliterated. 



iii. Within these lies a " vascular bundle " consisting of 

 elements essentially similar to those composing the central 

 bundle of the stem. 



2. In the sections of the upper part of the leaf note that the 

 arrangement of the tissues is for the most part similar to that 

 in the above sections, but rather more bulky, while opposite 

 each of the cells at the upper surface is seen to be attached a 

 series of three to six chlorophyll-containing cells, which repre- 

 sent transverse sections of those longitudinal plates or lamellae 

 above observed on the upper surface of the leaf ; the uppermost 

 cell in each lamella is enlarged and forked. It is obvious that 

 these chlorophyll-containing lamellae are separate laterally from 

 one another : they constitute the chief assimilating tissue of the 

 plant. 



