VIIL] THE BEAN-PLANT. 79 



b. The root. 



a. Its main central portion (axis). 



b. The irregularly arranged rootlets attached to the 

 axis. 



c. The absence of chlorophyll in the root. 



d. The root-sheath, covering the tip of each rootlet: 

 this is difficult to get whole out of the ground in 

 the bean, but is readily seen by examining the 

 roots of duckweed (Lemna) with i inch obj. In 

 the latter plant it consists of several layers of 

 cells forming a cap on the end of the root, and 

 ending abruptly with a prominent rim some way 

 up it. 



c. The stem. 



1 . Erect, green, four-cornered, with a ridge at each angle ; 

 not woody; the gradual shortening of the internodes 

 towards its apex. 



2. Cut a thin transverse section of the stem through an 

 internode ; note its central cavity, and the whitish ring 

 of fibro-vascular bundles in it, which is harder to 

 cut than the rest : mount in water and examine with 

 i inch obj. : note 



a. The medullary or pith-cavity in the centre of the 

 section. 



b. The pith-cells., around the central cavity : large 

 and more or less rounded (parenchyma) : some- 

 times with dotted walls from spots of local thin- 

 ness on them. 



c. The epidermis: composed of a single layer of 

 somewhat squarish-looking cells, containing no 

 chlorophyll. 



