STRUCTURE OF THE CELL. 27 



The material must have been previously treated for 

 a short time with picric acid, so as to prevent their 

 swelling up and disappearing when they are mounted 

 in water or in dilute glycerine. 



The most suitable material is any tissue of which the cells 

 contain but few starch-granules ; the best is the tubers of the 

 orchid Phajus grandifolius, (Bletia Tankervillice). In this starch- 

 grains can be easily seen borne on the leukoplastids. 



5. Structure of Thickened Cell-walls and of 

 Starch-grains. 



a. Cell-walls. Cut a transverse section of an old 

 branch of Clematis Vitalba ; mount in water ; examine 

 with high power. 



Observe the thick-walled cells of the pith ; the wall 

 appears to consist of a series of concentric layers ; this 

 is described as the stratification of the cell-wall. 



Strip a piece of the bark from the branch, and 

 remove with a needle some of the fibrous internal 

 layer of the bark ; mount in water, tease out with 

 needles, and examine with a high power. 



Observe the dark lines running in the wall of the 

 fibre at an acute angle to the longer axis of the 

 fibre ; note that these lines run in different direc- 

 tions in different layers of the wall of the fibre ; this 

 may be seen by carefully focussing first the surface 

 and then the deeper layers of the wall; these lines 

 are described as constituting the striation of the 

 cell-wall. 



Observe the canals running transversely across 

 the cell-walls. Some of the cells will present their 

 upper walls (those nearest the observer) : on these pits 



