ON THE STRUCTURE OF THE ROOT. 71 



in the older root) are stained green and the phloem and cell-walls 

 brown. The iodine green may be left out, in which case the sec- 

 tions should remain in the Bismarck brown solution for an hour, 

 and then washed well in 70 per cent, alcohol. The phloem and 

 endodermis will be found to stain more deeply than the surround- 

 ing tissues. 



Preparation of Soluimis. 

 Alcohol : — 



30% Methylated spirit (unmineralised), 33 vols, water- 66 vols. 



5°% M 11 )) 



7% >> >) ij 



Iodine Green Solution : — 

 Iodine green 

 Water 



Bismarck Brown Solution : — 

 Bismarck brown 

 70% alcohol 



A section taken just behind the root-hairs — -say, one inch or a 

 little more from the tip of the root — should be first examined, as 

 it shows the typical structure. 



With the naked eye or a pocket-lens, two distinct parts can be 

 made out in the section : a central dark portion, which we call the 

 central cylinder, and an outer lighter portion, the cortical cylinder 

 (Fig. i). Examined under a magnification of 50 to 100 diameters, 

 we see that both the central and the cortical cylinders are made 

 up of cylindrical or polygonal cells, varying in size and appearance, 

 but all consisting of an outer wall or membrane and an internal 

 cavity. Even with such a low power it is seen that the cells are 

 differentiated into groups or bundles in the central cylinder, which 

 is clearly marked off from the cortex by a ring of black cells, 

 known as the endodermis (Fig. 2). 



Let us now examine under a higher power (a i/6th or i/4th 

 objective) a section double-stained with Bismarck brown and 

 iodine green. 



On the outside will be seen a more or less regular layer of cells, 

 somewhat dark and rather smaller than the cells just inside it. 

 This is the outer layer of the cortex (Fig. 2,//.), and its function 

 is to protect the delicate tissues inside it. For this purpose the 



