66 PRACTICAL BOTANY 



and examine under a low power : observe numerous highly re- 

 fraction globules scattered over the section, or in the tissue : these 

 will be globules of resin, and they are to be found chiefly in or 

 about the resin-passages. 



1. Irrigate with alcohol : the globules will be dissolved. 



2. Place another section, in which the highly refractive 

 globules can be seen, on a slide : cut a tangential section from the 

 dry Alkanna root and place it with the outer surface downwards 

 on the section to be tested : irrigate with 50 per cent, alcohol, and 

 cover with a cover-glass for about an hour : on removing the 

 Alkanna section and examining, the globules of resin will be 

 found stained red. Obviously the difficulty in this reaction is 

 that alcohol dissolves both resin and alkannin, but 50 per cent, 

 alcohol has only a very slow action on resin, while it dissolves 

 the alkannin sufficiently for the colouring to take place. The 

 sections might be directly stained with solution of alkannin (see 

 Appendix A), but the results are not so good as by the above 

 method. 



Note that iii the epidermis, collenchyma, and thin- 

 walled parenchyma of the cortex, there occur, especially 

 in stems growing apace, divisions of the cells in a 

 radial direction. Compare the girth of the stem at 

 the upper with that at the lower part of the plant, or 

 that of a young plant with that of an old one. The 

 conclusion will naturally be drawn that the stem 

 increases in girth as it grows older, and since the outer 

 tissues neither peel off, nor do the individual cells 

 increase greatly in width, longitudinal radial divisions 

 of the cells are the only alternative. 



Before leaving the cortical tissue it must be noticed 

 that the bundle-sheath, which is the inmost layer of 

 the cortical tissue, and which is easy of observation in 

 the younger stem, may be identified also in these 

 sections, though with difficulty (see below, p. 76). 



