90 PEACTICAL BOTANT 



to the centre and periphery of the vascular ring : these 

 are secondary medullary rays. 



Compare with the vascular arrangements of Helian- 

 thus, and with Fig. 8, A, B, and 0. 



Cut transverse sections through the axis of a bud, or of a young 

 twig, during the process of extension in spring ; treat with potash, 

 and mount in glycerine. In these sections the vascular system 

 will be found to be much less developed, but even here the 

 primary bundles will not be found to be as clearly distinct 

 from one another as in the young stem of Helianthus. In 

 ligneous Dicotyledons the interfascicular cambium begins to be 

 active at an earlier period than in those which are herbaceous. 



Examine the several tissues, above enumerated, in 

 detail with a high power : 



1. Epidermis : a single layer of cells, with the outer 

 wall thickened and cuticularized or corky (test with 

 the usual reagents, see p. 39) : stomata will be found 

 in a normal position in young twigs, in older ones they 

 are found at the apices of the lenticels (see below 

 p. 93). Note the form of the conical hairs, the walls 

 of which are silicified. 



To obtain proof of the latter fact, treat tangential sections of 

 the surface of the stem with potassium chlorate and nitric acid 

 dry them with blotting-paper and ignite on a cover-slip, or plati- 

 num foil ; mount the ash in water, and treat with nitric acid. 

 Silicified walls will after this treatment present the same outline 

 as they originally did. In this case complete skeletons of the 

 conical hairs will be found. 



Note on Silicified Cell- Walls. 



In many plants the cell-walls over extensive or limited areas 

 are liable to silicification, and by a simple method the silica- 

 skeletons of such walls may be prepared, and observed under the 

 microscope. The stems or petioles of the Cucumber afford good 



