54 ELEMENTS OF BOTANY. 



Examine each thin section first with a power of about 25 diameters, 

 then with a power of from 100 to 200 diameters. With the lower power, 

 sketch a one-year-old section, labeling in your sketch : 



(o) The corky layer of the bark. 



(6) The green layer. 



(c) The masses of bast fibres. 



(d) The wood, with the medullary rays and vessels. 



(e) The pith. 





A! 



^'■^ 



fJR 





Fig. 41. — Cros5-Scct;on of a Tliree-year-old Linden Twig. (Mueli magnified.) 



P, epidermis and corky layer of the bark ; Phi, liast ; C, cambium 



layer ; JR, annual rings of wood. 



After examining this section with the higher power and noting par- 

 ticularly the appearance of the wood-cells, replace it by a section of stem 

 at least four years old. ' 



Sketch this as seen with .the low power, then substitute the higher 

 power and study the inner hark, noting especially the masses of very 

 thick-walled bast-cells. 



What are the principal differences between the structure of the apple 

 twig, so far as you have examined it, and the structure of a linden twig, 

 as shown in Fig. 41 ? 



