SECONDARY THICKENING OF STEMS 



263 



Prepare cross -sections of the stem of pteris or aspidium. 



They should be cut very thin and stained with hema- 

 toxylin. Make a sketch showing 

 the arrangement of bundles. Then 

 with the highest power study a 

 single bundle and the sheath sur- 

 rounding it. Draw. 



427. Radial bundles are charac- 

 terized by having several strands 

 of xylem tissue radiating from 

 near the center, and each strand 

 is separated from the next by a 

 mass of phloem. This plan is 

 typical of young roots and root- 

 lets, in which there is but one 

 bundle. 



428. SECONDARY THICKENING OF 



404. Cross-section of fibro- vas- 

 cular bundle of moonseed vsee 

 Fig. 402). f, /, crescent-shaped 

 sheatbs of bast fi bre ; p, phloem ; 



cp, crushed phloem ; c, cam- . 



bium; d, xylem ducts ; t, xylem STEMS. Dicotyledonous (or 6XO- 

 tracheids; m, medullary rays 



of fundamental tissue; from p;enOUS) stems With OD6n Collateral 

 c to f (at bottom), xylem; I,/ .... 



end of first year's growth; , "bundles may increase in diameter 



end of second year's growth of . 



wood. each year. If they are perennial 



they may add a ring of growth each spring (Fig. 407). 

 These rings may be 

 counted on the smooth 

 cross-cut surface of a 

 tree, and the exact age 

 of the tree usually can 

 be very closely deter- 

 mined. All growth in 

 thickness due to the 

 formation of new cells 

 outside of the primary 

 wood is called second- 

 ary thickening. 



405. Part of cross-section of root-stock of aspar- 

 AS We nave agus, showing a few nbro-vascular bundles. 



