SECONDARY THICKENING OF STEMS 



2G3 



~- - ---;-;. 



P 



- c 



m 



Prepare cross -sections <>t' the stem of pteris or aspidium. 



They should be cut very thin and stained with hema- 

 toxylin. Make a sketch showing 

 the arrangemenl of bundles. Then 

 with the highest power study a 

 single bundle and the sheath sur- 

 rounding it. Draw. 



4^7. Radial bundles are charac- 

 terized by having several strands 

 of xylem tissue radiating from 

 near the center, and cadi strand 

 is separated from i he next bj a 

 mass of phloem. This plan is 

 typical of young roots and root- 

 lets, in which there is but one 



404. Cross-section of fibro-vas 

 cttlar bundle <>t' moonseed isee laindle. 

 Pig. 402). f, f, crescent-shaped 



sheathsofbaBtflbrejp.pl im; 428. SECONDARY THICKENING OK 



<//, crashed phloem : e, cam- 

 bium; d, xylom ducts; t, xylem STEMS. 1 >icnl \ |c | ( .n. hi- (or eXO- 

 tracheids; m, medullary rays 



of fandamental tissue; from geUOUs) stems with Open Collateral 

 t to i a1 bottom I, tylem : l. ' 



end of Brsi year's growth . bundles 1 1 1 ; i \ increase in diameter 



end "t second year s growth ol 



food. each year, [f 1 1 1 . \ arc perennial 



thej may add a ring of growth each Bpring (Fig. I 1 

 These rings ma\ be 

 counted >n t he smoot h 

 cross-cut Burface of a 

 t pee, and t be exact age 

 of the tree usually can 

 be very closelj deter- 

 mined. All growth in 

 thickness due to the 

 formal ion of new cells 

 oul Bide of i he primary 



wood is called second- P 



ary thickening. 



i _ . ' . A8 We h ; i \ e 



