SECONDARY THICKENING OF STEMS 2G3 



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



They shoukl be cut very thin and stained with hema- 



'^~^,cc> toxyliu. Make a sketch showing 



f the arrangement of bundles. Bicol- 



( V- -^ lateral bundles differ from the col- 



{^^ s ^v> lateral in having additional phloem 



M ^^ on the inner side of the xvlem 



'^ ,' I strand; well marked in pumpkins 



' i d and squashes. 



T^\- ;fc 427. In roots the phloem and 



^ ,* xylem are not definitely arranged 



^' in bundles, but in alternating 



radial strands or plates. This 



f plan is typical in young roots and 



rootlets, but is more or less ob- 



404. Cross-section ot tibro-viis- 

 cularUmdle of moonseed (.see sCUred lU oldcr OUCS. 

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



sheaths of hast fibre;i). phloem; 428. SECONDARY THICKENING OF 



cp, crushed phloem ; c, eam- 



biuinjd, xylem ducts: «, xylem STEMS. — DicotvIedoUOUS (or 6X0- 

 tracheids; m, medullary rays 



of fundamental tissue; from geUOUs) stcmS witll OpCU Collateral 

 c to / (at bottom), xylem; 7, "^ ' 



end of first year's growth; 2 buudlcs may Increasc 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 .^KKM~ 



counted on the smooth j^^^^^^m.- 

 cross-cut surface of a J^^^^^^B^" ^ ' 

 tree, and the exact age /^9H|pH|^: 

 of the tree usually can ^^n-r-r^S ^ ' . \. 

 be very closel}' deter- --^^^ -. 

 mined. All growth in ' ;i . 



thickness due to the 

 formation of new cells 

 outside of the primary \ 

 wood is called second- 

 ary thickening. 



.QQ . 1 405. Part of cross-sectiiMi (f rni.t stock of aspar- 



4ZJ. As we Iiave agus, showing a few til.io-v^iMuhir bundles. 



