PROOF THAT FOOD PASSES DOWN THE STEM 89 



L 



Proof that Food passes down the Stem. — If a freshly cut wil- 

 low twig is placed in water, roots soon begin to develop from that 



part of the stem which is under 



water. If now the stem is girdled 



by removing the bark in a ring just 



above where the roots are growing, 



the latter will eventually die, and 



new roots will appear above the 



girdled area. The passage of food 



materials takes place in a downward 



direction just outside the wood in 



the layer of bark which contains the 



bast fibers and sieve tubes. This 



experiment with the twig explains 



why trees die when girdled so as to 



cut the sieve tubes of the inner bark. 



Many of the canoe birches of our 



forests are thus killed, girdled by 



thoughtless visitors. In the same 



manner mice and other gnawing 



animals kill fruit trees. To a much 



smaller extent food substances are 



conducted also in the wood itself, and food passes from the inner 



bark to the center of the tree by way 

 of the pith rays. If the pith rays are 

 tested for foods, it is found that much 

 starch is stored in this part of the tree 

 trunk. 



Structure of a Monocotyledonous 

 Stem. — A piece of cornstalk examined 

 carefully in cross section and longitudi- 

 nal section shows us that the main bulk 

 of the stalk is made up of pith, through 

 which are scattered numerous stringy, 

 tough structures called fihrovascular 

 bundles. The latter are the woody 

 bundles of tubes which in this stem pass 

 through the pith and run into the leaves,. 



Experiment to show that food 

 material passes down in the inner 

 bark. 



A broken cornstalk, with 

 cross section (at left) : A^, 

 node ; R, r, rind ; P, p, pith ; 

 FV, fv, fihrovascular bundle. 



