MONOCOTYLEDONOUS STEM 



169 



Demonstration 14. To show that food passes downward in the 

 bark. If a freshly cut willow twig is placed in water, roots develop 



under water. If the stem then 



from that part of the stem which i 

 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 ma- 

 terials takes place in a downward 

 direction 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 birches of our forests 

 have been killed, as a result of 

 being girdled by thoughtless 

 visitors. In the same way gnaw- 

 ing animals frequently kill fruit 

 trees. To a small extent food 

 substances are conducted in the 

 wood itself, and food passes 

 from the inner bark to the cen- A cornstalk WLat are the differences in the 



ter Of the tree byway Of the pith arrangement of the fibrovascular bundles here 



and in a dicotyledonous or vascular stem ? 



rays m which starch is stored. 



Structure and growth of a monocotyledonous stem. A piece of 

 cornstalk is made up of pith, through which are scattered numer- 

 ous stringy, tough structures called fibrovascular bundles. The 

 latter are the woody bundles of tubes and fibers which pass 

 through the pith and run into the leaves, where (in young speci- 

 mens) they may be followed as veins. The outside of the corn 

 stem is formed of large numbers of fibrovascular bundles, which, 

 closely packed together, form a hard, tough outer rind. Thus the 

 woody material on the outside gives mechanical support to an 

 otherwise spongy stem. In a very young stem epidermis is present. 



In the monocotyledonous stem the bundles are scattered and 

 the cambium layer is absent. The bundles increase in number as 



fibrovascular 

 DuncLle, 



