108 THE STRUCTURE AND WORK OF THE STEM 



Structure of a Monocotyledonous Stem. A piece of cornstalk ex- 

 amined carefully in cross and longitudinal section shows us that the main 

 bulk of the stalk is made up of pith, while scat- 

 tered through the pith are numerous stringy, tough 

 structures. To these the name fibrovascular bundles 

 has been given. The latter are the woody bundles 

 of tubes which in this stem are scattered through 

 the pith and run into the leaves at the nodes, 

 where (in young specimens) they may be followed 

 as veins. The outside of the corn stem is formed 

 of large numbers of these bundles, which, closely 

 packed together, form an outer rind. Thus the 

 woody material gives mechanical support to an 

 otherwise spongy stem. 



Structure of Fibrovascular Bundle in a Mono- 

 cotyledonous Stem. A fibrovascular bundle in 

 a cross section under the microscope shows this 

 arrangement : Around the outside of the bundle 

 is a collection of thick-walled, woody cells. 

 These cells serve to support the bundle. Inside 

 of these cells are found a number of tubes of 

 different diameters, some for conduction of water, 

 others for air, and still others for liquid food 

 material sent down from the leaves. These 

 tubes were formed by the elongation of certain 

 cells of the bundle which in their growth have 

 divided so as to 

 form a string of 

 cells. The con- 

 tents of some of 

 these cells die; 



a hollow tube' of cellulose remains, which 

 admits the passage of material from one 

 level of the stem to another through the 

 open ends of the cells. The conducting 

 tubes have various functions. Some 

 carry soil water and air up the stem, 

 while others take food material down 

 toward the roots. The bundles elon- 

 gate rapidly, but are limited in their 



growth outward by the hard-walled, 



j A ,, Monocotyledonous fibrovascular 



woody cells. An old stem of a mono- , 



bundle : ph, region in which food 



cotyledon contains more bundles than passesdown . d , woody portion 



does a young stem, the bundles growing or bundle ducts which carry air 



out as veins into the leaves. and water ; p, pith cell. 



Longitudinal section of 

 cornstalk, showing 

 some of the fibrovas- 

 cular bundles passing 

 outward at the node 

 just above the roots. 



