106 



BOTANY 



Compare it with the layer just under it as to color. How does the inner 

 brown layer compare with the others in thickness? This inner layer is 

 called the bast. It is made up largely of long, tough cells, somewhat like 

 fibers in appearance. 



Notice the lines which radiate from the pith to the bark or cortex. 

 They are called medullary rays. 



Compare a cross section of this stem with an- 

 other made just above, where last year's bud w^as 

 located. What differences do you find in the 

 two cross sections ? Show by means of a drawing 

 the structure of a horse-chestnut stem. This 

 may be taken as a type of dicotyledonous stem. 

 Study of a M onocotyledonous Stem. — Compare 

 the above section with that of a monocotyledon, 

 a piece of cornstalk, for example. Notice that 

 in the corn the whole stalk is more or less pithy 

 and that the w^ood is scattered through the pith 

 in the form of structures which look like little 

 dots. Cut a piece of dry cornstalk lengthwise, 

 and you will find that these dots are the ends of 

 long stringy threads. If the longitudinal section 

 be made through a node of the cornstalk, some 

 of these long structures will be found to curve 

 outward at the node to enter the leaf, where they 

 can be followed as veins. The threads are the 

 same structures that we have seen as the little 

 dots in the leaf scar of the horse-chestnut twig, 

 the fibrovascular bundles. They are composed 

 of bundles of very tiny tubes, supported by other 

 thick-walled, tough, woody cells, which give me- 

 chanical support to the tubes. Notice that the 

 exterior of the corn stem is composed of a great 

 number of these bundles, which have grown close 

 together and become harder and thicker. This 

 outer covering of the cornstalk is called a rind. 

 Of what use might the rind be to the stem ? 



Structure of Fibrovascular Bundle in a Mono- 

 cotyledonous Stem. — The stem of a monocotyledon 

 consists largely of pith, with, as we have seen, 

 slender bundles of wood inclosing hollow cells 

 placed end to end to form ducts. These are col- 

 lected around the outer part of the stem, thus 

 gi^^ng better support to the whole stem. 



A single fibrovascular bundle in a cross section 

 under the microscope shows the following arrange- 

 ment. Around the outside of the bundle is a 

 collection of thick-walled, w^oody 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. 

 All of 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 contents of some of these cells die ; thus a hollow tube of cellulose re- 

 mains, which admits the passage of material from one level of the stem to 



Longitudinal section of corn- 

 stalk, showing some of 

 the fibrovascular bundles 

 passing outward at the 

 node just above the roots. 



