108 



BOTANY 



Transverse section of a stem of burdock, 

 showing fibrovascular bundles not com- 

 pletely united into a ring. 



Young Monocotyledonous Stem. — Almost the entire stem of a very 

 young, green monocotyledon is composed of soft, thin-walled cells. These 



are the cells making up the paren- 

 chyma (parent tissue.) Later in 

 the life of the stem their contents 

 are used up by the plant, and we 

 find them empty and dry in the sec- 

 tion of cornstalk. In some mono- 



J til^^ :J^^fe I cotyledonous stems their walls 



"^: •^SSL *'-**^.*v*. \ become hard, while in other cases, 



as in the bamboo, they entirely dis- 

 appear. 



Structure op a Young Dicoty- 

 ledonous Stem. — In the stem 

 of a young dicotyledon the pith is 

 found to occupy relatively a small 

 space in the middle of the section. 

 This pith is composed of the paren- 

 chyma cells such as we found in the 

 cornstalk. Surrounding the pith is 

 the wood. The wood consists (in 



the cross section) of the ends of many tubes and the woody walls of 



supporting cells which occupy that 



area. If the section is several 



years old, we shall find a definite 



ring in the wood for each year's 



growth. 



Medullary Rays. — Radiat- 

 ing from the center of the stem 



outward are found a number of 



tiny lines. These are more promi- 

 nent in stems of older growth. 



The medullary rays, as they are 



called, are seen to begin at the 



pith and pass out to the bark. 



They are, indeed, formed of the 



pith. They are part of the original 



parenchyma which at first formed 



a large part of the whole stem. As 



the stem grew in size, fibrovascular 



bundles appeared. These bundles 



were not scattered through the 



Section across a twig of box elder three years 

 old, showing three annual growth rings, in 

 the vascular cylinder. The radiating lines 

 (m), which cross the wood {w), represent the 

 pith rays, the principal ones extending from 

 the pith to the cortex (c). (From Coul- 

 ter, Plant Relations.) 



stem, but were arranged more or less completely in a circle. Growth of 

 these woody bundles took place along the outer edge. This caused the 



