50 Elementary Botany 



owing to its capability of distension, the bark presents a smooth 

 appearance. 



8. The medullary rays uniting the pith with the middle 

 bark, and separating the fibro-vascular bundles. They are 

 generally made up of flattened, six-sided cells, arranged like 

 bricks in a wall {rimriform parenchyma). 



The rays are rarely continuous from the top to the bottom 

 of the stem, being separated by the fibro-vascular bundles {st, 



fig- 7i)- 



The medullary rays form what is known as the ' silver grain ' 

 of the cabinet makers. The use of the rays is to distribute the 

 elaborated sap from the liber through the other parts of the 

 stem. 



In most cases the fibro-vascular bundles are continuous 

 with those which form the veins of the leaves, and are known 

 as ' common bundles,' i.e. common to leaf and stem. At the 

 nodes (the point of attachment of the leaf to the stem) these 

 bundles enter the stem and run down parallel to one another, 

 usually through two or three internodes. At the nodes the 

 bundles branch and interlace in various ways, whilst they thin 

 out the further they descend. In some few water plants, as the 

 Hippuris or Mare's tail, the Myriophyllum or Milfoil, and a few 

 others, the fibro-vascular bundles run simply through the stem, 

 or are cauline, the foliar originating later. 



The direction of the fibro-vascular bundles can be readily 

 traced by making a longitudinal section of a young stem, 

 taking care to cut through at least two nodes, and then soaking 

 it for five or ten minutes in a solution of aniline sulphate 

 acidulated with a few drops of sulphuric acid ; the lignin of 

 the bundles will be stained yellow, and hence their position 

 can be noted. 



STRUCTURE AND FUNCTIONS OF THE PARTS OF A MONO- 

 COTYLEDONOUS STEM. 



Even in their extei nal form monocotyledonous stems present 

 a different appearance from the dicotyledonous stems described. 

 In this country we possess no indigenous monocotyledonous 

 tree (the Butcher's Broom is the only indigenous shrub), but 



