CONDUCTING TISSUE 



139 



studying: first, the cross-section of the plant; secondly, the 

 radial section; and, thirdly, the tangential section. 



Students should be careful to distinguish between the medul- 

 lary ray bundle, the medullary ray, and the meduUary ray cell. 

 In some plants the bundles are only one cell wide, but in other 

 plants the medullary ray bundle is more than one cell wide, 

 frequently several cells wide. 



THE MEDULLARY RAY BUNDLE 



The medullary ray bundle is made up of a great many medul- 

 lary ray cells. These bundles (Plate 106, Fig. 5) are of variable 

 length, height, and width. The bundles are isolated, and they 

 occur among and separate the other cells of the plants in which 

 they occur. Tangential sections show the medullary ray 

 bundle in cross-section. Such sections are lens-shaped, and 

 they show both the width and the height of the medullary ray 

 bundle. The length of the medullar^ ray bundle is shown in 

 cross-sections. 



THE MEDULLARY RAY 



The medullary ray (Plate 47) is a term used to indicate 

 that part of a medullary ray bundle which is seen in cross- 

 sections and in radial sections. In cross-sections the length 

 of the ray will be as great as the length of the bundle, and the 

 width of the ray will be as great as the width of the medullary 

 ray bundle at the point cut across. In longitudinal sections the 

 medullary ray will differ in height according to the thickness of 

 the bundle at the point cut. 



When the medullary rays extend from the centre of the stem 

 to the middle bark, they are termed primary medullary rays; 

 when they extend from the cambium circle to the middle bark, 

 they are termed secondary medullary rays. As the plant grows, 

 the diameter of the organ becomes greater and the number of 

 medullary rays are increased. In each of these cases the medul- 

 lary rays may be one or more than one cell wide, according to 

 whether the medullary ray bundle is one or more than one cell 

 wide. Even in the same plant the width of the medullary rays 

 will vary if the bundle is more than one cell wide, according to 

 width of the medullary ray bundle at the point cut across. 



