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 medullary 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 medullarv 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. 
