MISSISSIPPI EXPERIMENT STATION. 37 



of a wedge, project into it, for a greater or less distance (fig. 48). The 

 individual bundles in this aggregation are very wide radially, but nar- 

 row in the direction of the stem circumference. They are separated by 

 about three rows of elongated, thin-walled parenchyma cells which 

 resemble medullary rays. The groups of bundles are separated by 

 similar rows of cells, usually about five. The groups vary considerably 

 in size and their limits are not well marked out in some cases. 



In Kohlrabi, the woody ring of the stem is very rudimentary; this 

 is formed of small, irregular, scattered xylem strands; these are widely 

 separated by parenchyma cells (fig. 49). In the hybrid, the xylem 

 strands are varied; some are small and irregular, as in Kohlrabi, while 

 others are much larger, the majority being several hundred microns 

 in length (in the direction of the stem radius). (Fig. 50.) The larger 

 strands are grouped much as in the Radish stem. 



The phloem strands in the Radish stem are prominent, and well 

 defined. Each possesses sieve tubes 18 - 25m in diameter, and companion 

 cells 3 - 5iu. Peripheral to each phloem strand, is a strand of sclerenchyma 

 tissue which appears crescent-shaped in cross section. In the Kohlrabi 

 stem, the phloem is scarcely distinguishable, appearing merely as a 

 few smaller, more deeply staining cells standing peripherally to the 

 xylem strands. In the hybrid there are prominent long, narrow phloem 

 strands. 



The xylem of the stem bundles of each of the three plants contains 

 scattered pitted vessels 25 - 75m in diameter and tracheids 10 - 30m in 

 diameter. The difference in the xylem strands of the three plants lies 

 in the number of cells that enter into their structure rather than in the 

 form of the individual cells. 



The pith or ground tissue cells in the central part of the stem are 

 sub-spherical in each of the three plants and about the same size. The 

 walls are cellulose, thin, being apparently somewhat thinner in the 

 Radish stem than in the other two. They contain numeroiis large pits, 

 or thin places. 



Root Structure: (A cross section of the largest part of the root 

 of each of the plants under comparison was used.) Radish root has a 

 cortex 3650m thick. Kohlrabi 1800m, and the hybrid 5000 - 7000m. 



Within the primary cortex of the Radish root there is a ring of 

 well defined vascular bundles, but their xylem elements are lignified 

 only slightly; the rest of the root tissue is made up of parenchyma cells. 

 All of the tissue within the cambium of the root of the hybrid appears 

 to be woody, when viewed macroscopically, but a closer inspection 

 shows that it contains tissues other than the woody. All of the root 

 of the Kohlrabi, except the part without the cambium, is made up of 

 dense wood. 



Taking up the structure of the roots in detail, we find that in each 

 plant the epidermis has disappeared, the outer part of the root being 

 covered by a periderm several layers of cells thick. The ground tissue 

 cells of the cortex, in each plant, are thin-walled and sub-globular, 

 with a tendency to elongation in the direction of the circumference of 

 the ropt. In Radish, these cells average about 150m in length, in 

 Kohlrabi 50m, and in the hybrid 60m. The cortex of Kohlrabi contains 



