MISSISSIPPI EXPERIMENT STATION. 13 



N. Tabacum about 2000m, and in the hybrid about 2,600m. It is made 

 up principally of tracheids of rather uniform size; scattered among 

 these are rows of large pitted vessels, together with a few small spiral 

 and annular vessels. Between the xylem strands are numerous 

 medullary rays (figs. 7, 10 and 11). The pitted vessels average about 

 65m in diameter in N. Tabacum, 50m in N. sylvestris, and 80m in the 

 hybrid. (Longitudinal sections were used only in identifying the 

 elements of the tissues.) The tracheids are about 20m wide in N. 

 Tabacimi, 19m in N. sylvestris, and 30m in the hybrid. The walls of 

 the tracheids are 2.5m thick in N. Tabacum, 1.5m thick in N. slyvestris, 

 and about 2.2m thick in the hybrid. The medullary ray cells are about 

 the same size in the two parents, 18m wide and 45m long, but in the 

 cross, they are considerably larger, being about 30m wide and 60m long. 



The central part of the stem in each plant contains a compara- 

 tively large mass of pith tissue; this is made up of large, thin-walled 

 parenchyma cells. These cells are large and sub-spherical in each of 

 the three plants, being about 150 to 230m in diameter in N. Tabacum, 

 107 to 230m in N. sylvestris, and 275 to 460m in the hybrid. 



In the outer portion of the pith are small clusters of fibers similar 

 to the bast fibers discussed above. They are found here in each plant 

 in about the same proportion as in the bast, and the individual fibers 

 ha\-e about the same relative size. 



Roots: Each plant possesses several rather large cylindrical 

 roots, each being .6 to 1 cm. in diameter, and of a nearly uniform diam- 

 eter for a length of 20 cm. or more. The structure of the roots of the 

 three plants, compared, presents no very important differences. Each 

 has a comparatively thick cortex made up of thin-walled parenchyma 

 cells. These cells are well filled with starch. The cortex in N. sylves- 

 tris is about 1050m thick, 1,200m in N. Tabacum and 1325m in the hybrid. 

 The cqntral part of the roots is made up of woody tissue. This tissue 

 consists of rather large pitted vessels, tracheids and numerous medullary 

 rays, the last forming one-third to one-half of the tissue of this part of 

 the root. The medullary ray cells are richly stored with starch. The 

 vessels are 45 - 105m in diameter in N. sylvestris, 60 - 135m in N. Taba- 

 cum and 75 - 135m in the hybrid. The walls of the tracheid cells are 2m 

 thick in N. sylvestris, 3.5m thick in N. Tabacum, and 2.5m thick in the 

 hybrid. 



Starch Grains: In N. Tabacum the simple grains are circular 

 in outline (fig. 27); in N. sylvestris they are elongated considerably, 

 many appearing elliptical to ovate in outline and they averaged con- 

 siderably larger than in N. Tabacum (fig. 29). In the hybrid there 

 are grains of both shapes, circular ones predominating, and the grains 

 are much larger than in either parent (fig. 28). 



Leaf Structure: As stated above, the N. Tabacum leaves are 

 considerably thicker than the leaves of either of the other two plants 

 under comparison. In N. Tabacum they are 330 - 370m thick, 225 - 

 270m in N. sylvestris, and 225 to 285m in Tabacum x sylvestris (figs. 

 24, 25 and 26). The palisade cells in N. Tabacimi are regular and 

 about 140m long; in N. sylvestris they are somewhat more irregular, 



