STRUCTURE. 35 



shows a greater amount of xylem tissue in species with a greater extent of 

 leaf surface. The first-formed and smallest xylem tracheae are in contact 

 with the pericambium ; one such trachea occurs in each xylem plate, and 

 from it a single row of tracheae continues radially inward. These are in 

 direct contact with one another in N. tuberosa, clegans-zanzibariensis, flavo- 

 virens X zanzibariensis, and sometimes in N. odorata ; parenchyma cells 

 may be interposed between them in N. marliacea X chrotnatella, lotus, and 

 odorata ; in N. flava the radial lines are quite irregular and difficult to 

 follow, the tracheae having much the appearance of being scattered through- 

 out the parenchyma. Only spiral and spiro-reticulate tracheae have been 

 observed (IV. tuberosa, elegans X zanzibariensis). Large phloem patches 

 with sieve-tubes and companion cells, but no fibers, lie between the xylem 

 plates ; they are rounded on the inner side and do not extend so far 

 toward the center of the bundle as the xylem plates ; that is, the inner- 

 most tracheae project more deeply into the pith than do the inner bound- 

 aries of the phloem masses. All of the cells of the bundle appear polygonal 

 and nearly isodiametric in transverse section, but even the parenchyma is 

 much elongated in the direction of the axis of the root. 



Contractile roots differ from the more numerous nutritive roots in 

 that the xylem is less in quantity in the former, and the air-canals of 

 the cortex are much smaller in proportion to the cells bounding them. 

 In a root of an adult plant of N. flavo-virens (Fig. 9) taken up in 

 October, the false epidermis and exo-cortex are apparently normal. 

 The outer part of the medio-cortex is collapsed wherever a transverse 

 wrinkle occurs, but it consists for the most part of comparatively dense 

 tissue, in which the intercellular spaces are not larger than the sur- 

 rounding cells. The vascular system is cut off from the cortex by a 

 distinct endodermis. Each of the 5 to 9 xylem rays consists of a 

 single line of tracheae occupying about one-third of the diameter of 

 the vascular cylinder. The pith in the center of the cylinder is sharply 

 marked off from the surrounding tissues by an endodermis with very 

 distinct radial thickenings. This occurrence of an intrafascicular endo- 

 dermis seems very anomalous in a flowering plant, but was constantly 

 found in contractile roots of adults of N. flavo-virens. In non-contractile 

 roots of this species the xylem rays are often two cells wide, they 

 extend farther toward the center of the vascular cylinder, and there 

 is no inner endodermis. In none of the roots of other species was 

 an inner endodermis found. 



As stated before, a radial vascular bundle system traverses the entire 



