﻿312 Rowlee and Dohertv : The Histology 



from an initial cell, at the apex of the root. In examining many 

 sections, it was found that in no case did the dermatogen and 

 periblem originate from a common initial cell. On the contrary, 

 the initial cells of the two tissues appear quite distinct as do also 

 the cells resulting from their divisions. This observation does not 

 agree with that of Bowers (Prac. Bot), who finds that "the per- 

 iblem and dermatogen merge into a single layer of cells at the 

 apex of the plerome. Thus the extra-stellar tissue of the root of 

 maize comes primarily from a single initial row of cells/' 



Plerome. At the growing point the plerome consists of closely 

 packed tissue. It soon differentiates into a peripheral region, the 

 procambium, and a central region — the medulla. At a short dis- 

 tance from the apex of the plerome and well within the medulla 

 appear rows of larger cells. Traced backward from the tip, these 

 rows of cells are seen to increase in size much more rapidly than 

 the surrounding tissue and simultaneous with their enlargement 



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their nuclei break up and finally disappear altogether. After at- 

 taining full size and losing almost all their contents, their walls be- 

 gin to show reticulated thickenings. Whether the agency in the 

 thickening is the cell of the row or the cells abutting on the row 

 is not at present certainly known. Their appearance led at first to 

 their being regarded as in no way related to vascular elements but 

 the characteristic thickening points toward that relationship. The} 

 are very unique structures and so far as we can find they are con- 

 fined to the grasses. Most species of grasses, if indeed not all. 

 have them, some having only a single one in the center of the 

 root while others, such as the species we are studying, has several 

 of them. 



The Root. For a study of the permanent structure of the root 

 cross-sections taken at some distance from the growing tip serve 

 best. This shows the xylem and phloem arranged radially (fi£- 

 2). Strongly thickened and narrow-lumened cells make up the 

 ground tissue which surrounds the xylem and phloem strand 

 Every ray of xylem consists, in its outer part, of a group of vet} 

 small elements, and, in its inner part, of very large and isolate 

 ducts (pseudo-vessels). In each ray of phloem the sieve tuW 

 stand principally in a circle, the companion-cells occupying the 

 ner part. The xylem and phloem are separated laterally by se 



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