Box. -Vol. I.] CANNON— AVENA FATUA. 349 



with the sudden shooting up of the cotyledon, and doubt- 

 less these elongated cells function as food carriers from the 

 cotyledon to the developing plumule. *The differentiation 

 of corresponding tissues in the root occurs at a much later 

 period, but the primary root tissues are already well defined. 

 The group of cells at the end of the plerome cylinder, 

 which give rise to the root-cap and to the dermatogen and 

 cortex of the root, can be easily distinguished. The ple- 

 rome initials are not developed until later; they do not, in 

 fact, appear until one or two of the leaves of the plumule are 

 cut off, and the embryo is approaching maturity. The 

 plerome of the root is continuous with that of the cotyledon 

 and that of the stem; the cortex and dermatogen of the 

 root are continuous with the periblem of the embryo. A 

 cross-section of the embryo through the region of the root 

 shows a rather large central plerome-complex, as yet 

 undifferentiated, surrounded by a periblem two or three 

 cells in thickness, the outermost layer of which is begin- 

 ning to take on the characters of epidermis. Surrounding 

 all of these are one or two rows of cells which are the sub- 

 epidermal cells of the embryo, enclosed in the dermatogen 

 of the embryo. 



A somewhat older embryo shows the third stage of 

 growth, that is, the elongation of that part of the embryo 

 within which is the root. This exhibits at the same time a 

 further differentiation of the tissues of the root. A greater 

 number of cells than before has now been cut off from the 

 initials which give rise to the root-cap (by which this organ 

 is very well marked) and to the extra-plerome root tissues. 

 The plerome initials also have become well defined. 



The tissues of the radicle correspond to those of the root- 

 tip of the mature plant, and agree closely with those of 

 Hordeum figured by Strasburger (1898). The region 

 below the root-cap is made of large, irregular cells, with 

 little protoplasmic contents. These form the coleorhiza of 

 the mature embryo. 



After the epiblast has been cut off, there appears on the 

 cotyledonary side of the plumule, and just within the ridge 

 of tissues spoken of above as surrounding the stem-apex 



