348 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



After the primary tissues are differentiated there are 

 three very well-marked periods of growth, in which different 

 sections of the embryo take part. The first is that succeed- 

 ing the differentiation of the primary tissues and extends to 

 the appearance of the stem-apex as a rounded elevation of 

 the epidermis on the posterior surface of the embryo, and 

 includes mainly the descendants of segments I and III. 

 The second period is the elongation of that portion of the 

 embryo which is above the stem-apex, the cotyledon, which 

 takes place soon after the appearance of the plumule and 

 plumule-sheath. The third occurs in the region of the 

 radicle, in the descendants of the second segment of the 

 proembryo, and takes place after the cotyledon has elonga- 

 ted. After these periods of growth and before the matura- 

 tion of the embryo, plumule, cotyledon (scutellum), and 

 root develop and grow simultaneously, retaining the same 

 proportions. These periods of growth correspond very 

 well with the further differentiation of the tissues of the 

 embryo. 



When the stem-apex is first discernible (fig. 41), the 

 embryo is already relatively large, and all traces of its 

 primitive segments are quite obliterated. The embryo at 

 this period is oval with an obtuse cotyledonary and a sharp 

 tapering radicle end. In cross-section, the embryo has 

 changed from circular to oval. The stem-apex appears on 

 the posterior side of the embryo about one-third the dis- 

 tance from the cotyledonary end, and becomes surrounded 

 by a low ridge of tissue. From this, on the side of the stem- 

 apex opposite, springs the epiblast, and on the cotyledonary 

 side merely an augmentation of the boundary which sepa- 

 rates cotyledon and plumule (fig. 41)- An examination of 

 the tissues at this stage of the development of the embryo 

 shows several rows of elongated plerome cells in the cotyle- 

 don enclosed by a periblem of large and rather irregular 

 cells. The epidermis of the cotyledon is beginning to 

 assume the palisade-like structure it later develops as an 

 absorbing organ. The elongation of the plerome cells is 

 continued to the region of the stem-apex, but is not found 

 in the root. This differentiation of the plerome is connected 



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