THE EMBRYO 



49 



Probably the large cell (st) is the apical cell, but this is not absolutely certain. In 

 form and position this cell resembles the apical cell figured by Jeffrey (Jeffrey I, 

 fig. 48) in an older embryo. It may be said that in Jeffrey's figure the beginning of 

 the cotyledon, situated between the stem apex and the root, although not lettered, is 

 evidently present. 



As the embryo grows the root rapidly increases in length, and together with the 

 foot comprises the greater part of the embryo. The foot is very large, usually 

 nearly hemispherical in form, but not infrequently a good deal elongated (fig. 35), 

 and penetrates deep into the prothallial tissue. 



I he apical cell of the root is very conspicuous and can be made out without 

 difficulty as a large cell, triangular in form, whether it is viewed in longitudinal or 

 transverse section. Segments are cut off in regular sequence from all of the four 

 faces, and as these are relatively large and contain but little granular contents the 

 apical meristem shows very clearly against the smaller celled and more deeply stained 



rot 



Fie. 34. 



A-C. Three horizontal sections of an old embryo of Botrychium virginianum, X75. 

 D. The stem region. X150. 



adjacent tissue. In the earlier stages the divisions in the segments cut off from the 

 apical cell do not show absolute uniformity. The first wall in the young segment is 

 probably anticlinal, and seems to be followed by a periclinal wall or walls, cutting 

 off inner cells which contribute to the very large plerome cylinder of the young root. 

 The root cap, which is very massive, is derived in part from segments cut off directly 

 from the apical cell, and in part from cells separated by periclinal walls from the 

 outer part of the lateral segments of the apical cell. 



The development of the plerome begins very early, and it soon forms a con- 

 spicuous massive strand of procambium cells extending from the root apex to the 

 junction of the root and foot, where it ends abruptly on the lower side, but bends 

 upward on the upper side, and is extended as a simple axial strand of tissue into the 

 cotyledon. 



The cotyledon, which is first recognizable at an early stage in the development 

 of the embryo, at the stage in question (fig. 35, A) projects slightly as a broad, flat- 

 tened cone, strongly bent away from the root. Fig. 34, A , shows the cotyledon cut 

 4 



