Nov., 1906.] The Embryology of the Shepherd's Purse. 3 



the embryo than in the lower quadrant. Frequently the forma- 

 tion of anticlinal walls in the dermatogen is delayed until after 

 the division of the upper suspensor cell (Fig. 20). 



The entire embryo is, therefore, developed from the two cells 

 at the outer tip of the proembryo, the terminal cell, as will ap- 

 pear later, giving rise to the cotyledons, stem tip and hypocotyl 

 while the basal cell after cutting off one suspensor cell gives rise 

 to the calyptrogen, root tip, and calyptra. 



The further development of the embryonic sphere is by the 

 division of the lower tier of inner cells by transverse walls. 

 There are then three tiers of cells in the sphere besides the der- 

 matogen (Figs. 24-28). These divisions were established by the 

 presence of spindles in the lower tier as in Fig. 24. Frequently 

 also the outer tier of cells takes a much deeper stain than the 

 other two tiers as is shown in Fig. 28. When these divisions 

 begin the basal embryo cell divides by a transverse wall, the re- 

 sulting cells being the incepts of the growing point of the root 

 tip and the calyptrogen (Figs. 24-26). The upper cell, usually 

 lenticular in shape, divides first forming a plate of four cells by 

 the development of longitudinal walls (Fig. 28). These four cells 

 remain dormant for some time but continue division before the 

 embryo reaches maturity. 



The two lower tiers in the embryonic sphere now begin to 

 differentiate into a central plerome and the surroimding periblem 

 and at the same time the lowest cell of the embryo proper or the 

 incipient calyptrogen cell divides by longitudinal walls into four 

 ■cells followed by transverse or periclinal walls which differentiate 

 the primary layers of the calyptra and calyptrogen (Figs. 29-32). 

 This periclinal division extends out into the dermatogen cells 

 derived from the primary octant as will appear from an inspec- 

 tion of Figs. 29, 30 and 32. The calyptrogen thus is developed 

 from cells derived from both he terminal and basal embryo cell. 

 As stated before the embryo proper is developed entirely from 

 two original tiers of cells each developing octants, but the ter- 

 minal cell divides first by longitudinal walls and its develop- 

 ment is far advanced before the basal cell begins its division 

 which is always transverse in Bursa although longitudinal in 

 Alyssum. 



The cotyledons and plumule have their origin from the outer 

 ■cells of the original octant while the hypocotyl is developed 

 from the basal octants. The nearly spherical embryo begins to 

 take on a flattened appearance at the outer end (Fig. 28) and 

 later, by the appearance of two large protuberances, the inci- 

 pient cotyledons, it is heart-shaped in properly cut longitudinal 

 sections. The embiyo remains straight until by rapid elongation 

 the cot3dedons are forced around by the curved wall of the ovule 

 (Figs. 34, 35). When the embryo is approaching maturity the 



