274 



MORPHOLOGY 



Among certain plants without chlorophyll (Monotropa, etc.) the 

 embryo is very simply organized, consisting of a few cells, the differ- 

 entiation into body regions proceeding during germination. 



Monocotyledons. — The embryo of Alisma or of Sagittaria is most 

 commonly used to represent the monocotyledon >us embryo. The pro- 

 embryo is usually a filament of three cells (figs. 609, 610), the terminal 

 cell forming the cotyledon. The middle cell begins a series of divisions, 

 some of the resulting cells forming the stem tip, hypocotyl, and root 

 tip, and the others belonging to the suspensor (figs. 611-617). In 



Figs. 614-617. — Further development of embryo of Sagittaria: stages later than 

 the series shown in figs. 609-613; x, enlarged basal cell; y, middle cell, giving rise 

 to stem-tip (s), hypocotyl (Ji), and some suspensor cells; z, the terminal cotyledon. 

 — After Schaffner. 



this case, also, the boundary between embryo and suspensor is es- 

 tablished by a division of the second cell of the proembryo, which con- 

 tributes both to the embryo and to the suspensor (fig. 617); but in this 

 monocotyledon type the whole of the embryo except the terminal coty- 

 ledon is derived from the second cell. The basal cell of the proembryo 

 usually becomes very much enlarged, forming the conspicuous part of 

 the suspensor (figs. 616, 617). The notable monocotyledonous feature 

 is the terminal cotyledon and the laterally developed stem tip, which 

 appears in a notch developed in the side of the axis of the embryo (fig. 



