390 PLANT MORPHOLOGY 



the upper tier completing the dermatogen and the lower tier forming the 

 first layer of the rootcap (Fig. 337/). This stage is further marked by the 

 appearance of the two cotyledons, one on each side of the stem tip, which 

 lies at the upper end of the hypocotyl. Thus the stem tip is terminal and 

 the cotyledons are lateral. 



Many dicotyledons follow the general course of embryogeny as seen in 

 Capsella, but there are a number of departures from it. In the Nymphae- 

 aceae, for example, a globular proembryo is developed and generally no 

 suspensor is formed. In the Myrtaceae a massive proembryo fills the 

 micropylar end of the embryo sac and several embryos may be differen- 

 tiated from it. In the Ilubiaceae and Leguminosae the suspensor is 

 enormously elongated. 



Sagittaria. The development of the embryo of Sagittaria, one of the 

 Alismaceae, is representative of the more primitive families of monocotyle- 

 dons. Here the proembryo is a filament of three cells^ — a large basal cell, a 

 middle cell, and a terminal cell (Fig. 338A, B). The basal cell enlarges 

 considerably but does not divide, constituting the greater part of the sus- 

 pensor. The middle cell, by a series of transverse and vertical divisions, 

 forms the stem tip, hypocotyl, root tip, and the rest of the suspensor (Fig. 

 338C-F). The terminal cell gives rise to the cotyledon. It divides first 

 by a vertical wall and then by walls in the two other planes, thus forming 

 octants (Fig. 338Z), E). The dermatogen arises in the cotyledon by the 

 formation of periclinal walls and proceeds toward the root end of the 

 embryo (Fig. 338(t). Later the periblem and plerome are differentiated. 

 The stem tip arises as a depression in the side of the embryo, thus being 

 lateral in position rather than terminal as in the dicotyledons (Fig. 

 338i/, 7). 



A number of modifications of the Sagittaria type of embryogeny have 

 been noted. For example, the Araceae have a massive proembryo and 

 lack a suspensor. The Liliaceae have a filamentous proembryo that soon 

 becomes massive. In the Orchidaceae the body regions are not differen- 

 tiated and, in many forms, a large suspensor becomes a haustorial organ. 



In Agapanthus, a member of the Liliaceae, dicotyledonous embryos are 

 occasionally produced. The proembryo is more or less massive. Its tip 

 broadens and a peripheral cotyledonary zone gives rise to two growing 

 points. The entire zone then grows upward, resulting in the formation of 

 a cotyledonary ring surrounding a depression from which the stem tip 

 develops. If both growing points continue to develop equally, a dicoty- 

 ledonous embryo results, but if only one continues, a monocotyledonous 

 embryo is formed. This indicates that dicotyledony is more primitive 

 than monocotyledony. It also suggests that the stem tip is really ter- 

 minal and the cotyledons lateral, even though only one cotyledon is 

 produced. 



