THE EMBRYO 321 



rapidly; the cotyledon tip remains within the seed coats, at least for 

 some time. In the Helobiales, Pandanales, the primitive Liliales, and 

 some other taxa, the main root develops only weakly or not at all, and 

 the embryo is at once freed from the seed coats. Epigeal germination is 

 doubtless the primitive type. Both types and many transitional forms 

 are present in the Liliaceae. Some genera have epigeal; others, hypogeal; 

 the Scilleae have both types and also transitional types. 



The Angiosperm Embryo at Germination and in 

 Early Seedling Stages 



Interpretation of the structure of the embryo at time of germination 

 is difficult, because the various organs are incompletely differentiated 

 and the vascular system is immature. The morphology of the embryo, 

 especially that of the monocotyledons, can be determined only by com- 

 parison with early seedling stages. Although the embryos and seedlings 

 of angiosperms have received much study since about 1800, there is 

 still disagreement in interpretation of the nature of some of the parts; 

 terminology is much confused, and definitions often conflict seriously. 



The term mature embryo has been loosely applied — to the embryo 

 in the ripe seed; to the embryo at time of germination (often very 

 different from that in the ripe seed); and to embryos with organs 

 distinct (in many taxa, a doubtfully recognizable state). Obviously, 

 the embiyo that continues growth within the seed during an after- 

 ripening period should not be called "mature" when the seed is shed. 

 No line can be drawn between proembryo and "mature" embryo, or be- 

 tween mature embryo and seedling. The structure of the later stages of 

 embryos within the seed and that of seedlings are here discussed to- 

 gether, because, only in this way, can the morphological nature of all 

 three be determined. 



Few terms descriptive of the entire embryo have been used, but four 

 applied to form of the monocotyledonous embryo need mention, be- 

 cause they are used in two ways. These terms were probably first ap- 

 plied in 1811. Two of these — straight and curved — have continued in 

 occasional use; two others — remotive and admotive — which apply chiefly 

 to relation of embryo form to type of germination, are probably ob- 

 solete. Straight and curved were originally applied to the form of the 

 vascular system of the embryo (straight and curved from root tip to 

 plumule), but have largely been used to describe the embryo itself 

 (Fig. 122). Used for general description, they have little significance, 

 because embryo form is often correlated with seed form, and distinc- 

 tion between a straight and a curved embryo may be difficult to make 

 (Figs. 123 and 124). An embryo was considered remotive when the 

 vascular axis of cotyledon-hypocotyl-radicle was straight and parallel 



