THE EMBRYO 



323 



until germination. Some of the higher monocotyledons bring further 

 difficulties into the interpretation of the axis of the embryo by the 

 presence of a compound structure, the inesocotijl — the middle part of 

 the axis in some grasses and other monocotyledons. It has been urged 

 that this term be dropped as superfluous, since "this part is eitlier 



Fig. 123. Median longitudinal sections of embryos of grass genera showing variety 

 of basic form. A to F, axis curved, cotyledon pseudoterminal, plumule so-called 

 lateral; G to /, axis nearly straight and plumule terminal. A, Fcstuca elatior; B, 

 Dactijlis glomerata; C, Deschampsia flexuosa; D, Phalaris arundinacea; E, Phleiim 

 alpinum; F, Panicum clandestinum; G, Brachiaria platyphijlla; H, Echinochloa 

 crusgalli; I, Setaria italica; J, Sorghum vulgare. {After Reeder.) 



hypocotyl or epicotyl." But study of anatomy and comparative form 

 shows that the mesocotyl is a compound structure, the hypocotyl, with 

 an adnate part of the cotyledon. (Discussion later in this chapter.) 



The Hypocotyl. Morphologically, the hypocotyl is the part of the 

 embryo and seedling in which there is anatomical transition from root 

 structure — with radial arrangement of alternating strands of phloem 

 and exarch xylem — to stem structure, with vascular strands consisting 



