340 MORPHOLOGY OF THE ANGIOSPERMS 



made up of the united stipules of the cotyledon and stipules of the 

 first leaf. Comparison of embryos throughout monocotyledons, together 

 with their anatomy (Fig. 134), especially that of the various types 

 among grasses, shows that the coleoptile is a part of the cotyledon. 

 With the specialization of the tip of the cotyledon as an absorbing 

 organ and the adoption of hypogeal germination, the cotyledon be- 

 came bent downward. Its tip is retained in the seed below the ground; 

 the part adjacent to the seed coats is extended as a "neck," often long 

 and slender (Fig. 129). In some of the more advanced families where 

 the parallel-lying upper and median parts of the cotyledon have be- 

 come connate, the free tip of the neck and the scutellum seem to form 

 an appendage of the lower part of the cotyledon ( Fig. 129H ) . The length 

 of the neck — controlled in part by depth of soil — determines the posi- 

 tion of attachment of the free tip, but this position depends, in large 

 part, upon shape of the cotyledon and is a family or generic character. 

 The point of reversal of the top of the cotyledon is usually just above 

 the tip of the plumule — which may be completely enclosed within the 

 sheathing cotyledonary base or exposed within a slit. The closed part 

 of the cotyledon below the plumular opening and all the part above the 

 attachment of the scutellum, where there is lateral fusion of the base 

 and the inverted neck of the cotyledon, is the cotyledonary sheath, 

 sometimes called the Jigiihr and the stipiilar sheath. The cotyledonary 

 sheath is simple in more primitive taxa but forms, where fused with the 

 hypocotyl, a compound structure, the mesocotyl (Figs. 130 and 134) in 

 highly specialized embryos. 



In the doubled-back cotyledon, fusion of the neck to the basal part 

 may bring the suctorial tip to the base of the cotyledonary sheath; in 

 highly specialized embryos — those of some grasses and a few other taxa 

 — the downbent neck extends beyond the base of the sheath and is 

 fused with the hypocotyl also. This adnation of the cotyledonary tip to 

 the hypocotyl places the apparent attachment of the scutellum at the 

 base of the hypocotyl. Failure to recognize the existence of this adna- 

 tion and the secondary position of the scutellum has been the cause of 

 much of the difficulty in the interpretation of grass embryos. 



Stages in the connation of the neck and the sheathing base are com- 

 mon, and fusion of the neck with the hypocotyl is evident in the 

 presence of a ridge along the side of the hypocotyl (Fig. 134G, H). In 

 more advanced taxa, external evidence of the fusion has disappeared, 

 but downward-extending, inveHed vascular bundles of the connate 

 neck are present in the outer tissues of the lower sheath and in the cor- 

 tex of the hypocotyl. Anatomy (Fig. 134) gives internal proof of the 

 adnation in at least three unrelated families; the fusion must have 

 occurred independently more than once. In most taxa that have meso- 

 cotyls, there is little or no external evidence of this adnation; only 



