356 MORPHOLOGY OF THE ANGIOSPERMS 



a whole, during development of the seed or during after-ripening. There 

 are, of course, many modifications of this simple type; some forms are 

 asymmetrical — one cotyledon smaller than the other or even lost. In 

 the mature seed, the entire embryo may be very simple, as in reduced 

 forms. 



The Monocotyledon Embryo. Though monocotyledon embryos vary 

 greatly in form, the axis is basically like that of typical dicotyledons. 

 Some monocotyledons have the straight axis and terminal plumule, 

 typical of the dicotyledons, but the great majority are described as 

 having the stem tip (plumule) lateral. The so-called lateral position of 

 the plumule is morphologically terminal. The shoot apex has been 

 crowded out of its normal position by strong development of the huge 

 cotyledon, survivor of an ancestral two. The displacement may be 

 ontogenetic but has become congenitally established in the highly 

 specialized embryos of the grasses and some other families. In these 

 embryos, the cotyledon becomes pseudoterminal, even ontogenetically 

 terminal, as are some other falsely terminal organs — leaves, carpels, 

 stamens, and ovules. The axis is commonly strongly curved where the 

 cotyledon is falsely terminal, and the plumule is described as lateral. 

 The root may, similarly, be pushed into a lateral position. It is prob- 

 ably unavoidable that the morphologically obscure terms — scutellum, 

 coleorhiza, epiblast, and coleoptile — be used in descriptions of the em- 

 bryos of monocotyledons, but they are not helpful in the teaching of 

 comparative form. In monocotyledons, the hypocotyl may be prominent 

 but, in the higher taxa, is usually short. Though the transition region 

 may be reduced to little more than a vascular plate in these embryos, 

 as in some of the higher grasses, the statement that the hypocotyl is 

 "nonexistent in the monocotyledons" is not correct. The mesocotyl, 

 present in only highly specialized monocotyledon embryos, is a com- 

 pound structure, consisting of the hypocotyl and an adnate part of the 

 cotyledon. 



Monocotyledon embryos commonly differ from those of the dicotyle- 

 dons not only in number of cotyledons but also in the apparent position 

 of the cotyledons. The two cotyledons of the dicotyledons are borne 

 laterally, as an opposite pair at the top of the hypocotyl; the solitary 

 cotyledon of the monocotyledons is commonly described as terminal, 

 with the axis "lateral on the side of the cotyledon." The cotyledon 

 appears to be terminal in most monocotyledons, because it extends be- 

 yond the plumule. Ontogenetically, it seems to be terminal in some 

 taxa, but is obviously lateral in other taxa. Little is known of the 

 histological details of cotyledonary origin in the monocotyledons; de- 

 termination of point of origin on the rounded promeristem is difficult. 

 In Ottelia, the apices of the cotyledon and the shoot are described as 



