THE EMBRYO 325 



In greatly reduced embryos, without clear distinction of parts, as in 

 the orchids and some parasitic plants, the hypocotyl is not apparent. In 

 many aquatic taxa and in some biennials, the hypocotyl, vvitli adjacent 

 parts of root and stem, serves as a storage organ and is greatly dis- 

 torted. This distortion occurs, in some taxa, within the seed — Potamo- 

 getonaceae, Verbenaceae, Cactaceae, Myrsinaceae, Rhizophoraceae — 

 and, in other taxa, after germination — some Cruciferae, Beta, Raphanus, 

 Brassica, Umbelliferae, Amaranthaceae. Hypocotyls, similarly distorted, 

 are common in taxa with cormous habit. In seeds in which the hypo- 

 cotyl is greatly enlarged, the cotyledons are small. An enlarged hypo- 

 cotyl, serving as a storage organ, characterizes the seeds of many 

 aquatic genera, which have little or no endosperm and a reduced 

 embryo. 



The hypocotyl of most monocotyledons, together with the plumule, 

 is commonly described as lateral and oblique. But there are some taxa, 

 especially the more primitive, which have the hypocotyl median and 

 erect and the plumule clearly terminal. (Too much emphasis has been 

 placed on the grass embryo as typical of monocotyledonous embryos.) 



The term hypocotyl has been more loosely used than any other ap- 

 plied to the embryo. It has been defined as "all the embryo below the 

 cotyledons," "the primitive stem and root," "the nascent axis," "the first 

 stem," "the axis of the embryo," "the caulicle," "the upper part of the 

 caulicle." Confusion in the application of the term hypocotyl is, in part, 

 related to the interpretation of the embryonic root, the radicle. 



In vascular structure, tetrarchy seems to be the basic type; triarchy 

 and diarchy, derivative types. Tetrarchy characterizes massive seedlings, 

 and these are associated with large seeds and the tree habit. Since the 

 tree habit is undoubtedly primitive, the tetrarch habit seems primitive 

 in angiosperms. But vascular form in hypocotyls varies greatly; a type 

 may be "stable," constant throughout a taxon, or "unstable," varying 

 even in individuals of a species. Any type may be unstable; the derived 

 types are especially so. 



The Epicotyl. The part of the stem that develops above the cotyledon- 

 ary node — the level of attachment of the one or two cotyledons — is the 

 epicotyl. This part of the axis — with true stem structure — is developed 

 by the axial growing point at the top of the hypocotyl; it is present in 

 the plumule in meristematic stage. It is fairly well developed in the 

 seeds of some dicotyledons, as in some Leguminosae, Fagaceae, Cru- 

 ciferae, but is rarely present in monocotyledons until after germination. 

 It is better developed in taxa with epigeal than in those with hypogeal 

 germination, where the early stages of the seedling develop below 

 ground. 



