THE TWO MAJOR TYPES OF ANGIOSPERM EMBRYOS 355 



THE TWO MAJOR TYPES OF ANGIOSPERM EMBRYOS 



The embryos of angiosperms have long been rather fully described 

 in elementary textbooks.* The differences are chiefly those of ontogeny 

 and of the mature embryos; the proembryos differ in no important way. 

 Under phylogenetic reduction, the mature embryo may consist of only 

 a few cells and show little or no differentiation into the usual organs. 

 These reduced embryos characterize many parasitic and mycorrhizal 

 taxa and also highly specialized taxa — Orchidaceae, Apostasiaceae, 

 Burmanniaceae. The highly specialized embryo of the orchids shows 

 little differentiation of organs. Similarly, in Orohanche, the embryo in 

 the seed at shedding time is a mere sphere of cells. But this embryo, in 

 contrast with that of the orchids, where there is no endosperm, is em- 

 bedded in endosperm. In these very small embryos, where there is no 

 after-ripening growth within the seed coats, differentiation of early 

 stages of organs may accompany or follow germination. 



The Dicotyledon Embryo. The mature embryo of the dicotyledons is 

 simple in structure; its parts are distinct and usually readily recogniz- 

 able — an axis with two lateral appendages, the cotyledons. At one end 

 of the axis is the terminal bud, the pliwnde; at the other end, a root tip, 

 the radicle. The axis between the cotyledons and radicle is the hypocofijl 

 — the transition region between the markedly different structure of 

 stem and root.f In dicotyledons, the hypocotyl is commonly prominent; 

 the line between the hypocotyl and the root may be evident externally 

 by differences in diameter and in nature of the epidermis, but examina- 

 tion of internal structure is usually necessary to delimit these parts. The 

 radicle may be represented at germination by only a growing point or 

 it may be well developed, a few millimeters long. The cotyledons are 

 large and prominent, commonly alike and strictly opposite. During 

 germination they become either epigeal or hypogeal. Well-developed 

 plumules — the basal one or more immature internodes of the stem and 

 their appendages — may be apparent within the seed; the stem within 

 the bud (sometimes prominent) is morphologically the epicoti/l. (The 

 terms radicle, hypocotyl, and epicotyl have, unfortunately, often been 

 loosely used, to the confusion of beginning students. ) Endosperm is not 

 so common in the dicotyledonous as in the monocotyledonous seed; 

 the early-formed endosperm is commonly absorbed by the embryo as 



* Unfortunately, the embryo usually chosen to represent the monocotyledonous 

 type is that of the grasses, especially that of Zea, one of the most specialized 

 embryos in the family; grass embryos, as a whole, are difficult of interpretation. 



f The term caulicle was long applied to the axial part of the embryo, and con- 

 sidered the embryonic stem between the radicle and the plumule. But this part of 

 the axis is hypocotyl, not stem, morphologically, and therefore the term cauhcle is 

 inaccurate and has become obsolete. 



