ANATOMY OF THE EMBRYO AND YOUNG SEEDLING 351 



cotyledons have no hypocotyl. The basis for this statement is, perhaps, 

 the shortness of the hypocotyl of many of these plants, especially that 

 of certain grasses that are commonly used to illustrate monocotyle- 

 donous embryos. But the grass embryos are far from typical of mono- 

 cotyledon embryos; they represent highly specialized types. Interpre- 

 tation of the hypocotyl as the caulicle has also formed a basis for the 

 statement that a hypocotyl is lacking. Very short, almost platelike 

 hypocotyls have received little attention anatomically, because of the 

 complexity of structure where the transition occurs abruptly and the 

 vascular strands lie almost horizontally. The hypocotyl of some mono- 

 cotyledons has been described as "often platelike, hardly existent"; in a 

 few, it is long and prominent in the embryo (Fig. 135C, D, F), espe- 

 cially where it is part of the mesocot}'l (with adnate neck of the cotyle- 

 don). The shortening of the hypocot\d is a part of a general shortening 

 at the base of the shoot in many monocotyledons in which the basal 

 internodes are largely suppressed. The hypocotyls of dicotyledons are, 

 in general, longer than those of the monocotyledons. The normal, cylin- 

 drical or platelike hypocotyl may be greatly distorted; it tends to be 

 ovoid or spherical in seedlings of cormous taxa — Crocus — in fleshy an- 

 nuals or biennials — Raphanus — and in poorly differentiated embryos — 

 orchids, saprophytes, and some parasites. 



The types of transition in vascular structure between root and stem — 

 from radial and exarch to collateral and endarch — are not discussed 

 here; they are well treated in textbooks of anatomy. But important in 

 the interpretation of the morphology of the embryo is the relation of 

 the vascular structure of hypocotyl to that of the adnate "neck" of the 

 cotyledon in the formation of the mesocotyl. (Failure to recognize this 

 adnation has been responsible for major errors in the interpretation of 

 the nature of the coleoptile. ) 



Anatomy of the Seedling Root. The vascular cylinder of the primary 

 root of seedlings is commonly diarch or tetrarch; other types seem to 

 represent modifications of these basic types. Monarchy is rare in primary 

 roots; polyarchy, frecjuent in the monocotyledons. It has long been 

 argued that diarchy is basic, tetrarchy derived, and vice versa. Tetrarchy 

 has been considered the basic type because it is associated with arbores- 

 cent taxa (and diarchy is associated with herbaceous). (In the Legumi- 

 nosae, tetrarchy is characteristic of woody taxa; diarchy, of herbaceous 

 taxa.) The Ranunculaceae have diarch; woody families have tetrarch 

 primary roots. The Compositae have both types and give no evidence 

 that one is more primitive than the other. In the dicotyledons, there 

 are few variations from diarchy or tetrarchy; in the monocotyledons, 

 there are many variations; polyarchy — double or triple the dicotyledon 

 numbers — is common in the monocotyledons. The simple transition 



