326 MORPHOLOGY OF THE ANGIOSPERMS 



The Plumule. The phimule is the bud of the embryo, the meristematic 

 base of the epicotyl with leaf primordia. The sheathing coleoptile is 

 often included as a part of the plumule, but this is inaccurate, because 

 the coleoptile is a part of the cotyledon. Within the seed, the plumule 

 is further developed in hypogeal than in epigeal taxa; the first foliage 

 leaf also develops earlier in the hypogeal type. The plumule is promi- 

 nent in most highly specialized embryos, as in grasses. 



The Radicle. Compared with the other organs of the embryo, the radi- 

 cle has received rather little attention. In the embryo within the seed, 

 it usually is represented by a primordium only; a true root is formed 

 only at germination. The histological origin of the root is difficult to 

 determine; the primordium arises late in the development of the embryo 

 and is clearly endogenous in many taxa. The relation of the root pri- 

 mordium to the differentiating axis of the embryo is obscure. The pri- 

 mary root, formed by the endogenous primordium, often appears some- 

 what lateral, but it is generally considered to form one end of the 

 embryonic axis, because it appears to be attached directly to the end of 

 the hypocotyl. Yet the view has sometimes been held that the root is, 

 like the cotyledon, a lateral structure; that the primitive axis was root- 

 less. The lateness of appearance of the root and its lateral position, as 

 well as its failure to function in some genera — Nelumbo — have been 

 believed to support its secondary nature. Root origin needs much 

 critical study; the nature of the root may be important in the search 

 for the ancestry of the angiosperms. 



Characteristically, in monocotyledons, a radicle is not present before 

 germination. At germination, the radicle may develop rapidly, slowly, 

 or be greatly delayed. In some taxa, it is filamentous and ephemeral; in 

 others, it persists as a taproot — palms, dracaenas, yuccas — and, rarely, 

 it is not formed. In the higher monocotyledons, the primary root is 

 late in development and seems to be a disappearing part of the embryo. 

 Lateral, adventitious roots are formed where the primary root is weak 

 or lacking, as in some orchids and some aquatic and parasitic families. 

 It is difficult to distinguish the presence or absence of a root in the 

 embryo before germination. It is frequently stated that, in germination, 

 the root elongates and breaks through the seed coat, but, in some taxa, 

 rupture of the seed coat results from elongation of the hypocotyl and 

 of the cotyledonary sheath, rather than of the radicle. Some descrip- 

 tions of embryo structure emphasize the radicle as an important part, 

 inaccurately interpreting the axis as consisting of plumule at one end 

 and radicle at the other, and omitting mention of the hypocotyl. 



The Coleorhiza. In the highly specialized embryos of the monocotyle- 

 dons, a sheath of tissue surrounds the base of the root (Fig. 130). This 

 sheath has been interpreted in several ways — as a part of the scutellum, 



