THE EMBRYO 337 



and likened to the two lobes of the cotyledon in the dicotyledons, but 

 division in the dicotyledons is into lateral, not basal and distal, parts. 

 In the monocotyledons, the suctorial part is the distal part; the "neck" 

 consists of the median part, often with lateral, stipulelike margins extend- 

 ing upward above the point of downbending (Fig. 129G, .r). The 

 upward-extended stipular margins have been considered to form a ligule 

 like that of the grass leaf, and the sheathing base of the cotyledon has 

 been commonly called a "ligular sheath." There has been much confu- 

 sion in the terminology and descriptions of these prolongations. The 

 terms stipular and ligular have both been used loosely for the distal 

 parts of the sheath, which, as prolongations of the margins of the sheath, 

 are surely stipular. The term ligular should be used only for the collar- 

 like, transverse expansion on the ventral side of the cotyledon neck at 

 the point of downbending (Fig. 129G, y); this structure is the morpho- 

 logical equivalent of the ligule of the grass leaf. That the coleoptile, 

 commonly, is a stipular structure in part has not been generally recog- 

 nized. The apparent structural separation of the coleoptile from the 

 scutellum (Fig. 130) — often considered the cotyledon — has seemed to 

 support the view that the coleoptile is the first leaf. The frequently bi- 

 lobed tip of the sheath and its paired veins, in many taxa, are strong 

 support for its two-stipule nature. (The bilobed form persists in cole- 

 optiles with marked "shoulders" — Bromus, Ammophila, Festuca, some 

 forms of Zea. ) 



There is great variety in form of the stipular sheath (Figs. 131 and 

 132). Its margins vary in extent of fusion; the sheath may be tubular to 

 the top or ventrally open for short or long distances. Distal "append- 

 ages" — free tips of the stipules — may be prominent, lanceolate, auricu- 

 late, or absent. The sheath, as a whole, is commonly cylindrical but may 

 be ovoid, or subspherical. Where there is lateral adnation to the neck, 

 its form is related, in part, to the position of the scutellum and to the 

 degree of completeness of the fusion (Fig. 131B, F, H). 



Stipular sheath is a good term, morphologically suggestive of the 

 nature of this structure. The stipules of the typical monocotyledon leaf 

 are thin marginal wings, often with free distal parts; the cotyledon 

 shows similar stipules in some taxa with lanceolate or auriculate tips. 

 The point of dorsal downbending of the cotyledonary neck is the base 

 of the free stipule tips and the tips remain erect (Fig. 129G, H). In 

 this position, they shelter the plumule and, in further specialization of 

 the cotyledon, fuse by their margins and form a protective cap over the 

 plumule. This cap, together with adjacent parts of the sheath — the apex 

 of the downbent cotyledon — forms the coleoptile (Fig. 130, ct). Morpho- 

 logically, the coleoptile is a median part of the cotyledon, the "elbow" 

 section of the bent "neck," together with the distal parts of the stipules. 



