276 EMBRYOGENESIS IN PLANTS 



PaspaJum has one foliage leaf but no epiblast. Some of Bennett's 

 illustrations suggest that the scutellum is lateral in origin. 



The coleoptile — now a very familiar object because of its extensive 

 use in auxin studies — has been interpreted in several different ways: 

 (i) It is the first leaf of the plumule, and the second leaf of the plant, the 

 scutellum being the first leaf or cotyledon. If the epiblast is regarded as 

 a rudimentary cotyledon, then the coleoptile is the third leaf, but the 

 first plumular leaf, (ii) The scutellum and the coleoptile together form 

 the cotyledon, the coleoptile being variously interpreted as a ligule, as a 

 pair of fused stipules, or as an extension of the cotyledonary sheath, 

 (iii) The coleoptile is the cotyledon, the scutellum being an outgrowth 

 of the radicle or of the axis, (iv) The coleoptile and scutellum are 

 structures peculiar to the embryo and morphologically distinct from 

 the foliage leaves. 



Weatherwax (1920), Percival (1927), Avery (1930), Boyd (1931), 

 Randolph (1936), Jakovlev (1937), Yung (1938), and Kiesselbach (1949) 

 have maintained that the coleoptile is homologous with a leaf. Nish- 

 mura (1922), Soueges (1924), Howarth (1927), Reznik (1934), and 

 Arber (1934), on the other hand, consider that the scutellum and 

 coleoptile are parts of one structure — the cotyledon. The third sug- 

 gestion seems to have had no adherents in recent years, while the last, 

 which was proposed by Merry (1941), has obtained little support. 



Reeder (1953) has pointed out that investigators of the coleoptile 

 have, to some extent, made their own difficulties, in that they have 

 mainly worked with the somewhat specialised agricultural grasses such 

 as Zea, Hordewn, Sorghum, Avena, Triticum and Oryza. If the less 

 specialised, or more primitive, genera are investigated, the nature of 

 some of the embryonic parts becomes fairly evident. This in Strepto- 

 chaeta spicata, one of the most primitive of the grasses — taxonomically 

 isolated but probably allied to Bambusa — the coleoptile is not a closed 

 conical organ, as in Zea or Triticum, but is quite evidently a foliar 

 structure, open on the side away from the scutellum and standing in a 

 normal phyllotactic relationship to the first plumular leaf. Its foliar 

 character is confirmed by the details of its vascular system. This 

 consists of five vascular strands, one of which is median. Reeder has 

 thus disposed of the main difficulties of accepting the coleoptile as a 

 foliar organ. 



As it seems to the present writer, the grass embryo, though admit- 

 tedly complex and specialised, is perhaps less difficult to understand 

 than is sometimes thought. In grasses, as in other monocotyledons, the 

 distal region of the pear-shaped proembryo becomes a terminal foliar 

 member and the shoot apex eventually becomes recognisable as an 

 inconspicuous indentation on the side of the embryo facing away from 



