272 EMBRYOGENESIS IN PLANTS 



scutellum is the cotyledon, and although it may appear to be terminal 

 at a certain stage in the embryogeny, it is essentially a lateral organ; 

 (ii) that the 'ventral' scale when present is a ligule; (iii) that the epiblast, 

 when present, is not a rudimentary second cotyledon, and probably has 

 httle morphological significance; (iv) that the coleoptile is homolgous 

 with a foliage leaf, and is the second foliar member of the embryo ; (v) 

 that the elongated structure between the cotyledon and coleoptile in 

 maize and oat seedlings is the first internode of the axis; the same 

 region exists in wheat but is less elongated. In Avery's view the three 

 morphological types which van Tieghem (1872, 1897) distinguished are 

 fundamentally alike; they differ in the location of the meristematic 

 region of the first internode. Avery's illustrations show the mature 

 embryo of Zea with at least five leaves within the coleoptile, Avena 

 with two, and Triticum with three or four. Figs. 72, 73. 



In Triticum vulgar e, McCall (1934) considers that the embryo organs 

 can best be understood by referring to the positional relationships of 

 the organs and tissues of the adult plant, i.e. the assumption is that 

 essentially similar relationships obtain in the embryo and in the adult 

 plant. In the embryo of Triticum, the transverse procambial plate 

 which separates the primary root and the shoot is held to be the first 

 node of the young plant. Roots are present at this node. The epiblast 

 which is formed at this level, is interpreted as a vestigial first seedling 

 leaf. The scutellum, which originates at the second node on the oppo- 

 site side to the epiblast, is held to be the second seedling leaf. The 

 coleoptile, which in Triticum has an axillary bud, is difficult to interpret 

 because it arises on the same side as the scutellum. It is recognised as 

 the third leaf, homologous with a bud prophyll. 



As to the nature of the epiblast various views have been expressed. 

 In the view of Sargant and Arber (1915), and Avery (1930), it is a mere 

 outgrowth; to Celakovsky (1897) and Worsdell (1916) it is homologous 

 with the auricles of the seed leaves; while to Poiteau (1809), Warming 

 (1879), van Tiegham (1897), Bruns (1892) and Percival (1921), it is a 

 rudimentary leaf. It is not unlike the non-vasculated prophylls and 

 enations in some vascular cryptogams. 



Randolph (1936) has shown that the pear-shaped embryo sac of 

 Zea mays rapidly encroaches on the large nucellus and integuments, 

 the small linear proembryo lying obliquely at the anterior side, Fig. 72. 

 The initial small, ovoid proembryo enlarges into a club-shaped structure 

 with a well-marked gradation of cell size, the basal cells being large and 

 vacuolated. Apart from the localisation of rapid growth in the small- 

 celled distal tissue, the proembryo is histologically undifferentiated up 

 to the seventh day after fertilisation. Its peripheral cells divide by both 

 anticlinal and periclinal walls, and elsewhere divisions take place in an 



