336 



MORPHOLOGY OF THE ANGIOSPERMS 



(Fig. 129F to H). The cotyledon is doubled back upon itself, and its 

 proximal and distal segments — except for the absorbing tip — may be- 

 come laterally connate through part or most of their extent. Lateral 

 fusion of the two arms of the neck may extend any distance, from the 

 point of bending downward even below the base of the 

 cotyledon to and along the hypocotyl (Fig. 129G, H). 

 Complex structure is formed where fusion is extensive 

 — connation between separate parts of an organ, the cot- 

 yledon, together with adnation of these connate parts to 

 another organ, the hypocotyl. The compound structure 

 formed in this way by adnation of cotyledon to hypo- 

 cotyl has been termed the mesocotijl (Fig. 130). In em- 

 bryos in which there is a mesocotyl, this fusion of coty- 

 ledon neck and hypocotyl is evident externally in some 

 taxa by a dorsal ridge (Fig. 134G, H); in other taxa, 

 there is no external evidence of the adnation, but vas- 

 cular structure gives complete evidence of the double 

 fusion. All stages in this complex fusion exist; the grasses 

 provide examples of the most complete union, where 

 even the vascular tissues of the two organs are united 

 (Fig. 134). 

 Fusion of the vascular tissues of two organs under 



Fig. 130. Dia- 

 grammatic lon- 



gitudinal section adnation is a common condition in all parts of the plant. 

 Failure to recognize the presence of this fusion and to 

 intei*pret critically the anatomical complexity in mono- 

 cotyledonous embryos has been largely responsible for 

 varied and often extraordinary interpretations of the 

 embryo. If a series of embryos from more primitive 

 monocotyledons — the Liliales, Helobiales — to the ad- 

 vanced Palmae and Gramineae is considered compara- 

 tively, the story of evolutionary change is evident, and 

 the terminal form, that of Zea and other grasses, can be 

 correctly interpreted. Much of the evidence for this in- 

 terpretation of the highest forms of monocotyledonous 

 embryo was available as early as the 1870s, and, at that 

 time, the statement was made that "in the Zingiberaceae 

 and palms, there can be no question but that the cole- 

 optile, cotyledonary sheath, and scutellum represent 

 the cotyledon or part of it." The whole story became clear in 1915, but 

 various later interpretations, overlooking the evidence provided by the 

 vascular skeleton and the existence of adnation, have obscured the true 

 morphological make-up of the grass embryo. The sheathing base and 

 scutellum (connected by the neck) have sometimes been called "lobes" 



of a very young 

 seedling of Av- 

 ena sofiva to 

 show morphol- 

 ogy of grass em- 

 bryo, especially 

 the several parts 

 of the cotyle- 

 don, shaded. 

 ar, adventitious 

 root; cr, coleo- 

 rhiza; c^.colcop- 

 tile; e, epiblast; 

 m, mesocotyl; p, 

 plum vile; pr, pri- 

 mary root; s, scu- 

 tellum. ( After 

 Sargant and 

 Arber. ) 



