262 EMBRYOGENESIS IN PLANTS 



embryo has resulted from the fusion of the two originally separate 

 cotyledons, or that one cotyledon has been suppressed. In another 

 approach to the problem, investigators have attempted to ascertain 

 whether or not the single cotyledon is a truly terminal organ, or initially 

 a lateral organ which has come to occupy a distal position. The several 

 views have each received some factual support, but although many 

 relevant investigations have been carried out, the problem has not yet 

 been finally resolved. 



In Agapanthus umbellatus (Liliaceae), a widening, distal 'tubular 

 cotyledonary zone' with two primordia growing at its tip, and sur- 

 rounding the sunken shoot apex, is to be seen at certain stages in the 

 embryonic development (Coulter and Land, 1914). If both of the 

 primordia develop equally, a dicotyledonous embryo is formed; but, 

 as more frequently happens, one of the primordia soon stops growing 

 while the other, by growing rapidly, becomes a conspicuous terminal 

 organ, leaving the shoot apex in a relatively lateral position. Coulter 

 and Land (1915) have also interpreted the embryo of grasses along 

 similar lines, the scutellum being a functional cotyledon arising from 

 the peripheral cotyledonary ring, while the epiblast is held to be the 

 greatly reduced second cotyledon. They consider that this view is 

 supported by the presence of a conspicuous epiblast in species of 

 Leersia and Zizania (both Gramineae: Oryzeae). This view will be 

 supported if it can be shown that the dicotyledons afford examples of 

 partial or complete transition to a monocotyledonous condition. In 

 fact, seedlings with a single cotyledon have been found in species of 

 widely separated families, e.g. Ranunculus Jicaria {Ranales: Rammcu- 

 laceae), Corydalis cava (Rhoeadales; Fumariaceae) Abronia (Thyme- 

 laeales: Nyctaginaceae), Carum bulbocastanum (Umbelliflorae: Umbel- 

 liferae), BJumium elegans, Erigenia bulbosa, species of Gesneraceae, etc. 



The idea that the monocotyledons are an offshoot of some relatively 

 primitive dicotyledon stock has long been entertained, and seemingly 

 receives support from the monocotyledonous embryos of such species 

 as Ranunculus Jicaria and Corydalis cava. Fig. 67. Takhtajan (1945) 

 considers that the monocotyledons must be descended from some 



Fig. 66. Illustrating various anomalous embryos 



A, Toreniafournieri. Elongated zygote within the micropylar haustorial endosperm ; 

 the embryo sac has also a chalazal haustorium. Note remains of pollen tube, endo- 

 sperm, and large epithelial cells ( >s 250). B, Mimiiliis rigriniis. Endosperm breaking 

 down round embryo, the cotyledons of which are beginning to differentiate; a 

 conspicuous multinucleate micropylar endosperm haustorium can be seen ( x 333). 

 (A and B, after Guilford and Fisk.) C, Plaiitago lanceolata. Longitudinal section of 

 seed, showing embryo and conspicuous endosperm haustoria ( x 180, after Cooper). 

 D-G, Euphorbia rothiana. Four stages in the development of the embryo, the first 

 division being vertical ( x 320; after Srivastava). H-L, Acalyplia lanceolata (H, J, 

 X 900; K, X 630; L, x 450; after Thalhacher). 



