EMBRYOGENESIS IN FLOWERING PLANTS 261 



Fisk, 1951). The inner layer of the integument now becomes differen- 

 tiated as an endothelial layer. A common feature is the presence of 

 chalazal and micropylar endosperm haustoria, Fig. 66. In Torenia the 

 apical portion of the embryo sac grows through the micropyle and 

 along the funiculus, from which there may be a loss of cell contents 

 (Balicka Iwanowska, 1899); and in some cases it even reaches the 

 placenta. Starch is present in the developing and mature embryo sac 

 and may still be observed in the early stages of endosperm formation, 

 {see Guilford and Fisk (1951) for references to literature). 



In Plantago lanceolata. Fig. 66c, the endosperm divides to form two 

 cells, of which the chalazal cell becomes binucleate and functions as a 

 haustorium. The micropylar cell divides to form four cells, two of these 

 forming the endosperm proper, while the other two undergo a most 

 remarkable development as micropylar haustoria. They grow out as 

 hypha-like cells with swollen ends, and eventually separate the ovule 

 from the placenta, except for the vascular strands of the funiculus. 

 The developing endosperm is surrounded by a layer of specialised 

 cells that persist even in the mature seed. The zygote does not divide 

 until the endosperm has become multicellular (Cooper, 1942). 



In the embryogeney of Euphorbia rothiana (Euphorbiaceae), Fig. 

 66d-g, Srivastava (1952) has illustrated the first division of the zygote 

 by a vertical wall, followed by a transverse wall. During the subsequent 

 development, the embryo becomes pear-shaped, the suspensor being 

 ill-defined, if it could indeed be distinguished. This first division of the 

 zygote by a vertical wall is different from what has been observed in 

 E. exigua and E. esula by Soueges (1942, 1945), or in E. presli and E. 

 splendens by Weniger (1927). In Acalypha indica, as investigated by 

 Johri and Kapil (1953), the embryogeny is a variant of the Onagrad 

 type. These authors have also given a survey of the embryogeny in all 

 the investigated species of Euphorbia and have indicated considerable 

 variability in the details of development in different species. 



THE ORIGIN OF THE MONOCOTYLEDONOUS EMBRYO 



The origin of the monocotyledonous embryo has long attracted the 

 attention of botanists. Maheshwari has described it as the most 

 difficult problem in angiosperm embryology. In the dicotyledonous 

 embryo the plumule is typically distal and is situated symmetrically 

 between two equivalent cotyledons : in the monocotyledonous embryo 

 the shoot apex occupies a lateral indentation in the somewhat cylindrical 

 embryo and the cotyledon is terminal. Fig. 56f-k. Up to a certain 

 stage in the embryonic development, however, the two types of embryo 

 may be very closely comparable, both being cylindrical, or club- 

 shaped, axial bodies. It has been suggested that the monocotyledonous 



