224 EMBRYOGENESIS IN PLANTS 



A longitudinal axis of the embryo-sac may be specified as passing 

 through the chalazal and micropylar ends, or through the antipodal 

 group of nuclei, the central fusion nucleus and the egg apparatus. 

 The micropylar end is morphologically the upper or distal end of the 

 embryo sac. There is support for the view that polar gradients have a 

 real existence in the embryo sac and that they are probably important 

 in the development of the embryo. 



The embryo sac may be enclosed within a tapetum of specialised 

 cells, and these are probably important in the nutrition of the embryo 

 sac, the developing endosperm and the embryo. Although we know- 

 little about physiological conditions within the embryo sac — our 

 information being mainly of an inferential character, based on ana- 

 tomical and cytological observations— it seems improbable that the 

 distribution of metabolic substances in it is homogeneous. In Lobelia 

 tn'gona, for example, the nuclear condition and the cytoplasmic vacuola- 

 tion are noticeably different at the two ends of the elongated embryo sac. 



Maheshwari (1950, p. 86) has illustrated diagrammatically several 

 different types of embryo sac, these varying in the number and dis- 

 tribution of their nuclei (from four to sixteen). In that the nucleus 

 controls the metabolism of the cytoplasm — it certainly produces 

 important effects in its own immediate vicinity — metabolic differences 

 in different regions of the embryo sac are to be expected. It may be 

 noted that not all embryo sacs contain nuclei of identical chromosomal 

 and genetical constitution and that they are not always disposed in the 

 same manner. In fact, a considerable diversity in the distribution of 

 nuclei in the embryo sac is found in different species. Moreover, some 

 female gametophytes are monosporic, some are bisporic and some are 

 tetrasporic in origin, according to the fate of the four nuclei derived 

 from the division of the megaspore mother cell. The different patterns 

 of nuclear aggregation in the embryo sac may also affect the embryonic 

 development. 



As a working hypothesis, the embryo sac may be recognised as a 

 biochemical and biophysical system of considerable complexity. 

 Maheshwari (1950) has illustrated many different conditions in the 

 embryo sac. The supply of nutrients arriving by way of the chalaza 

 will tend to maintain a gradient from the antipodal to the micropylar 

 end. Notwithstanding the variety of detail in embryo sac formation 

 in different species, the eventual organisation is rather alike in all. At 

 maturity, the Polvgonum, Alliunu Fritillarla and Adoxa types, as 

 described by Maheshwari (1950), have all a three-nucleate egg 

 apparatus, an antipodal group, and two central, or polar nuclei 

 (fused); and in the other types the egg apparatus, at least, is of the 

 usual kind. In some genera, e.g. Peperomio, Plumbago, Plumbagella, 



