226 EMBRYOGENESIS IN PLANTS 



THE ENDOSPERM 



Contemporaneously with the zygotic development, another factor 

 becomes incident, namely, the developing endosperm. The central 

 fusion nucleus forms the endosperm on being stimulated by fusion 

 with a male nucleus. The endosperm is usually a triploid tissue but 

 instances are known of haploid endosperm, while tetraploid and 

 pentaploid endosperms are also known. In albuminous seeds the 

 endosperm typically grows much more quickly than the embryo. 



Although the endosperm will, of necessity, be treated rather briefly 

 here, this tissue is very important in the development of the angiosperm 

 embryo. Thus far, a majority of the investigations of endosperm have 

 been undertaken from the anatomical and cytological rather than from 

 the physiological point of view. The endosperm in the Gramineae, 

 especially that of maize, has been extensively investigated from the 

 genetical point of view. Maheshwari (1950) has contributed a full and 

 interesting chapter on endosperm to which the reader is referred. The 

 extensive review by Brink and Cooper (1947) on 'The Endosperm in Seed 

 Development,' with its bibliography of 283 titles, should also be 

 consulted. 



The evolutionary origin and morphological nature of the angio- 

 sperm endosperm are difficult subjects. It is doubtful if an understand- 

 ing of angiosperm endosperm can be gained by comparing it with that 

 in gymnosperms. The angiosperm embryo sac, whatever its phyletic 

 origin may have been, is a unique evolutionary specialisation; within it, 

 in the formation of endosperm, we may recognise a further innovation. 



Whereas the fertilised ovum becomes activated and forms a polarised 

 embryo, the fertilised polar nucleus, which is usually situated near the 

 centre of the embryo sac, shows no such development. The differences 

 in physiological conditions at the micropylar end and in the centre of 

 the embryo sac may be slight, but they must be considered highly 

 important. Even in the Onagraceae, where the embryo sac has only 

 four nuclei, with three forming the egg apparatus and one the polar 

 nucleus, the latter, though fertilised like the ovum, does not form an 

 embryo but an endosperm. As the ovum and the polar nucleus are 

 genetically identical, as are also the two nuclei from the pollen tube, 

 the zygote and fertilised polar nuclei are also identical. Indeed, irrespec- 

 tive of its chromosomal constitution, the central nucleus typically yields 

 endosperm — a formless mass of cells, bearing no resemblance to a second 

 embryo. The various theories of the endosperm as an anomalous 

 embryo, though interesting notions, do not appear to lead anywhere. 



The participation of a male nucleus in the inception of the endo- 

 sperm is a very general phenomenon. According to investigators such 



