228 EMBRYOGENESIS IN PLANTS 



and differentiation of the embryo and in these the endosperm is free- 

 cellular at first, only becoming cellular later, Maheshwari (1950) has 

 countered Rao's suggestion by stating that there are many examples 

 which do not support this correlation. Nevertheless, some such 

 correlation is perhaps to be expected, but it might be qualified, in 

 different species, by the nutritional status of the ovule. 



FERTILISATION AND EARLY EMBRYOGENY 



In such a large systematic group as the flowering plants, with all 

 their diversity of gynoecium, placentation and ovule shape, there is 

 naturally a considerable variety of ways in which the male gamete 

 eventually makes contact with the ovum. The relevant details have 

 been surveyed by Maheshwari (1950). Here our primary concern is 

 with the formation of the zygote and the factors that determine its 

 initial and subsequent conformation. Figs. 53-55. 



The actual gametic union seems to be a full and complete one and 

 considerably different from that recorded for many gymnosperms. In 

 some species, cytoplasm has been observed surrounding the male 

 nucleus. Male cytoplasm may also surround the ovum, and about this 

 time there may be disruption or disorganisation of the synergids. The 

 biochemical and biophysical situation at the time of fertilisation is 

 undoubtedly complex: many separate but inter-related events, some 

 at the microscopic level, but almost certainly many more that are quite 

 invisible, follow in rapid succession, e.g. the movement of the male 

 nucleus, and its changing shape as it comes into contact with, and is 

 absorbed into, the female nucleus. Yet it is an ordered complexity, for 

 out of it there regularly comes the characteristic development of the 

 embryo. Whether or not cytoplasm from the male gamete takes part 

 in the formation of the embryo — a matter of genetical importance — is 

 not definitely known. 



Syngamy is followed by a period of rest, or, more probably, of 

 reorganisation, during which the large vacuoles at the micropylar end 

 of the ovum gradually disappear, and the cytoplasm round the nucleus 

 becomes densely homogeneous. The primary endosperm nucleus 

 usually divides a little before the zygote. The so-called 'resting period' 

 of the zygote may be only a few hours, e.g. 4-6 hours in the Compositae 

 and Gramineae, or many days, e.g. 14-15 days in Theohroma cacao. 



It is a reasonable inference that polarity is determined in the un- 

 divided zygote, or even in the unfertilised ovum. The still undivided 

 zygote typically elongates in the axis of the embryo sac and small 

 vacuoles become uniformly distributed through the cytoplasm. With 

 some rare exceptions, the first nuclear division is followed by the 

 formation of a transverse wall ; i.e. as in pteridophytes, the first wall is at 



