CMBRYOGENESIS IN FLOWERING PLANTS 225 



Acalypha, etc., the organisation of the embryo sac is radically different. 

 In these genera it will, therefore, be interesting to see if the embryonic 

 development shows any special features, these possibly throwing light 

 on factors involved. 



The immediate environment of the egg, or ovum, includes the 

 micropylar passage, by which the pollen-tube enters, the two synergids, 

 the adjacent embryo sac cytoplasm, the nucellar tissue, and, at ferti- 

 lisation, the contents discharged from the pollen tube. The synergids, 

 which initially have a cellular organisation with a vacuolated cytoplasm, 

 usually degenerate before, at, or soon after, fertilisation of the egg, but 

 they may persist in some species. They do not usually extend beyond 

 the limits of the embryo sac wall, but cases are known in the Com- 

 positae where they elongate and penetrate into the micropyle, and even 

 through and beyond it. The egg itself consists of the female nucleus 

 and surrounding cytoplasm, enclosed in a thin wall or pellicle, and 

 attached to the embryo sac wall near the micropyle. A small vacuole 

 may be present near the point of attachment, the nucleus being typically 

 situated at the other end, i.e. away from the micropyle. Though at a 

 distance, the antipodal cells may have an effect on the fertihsed egg. 

 The central fusion nucleus (or polar nucleus), from which the endo- 

 sperm is formed, may have important and, in different species, variable 

 effects in the embryonic development. 



Embryo sacs with disturbed polarity have been described : Mahesh- 

 wari (1950) has summarised the relevant data. 



The embryo sac is usually regarded as being devoid of appreciable 

 food reserves, presumably because of its growth to large size and its 

 eventual vacuolation. The considerable enlargement, however, is 

 itself indicative either of rapid growth of the surrounding cells, or of 

 the presence of osmotically active substances in the embryo sac. 

 Starch is commonly present in the embryo sac of some species. The 

 investigation of the storage materials in the embryo sac has been rather 

 neglected by embryologists intent on other details (Dahlgren, 1939). 

 In some species, e.g. Arachis, Tilia, Penstemon and Acacia, the embryo 

 sacs are so full of starch as to make cytological observations difficult, 

 while in Styphelia the egg tends to be obscured by the starchy accumu- 

 lations. In most species that have been investigated the starch content 

 becomes maximal about the time of fertilisation, and gradually decreases 

 during the embryonic development. In some species large quantities are 

 still present during endosperm formation. Even the egg apparatus may 

 show evidence of starch accumulation. The nutrition of the embryo, 

 by the general mechanism of the embryo sac tapetum, or by special 

 haustorial structures, is considered later {see Maheshwari (1950) for 

 references to the literature). 



