THE ANGIOSPERMAE 1423 



The question of the function of the antipodals is bound up with the more 

 general question of the nutrition of the embryo sac. It will be remembered 

 that the surface of the nucellus and the inner surface of the integuments are 

 cuticularized, so that direct lateral passage of substances into the embryo 

 sac is very restricted or non-existent. This alone seems to us sufficient 

 reason for rejecting the name of " tapetum " as applied to the prominent 

 mantle-layer or endothelium developed by the inner integument in most 

 Metachlamydeae (see p. 1383). It is true that the growing embryo sac 

 frequently destroys and perhaps absorbs the tissue of the nucellus and that 

 it even encroaches on the inner integument in the same way. Nevertheless 

 it is clear that the main supplies of water and food materials must come from 

 the chalaza and enter the sac at the antipodal end. 



In the chalaza, and often near the base of the embryo sac, is the end of 

 the funicular vascular bundle. Between it and the embryo sac there are 

 frequently formed specialized tissues, sometimes lignified and tracheidal, 

 sometimes thin-walled and either starch-containing or mucilaginous, 

 sometimes even broken down into mucilage-filled cavities. In Podo- 

 stemaceae a very large lysigenous cavity is formed containing many free 

 nuclei, which pushes the nucellus up above the inner micropyle, and is 

 called the pseudo-embryo sac. In the chalaza also may be found the 

 thick-walled mass of cells, called by Van Tieghem the hypostase (see p. 1392), 

 which he considered to be a barrier to the growth of the embr>'o sac in 

 this direction, but which is much more likely to be concerned with the 

 passage of fluid both towards the growing embryo sac and also away from it 

 during the ripening of the seed. 



With these facts in mind it is not surprising to find that the antipodals 

 often become aggressive and invade the chalaza as haustoria, which is 

 quite in line with their supposed nutritive function. Similar aggressive 

 outgrowths, both lateral and basal, may be formed by the embryo sac 

 itself and it is noteworthy that these embryo sac haustoria are generally 

 associated with a weakly developed antipodal apparatus. We are here 

 dealing with haustoria formed before fertilization and we shall consider 

 haustoria formed by the endosperm and the suspensor cells later (see pp. 



1458 and 1471). 



There is a clear distinction between these various types of haustoria 

 but both antipodal and endospermal haustoria may be independently 

 formed by the same sac, as in some Amentiferae. Some of the best devel- 

 oped antipodal haustoria are found among the Rubiaceae and the Com- 

 positae. Either the lowermost antipodal cell or sometimes all three may 

 grow out into the chalaza, forming tubular or swollen expansions (caeca), 

 while in the Compositae, where there may be numerous antipodals, they all 

 take part in forming a long salient into the chalaza, {e.g., Senecio). In Aster 

 novae-angUae there is a large, muki nucleate caecum (Fig. 13 14) apparently 

 formed from the lowermost antipodals, but it has been suggested that this 

 caecum is really formed by the lower megaspores of the tetrad, which have 

 persisted and taken on the appearance of antipodal cells. This recalls the 

 N 



