1452 A TEXTBOOK OF THEORETICAL BOTANY 



nucleus, they should show greater power of development than the com- 

 paratively slight development which follows when a ventral canal cell in a 

 Gymnosperm is fertilized, might reasonably be expected. 



A further consideration has been urged, both by Thomas and Nemec, 

 namely that the incorporation of paternal genes in the primary endosperm 

 nucleus makes it genetically related to the embryo and that for this reason 

 the chemical nature of the nutritive substances provided by the endosperm 

 may be more closely related to the requirements of the embryo than would 

 otherwise be the case. Experiments on the transplantation of grass embryos 

 into unrelated endosperms seems to bear this out. 



As we mentioned earlier (p. 1430), Berridge dissented from the view that 

 the polar nucleus was the equivalent of a ventral canal cell. She regarded 

 the endosperm as an anomalous embryo and compared it with the apoga- 

 mous embryos which she observed in Ephedra, which originate by the fusion 

 of two archegonial jacket nuclei, to form a diploid nucleus which has the 

 power of forming an embryo, if it escapes into the archegonial cell. These 

 embryos only develop after fertilization has occurred and she suspected that 

 there was an actual union of the diploid nucleus w'ith a sperm nucleus before 

 development. The jacket cells in Ephedra come from an initial cell which is 

 closely related to the archegonium initial. As, in Berridge's view, the 

 antipodals represent the lower prothallus tissue in Gnetales, the partici- 

 pation of one of their group in triple fusion could only have a nutritive signi- 

 ficance. 



The Endosperm 



There are only two families of Angiosperms in which endosperm develop- 

 ment is effectively absent, the Orchidaceae and the Podostemaceae. In the 

 former the embryo remains a small undifferentiated mass of cells and its 

 further development depends on mycorrhizal nutrition. In the latter the 

 growing embryo is pushed down into the pseudo-embryo sac, below the true 

 one, which apparently fulfils the nutritive function of the missing endo- 

 sperm. Double fertilization has been show'n to occur in Orchidaceae and it 

 is not known why it produces no result. In all other families endosperm 

 development begins, though in some of them it is only transient and the ripe 

 seeds are non-endospermous. 



Endosperm development does not seem to be affected by the triploid 

 or polyploid status of its cells. There is, however, no immediate union of 

 component chromosome groups. At the first, and even at some subsequent 

 divisions, tripolar spindles are formed and the three chromosome groups 

 remain distinct, though the tripolar spindle has sometimes been seen to 

 pass over into a bipolar one. Certainly chromosome pairing is delayed, 

 but when it takes place, if at all, is not known. 



Although the endospermal fertilization is called triple fusion, the three 

 nuclei do not always unite simultaneously. The following are the variations 

 which have been observed in the order of events. 



