372 W. J. C. Lawrence Cytoiogia 2, 



tion of auto-polyploidy occurs and it is often impossible to draw any 

 line of distinction between the types. 



In the ideal allopolyploid chromosome pairing occurs within the 

 parental sets and never between them, therefore although such an in- 

 dividual may breed true it is in the strictest sense heterozygous in con- 

 stitution. Allopolyploidy therefore confers hybrid vigour (in a general 

 sense, and not necessarily to be measured by comparison of size). 



Moreover, since fertility is directly correlated with regularity of 

 the meiotic processes the true allopolyploid will show a maximum of 

 fertility, as each pair of conjugants is exactly identical, and different 

 from every other pair. Allopolyploidy therefore promotes fertility. 



This combination of vigour and fertility has a high survival value, 

 in consequence of which the great majority of seed-bearing polyploids 

 are allopolyploid. 



As we have seen, the condition of allopolyploidy requires that the 

 chromosome complements of the respective parents should be differen- 

 tiated to such a degree that pairing should rarely or never occur 

 between chromosomes from opposite parents (Cf. Primula kewensis, 

 Tetraploid, Newton and Pellew 1929). If it does occur, fertility is 

 invariably decreased. But the fact that two species will cross to give 

 even a sterile hybrid implies a certain relation between them, and as the 

 constitution of the chromosomes is mainly responsible for the identity 

 of the individual it follows that the chromosomes must be more or less 

 structurally related. Similarity in gross structure has been observed 

 and illustrated in numerous cases, while genetic evidence indicates that 

 genie homologies occur in like manner. 



Thus the ideal condition of allo-polyploidy demands that the cor- 

 responding chromosome types from the parental species shall be too 

 different to pair. Hence differentiation between the parental chromo- 

 some sets is vital to survival. In terms of pairing, complete differen- 

 tiation of the bivalents assures stability of the meiotic processes and 

 promotes fertility. If multivalent association occurs within the parental 

 sets of high polyploids it will almost always be of a regular kind in seed 

 bearing plants and moreover reduced to a minimum. Multivalent pairing 

 impairs fertility unless compensated for by accidents of structure and 

 behaviour which ensure regularity in the alternation of generations. 



Moreover in a (historically) new hybrid-polyploid, there will be a 

 tendency to conserve any structural differences between similar chro- 

 mosome types of the parental sets. For example, if slight variation 

 occurs among 4 identical chromosomes, so that they are most alike in 



