226 BOTANY 



in the tetraploid species. Again, intermediate forms are possible 

 in the tetraploid plant which are not found in the diploid. The 

 most important differences relate to the laws of segregation and 

 inheritance. Only one kind of heterozygote is possible in the 

 diploid. In the tetraploid, on the other hand, three kinds are 

 present. The genetics of plants heterozygous for several factors, 

 especially when these are linked, is very much more complicated, 

 but agrees with expectations if the factors be associated with 

 definite loci in the chromosomes. 



The conditions present in the allopolyploid are very different. 

 For example, in the cross between Primula floribunda and P. 

 verticillata a sterile hybrid has several times been obtained. This 

 problem was investigated by Newton and Pellew, who have 

 shown that the hvbrid, which contains one set of nine chromo- 

 somes from each parent, can become fertile by throwing tetraploid 

 branches. The seedlings produced from these branches are the 

 well-known Primula kewensis. Here there are nine sets of four 

 chromosomes, which may be called r^F^V^Vj, F2F2V2V2, and so 

 on, the floribunda chromosomes being called F^^Fg, etc., and the 

 verticillata V^Vg, etc. These chromosomes generally unite in 

 pairs, and not in fours, and since the plant breeds nearly true, it is 

 assumed that F^ generally pairs with F^ and V^ with V^, and so on. 

 Thus each gamete will have the composition F^ViFgVg, etc., and 

 the plant breeds true. Occasionally, Y^ pairs with V^, and off- 

 spring different from the parent will be produced. Several other 

 fertile and constant inter-specific hybrids are similarly allo- 

 tetraploids, and it is probable that this is one method whereby 

 new species arise in nature. 



Triploids. It has generally been considered that triploids have 

 arisen by the union of a diploid gamete with a haploid one. 

 Certainly triploids have been produced by crossing tetraploid 

 mutants with diploid species. Another possible origin for triploids 

 is the entrance of two male nuclei into the egg (dispermy). Such 

 a process has been recorded by Ishikawa for (Enothera. 



In Hyacinthus de Mol observed that diploid plants on occasion 

 produced a few functional diploid pollen grains. This result was 

 apparently due to the shortening of the growing season and was a 



