POLYPLOIDY 225 



hybrids of Solanum due to the fusion of nuclei in adjacent cells 

 near the wound. 



Among tetraploid wild species, the case of Spiranthes cernua 

 perhaps comes nearest to that of (Enothera gigas. This species 

 is widely distributed in North America and is stouter in all its 

 parts than related forms, and is apparently a cell giant. In the 

 genus Primula (see below), cell-gigantism can occur either with or 

 without polyploidy. Thus the simplest form of tetraploidy appears 

 to be that in which chromosome doubling is accompanied by an 

 increase in the volume of the nuclei and cells. 



The whole question of size relations between chromosomes, 

 nuclei, cells and organs of polyploid species requires further 

 investigation, in order to analyse completely the nature of the 

 changes which have occurred in each case. These size relationships 

 vary very much in different genera. Thus in Trifolium, species 

 are to be found with the same chromosome number, yet the 

 chromosomes may differ in size. Also in this genus there appears 

 to be no direct relationship between chromosome number and 

 plant size, but there is such a relationship between cell size and 

 plant size. Thus in T. campestre and T. glomeratum, which have 

 the same number of chromosomes (n = 7), the chromosomes are 

 also approximately the same size. T. arvense and T. pratense 

 have the same number of chromosomes, but these differ in size. 

 In T. repens the number of chromosomes has been doubled 

 (n = 14), yet the size of the chromosomes remains about the same 

 as that of related species. In T. minus the chromosome number 

 is again tetraploid (n = 14), but the chromosomes are smaller and 

 may have arisen through transverse segmentation. Lastly, in 

 T. pannonicum, the chromosome size is the same, while the 

 chromosome number is 14. 



In the tetraploid form of Primula sinensis there are 48 chromo- 

 somes, the diploid plant having 24. About thirty factors are now 

 known causing variations in the diploid, and nine of these also 

 occur in the tetraploid plant. 



The genet ical behaviour of these forms differs in three ways. 

 Thus the recessive factor, which causes intense crimping of the 

 leaves and flowers in the diploid, has a very much less intense effect 



K.A.M, 8 



