234 



THE BEHAVIOUR OF POLYPLOIDS 



are those giant varieties found within species (Table 37) which 

 can certainly be supposed to have arisen from simple doubling. 

 Similarly, Vallisneria spiralis [n = 10) is dioecious, and appears to 

 have given rise to V. gigantea (n = 20) of both sexes separately by 

 somatic doubling (j0rgensen, 1927). 



Secondly, there are allopolyploid species whose diploid parents 

 are known with certainty (Table 39). Amongst cultivated plants 

 there are also many forms of equivalent rank to new species, whose 

 origin can be inferred with high certainty. Such are the auto- 

 polyploid Petunia varieties (Dermen, 1930) and certain allo- 

 polyploid Rubi (Crane and D., 1927). 



In all these cases, therefore, there can be no doubt that a new 

 species has been produced in one generation. This is possible with 

 polyploidy and also with other changes which are in themselves of 

 greater evolutionary significance. Some of these will be discussed 

 in regard to mutation in (Enothera (Ch. IX). 



Table 39 

 Allopolyploid Species whose Origin can be Determined 



Genus Species 



1. ^sctilus (10) Hippo- {4x) X pavia {4X) = 



castanum 



2. Msculiis (10) carnea {8x) X Hippo- {4X) = 



castanum, 



3. Galeopsis (8) puhescens {2x) x speciosa {2x) — 



4. Phleum (7) pratense {2x) x alpinum {4X) = 



5. Spartina (7) alterniflora [lox) x stricta {8x) 



6. Prunus (8) divaricata {2x) x spinosa {4X) 



7. Pentstemon (8) IcbIus {2x) X azureus {6x) 



8. Agropyron (7) junceum {4X) x littoreum {6x) 



Hybrid 



carnea [8x), Hoar, 



1927. 

 plantierensis (6^), 



Upcott, 1936. 

 Tetrahit {4X), Miintz- 



ing, 1927, 1930 b. 

 pratense (America), 



{6x), Gregor and 



Sansome, 1930. 

 Townshendii {18 x), 



Huskins, 1931a. 

 domestica {^^)> 



Rybin, 1936. 

 neotericus {8x) , 



J. Clausen, 1933. 

 a c u t u ni (5^). 



Simonet, 1934. 



(iii) Somatic and Secondary Pairing. At metaphase of mitosis 

 the positions of the chromosomes seem to be governed by three 

 rules : (i) the centromeres lie within the spindle and in its 

 equatorial plane, which is the metaphase plate ; (ii) they lie 



