CHROMOSOME RACES IN CARDAMINE PRATENSIS 



173 



of Nasturtium microphyllum (uniseriatum) which proved to be an allopolyploid, rather 

 than an autopolyploid, of N. officinale (Manton & Howard 1946). In such cases, smallness 

 of the chromosomes makes it difficult to consider the lower percentage of multivalents 

 as a definite criterion of allopolyploidy, although the high fertility of the 2n = 56 race 

 is suggestive. Morphological differences between the two chromosomes races were quite 

 notable and will be dealt with statistically in a following study. 



Type of Flower 



From the cytological data it was clear that doubleness of flower was of a different 

 type in the two chromosome races. All sterile doubles including the Arabis type, were 

 2n = 30, while all semidoubles were 2n = 56. 



;/jioi 1 1 i)»«M»c«niit««« & 



Fig. 2. (a) Somatic chromosomes of 2n = 56 of a single and semi-double type; (b) Meiotic chromosomes 



of a 2n = 56. 



This may perhaps be considered as some indication that there is a genetic cause 

 for all types of doubleness. Both sterile doubles and the Arabis type occurred without 

 change under varying environmental conditions and were always vegetatively reproduced 

 by the usual leaf buds, but in case of semidoubles, it was noticed that the proliferation 

 occasionally, but not always, disappeared under greenhouse conditions in the following 

 years, and some seeds were produced. It may be suggested that there is a physiological 

 factor controlling the appearance of the semi-double condition. 



Summary 



(1) The variability of the species is indicated and a short review of literature on 

 the chromosome number given. 



(2) Three aspects of the British races are discussed, geographical distribution, 

 cytological differences and type of flower. 



(3) No chromosome races except 2n — 30 and 2n = 56 were found in the 113 

 plants examined. 



