478 



in such an assumption are appreciated. It has, however, been 

 possible to examine thc number ol somatic chromosomes in an 

 "extracted" sijlvestris which bas bied true for one generation after 

 the back-cross. The number of chromosomes is apparently 2x — 24 

 bkc the parent sijlnestris. The explanation of this situation given 

 above would involve the considerabie assumption tiiat in some 

 cases the 12 sijlnestris biv^alent parlners all pass to one pole in the 

 helerotypic anaphase. The difficullies of inlerpreting the recovery 

 of thc Tabaciim set of chromosomes is, obviously, grealer. It is 

 hoped in the near futurc lo present further cytological evidence 

 bearing on these matters. 



Sachsowa (c. R. Soc. Sc. Varsovie, 1917) and låter Woycicki (Dis. 

 Biol. Arch. Soc. Sc. Varsaviensis, 1921) have published parlially 

 complete accounts of the cytology of a Tabacum X sijluestris hybrid. 

 Apparently neither investigalor realized that Ihe parents of this 

 hybrid differ in chromosome number and this fact, perhaps, explains 

 their lack of appreciation of the phenomena which they describe. 

 Woycicki has figured many of the signilicant stages observed by 

 the present writcr. In the helerotypic metaphase he reports Iwenty- 

 four chromosomes of which twelve are largc and twelve smaller 

 but for some reason speaks of them as paired ("douze paircs de 

 chromosomes, etc")- Sachsowa lays emphasis upon what appear 

 to be abnormalities produced by fixation in early metaphase and 

 which are figured as a string of thirtcen chromosomes. He appa- 

 rently considers this to be Ihe hapioid number and furlher con- 

 cludes that the Drosera scheme docs not apply. He, however, figures 

 bivalent and univalent chromosomes behaving in a manner corres- 

 ponding to the stages illustrated and discussed by the present writer. 



The studies reported upon in this paper were carricd on in thc 

 Botanical Institute of the University of Stockholm while the author 

 held a Fellowship under the American Scandinavian Foundation. 

 It is a pleasure lo acknowledge my indebtedncss to Professor O. 

 Rosenberg and the members of the Botanical Institute for many 

 courlesies and much assislance and adyice. I am furlher indebted 

 to Professor R. R. Gates Ihroiigh whosc courtesy I was enabled 

 lo grow a number of culturcs in the Garden of the Royal Botanic 

 Sociely, Regents Park. 



Botanical Institute Universitv of Stockholm. 



