474 



edly the somatic mimber in the race cxamined will be found to 

 be elghteen. 



In the case of longiflora there is no such variation in chromosome 

 number in P. M. C. Ten chromosomcs have uniformally been 

 counted in heterotypic metaphase, anaphase and in homotypic 

 anaphascs. The situation described above for alata suggests one 

 way, at least, in which a ten chromosomed form might have 

 arisen. Further study of other races of this species must, also, 

 be made. 



A more detailed examination of suaveolens. original ly stated pro- 

 bably to have twelve chromosomes, indicates that x — 18 is the 

 correct number for this species. 



A number of varieties of Tahacum have been used in crosses 

 with sijlueslris and with other species, as follows. 



Syluestris X Tabaciim vars. macrophylla, angustifolia, macrophylla 



purpurea, "Maryland", "Cuba".^ 

 Tabaciim X paniciilata. 

 Tabaciim X glutinosa. 



It is the purpose of the ])rescnt prcliminary noté to comment 

 upon the cytological phenomena exhlbited by F^ hybrids of syl- 

 uestris X Tabaciim. Material of sylnestris X Tabaciim vars. angusti- 

 folia. "Cuba" and "Miradato"" has been examined; the most com- 

 plete series of stages being obtaincd in the cross wilh "Miradato". 

 The somalic number in these hybrids is Ihirty-six and twelve 

 bivalent and twelve univalent chromosomes appcar in heterotypic 

 metaphase in both P. M. C. and F. M. C. The distinction bctween 

 gemini and single chromosomes is sharp as indicated in fig. 1 

 c and d. 



In the first division of P. M. C. there are two successive stages, 

 one involving the separation of the partners of the bivalents and 

 the other the division of the univalents. In late metaphase the 

 bivalents are rather accurately disposed in the e([uatorial region, 

 some of the singles lying betw^een them but, for the most part, 



^ For a taxonomic discussion of the species and varieties referred to, see 

 Setchell, Univ. Calif Pub! Botany, vol. 5, no. 1,1912. 



^ Material of var. '"Miradato" was kindl}'^ put at my disposal by Dr. \V. Bateson. 

 and I am much indebted to Mr. Newton of the staff of the John Innes Horti- 

 cultural Institution for fixing material of this variety and of the cross with 

 sfilvestris. 



