,1904.] 



The Reduction Division in terns. 



91 



sativum, a result which is confirmed by my own observations upon 

 race-hybrids of Lathyrus odoratus, for the material of which I am 

 indebted to Mr. Bateson. 



Further light upon this question may be expected from the study 

 of hybrids between races which diner from one another either in the 

 morphology or number of the chromosomes. The only observations 

 upon this point are those of Rosenberg* on the hybrid Drosera longifolia 

 x Drosera rotundifolia. 



The number of chromosomes characteristic of the vegetative and of 

 the reduction divisions respectively in the former is 20 and 10 ; in 

 the latter, 40 and 20. In the vegetative cells of the hybrid 30 

 {i.e., 10 + 20) chromosomes appear. In the formation of the pollen 

 of the hybrid there occur three types of nuclei, all of which may occur 

 in the same pollen-sac. Commonly 15 bivalent chromosomes appear, 

 but in many cases there are 20 (as in D. rotundifolia), and in two 

 cases Rosenberg observed 10 (as in D. longifolia). He was, however, 

 unable to determine whether dissimilar numbers of chromosomes 

 appear in the daughter nuclei of the same pollen mother-cell. The 

 investigation is, therefore, not sufficiently complete at present to 

 permit a useful discussion of the results. 



The sterility which characterises many hybrids follows upon the 

 abortive development of the sex cells, and the suggestion has been 

 madef that this may be due to the inability of the hybrid to separate, 

 in the formation of the gametes, the characters which were united in 

 the hybrid zygote. It is well known that sterile plant-hybrids are 

 particularly characterised by abortive development of the pollen, or 

 (in the case of the hybrid Fern described by Farmer \) of the spores. 



Among the offspring of a race-hybrid of Lathyrus odoratus fertilised 

 with its own pollen, Mr. Bateson obtained a number of individuals 

 which failed to form good pollen. In the plants with coloured flowers 

 the sterility was, with a few exceptions, correlated with the develop- 

 ment of a somatic character — the sterile plants generally possessing 

 a green leaf axil, while the fertile coloured plants with rare exceptions 

 had red axils. In these plants the divisions of the vegetative cells are 

 quite normal, as are also those of the archesporium up to the forma- 

 tion of the pollen mother-cells. The irregularity makes its appearance 

 only in the heterotype division. 



The longitudinal fission of the spireme takes place quite normally, 

 but the segmentation into chromosomes is, if carried out at all, 

 irregular, and the pollen mother-cells degenerate. Since the equation 

 divisions are quite normal, this would seem to indicate that the union 



* " Das Verhalten der Chromosonien in einer hybriden Pflanze," ' Ber. d. 

 Deutsch. Bot. Gesellsch.,' 1903, vol. 21, p. 110. 



f Bateson and Saunders, Report to Evolution Committee, I, p. 148. 

 X ' Annals of Botany,' 1897, vol. 11, p. 533. 



