418 University of California Publications in Agricultural Sciences [Vol. 2 



pechenko, 1924). The other extreme is represented by hybrids in 

 which the affinity between the parental chromosomes is so strong that 

 they always pair. Tliis condition can be illustrated by the hybrids 

 Geuni rivale X nrhanum, both with w ^ 21 chromosomes, and Trago- 

 pogon pratensis \ porrifolius, both ?i = 6 (Winge, 1926, 1928), and 

 b}^ varietal hybrids. Very similar to this is the case where the two 

 parents have different chromosome numbers but all chromosomes from 

 the parent that has the lowest chromosome number, pair with their 

 homologiies from the other parents, as in Nicoiiana paniculata X fus- 

 tica (Goodspeed, Clausen and Chipman, 1926) ; and in all clear cases 

 following the Drosera scheme. Between these two extremes are the 

 cases described in the three Crepis hybrids. Apparently a large 

 gradation exists between these extremes as manifested by the different 

 percentages of failures to pair in different hybrids. Although in the 

 Crepis case the degree of sterility to some extent seems to follow the 

 degree of cytological irregularities and the degree of failure to con- 

 jugate, it does not always hold true, as shown by the Tragopogon 

 hybrid which was almost sterile notwithstanding complete pairing; 

 and similarly in Lamium hybrids described by C. A. Jorgensen 

 (1927). The sterility might be caused mainly by non-balanced inter- 

 action of genes and perhaps has not much connection with the degree 

 of mutual affinity between chromosomes. Only so far as mutual 

 affinity between homologous chromosomes is one of the factors respon- 

 sible for keeping up an already established balance between the genes 

 constituting a certain genotype, can it be said that affinity betw'een 

 chromosomes has any bearing upon the question of sterility. 



The present paper in all essentials was completed May 15, 1928, 

 but w^as delayed five months by overwhelming experimental work. 

 In the meantime two papers bearing on similar phenomena were 

 published, namely, Maeda's paper on Laihyrus (1928) and Selling's 

 paper on Lilium (1928i). Maeda also has applied the combination 

 of Carnoy's and Navashin's fixatives for bringing out very conspicu- 

 ously the spiral structure of the chromosomes. Belling 's description 

 of the chiasmas in Lilium (pp. 467-468) corresponds with the con- 

 dition in Crepis, but the figures, as well as the earlier figures of 

 Uvularia and Hyacinthus, apparently do not show such fine details 

 as Crepis shows. Whether this is due to a profound difference in 

 structure of the chromosomes or to the squeezing applied in Belling 's 

 investigations cannot be told. 



