154 BOTANICAL GAZETTE [FEBRUARY 
NOTES FOR STUDENTS 
Species hybrids.—According to one definition of species, which maintains 
that crossing is impossible beyond the species boundary, there can be no such 
thing as a species hybrid. The experience of many breeders, however, shows 
numerous cases of successful crossing between forms that are commonly 
regarded as distinct species. As might well be expected, many of these species 
crosses exhibit peculiarities, and it is of considerable theoretical interest to 
analyze them. In this connection it is enlightening to consider some of the 
ideas which were suggested to East and Hayes’ by their experiments on 
hybrid vigor, and which may be freely interpreted as follows. As the width 
of a cross increases, four characteristic stages are encountered. (1) Very 
narrow crosses commonly bring little more than the redistribution of a few 
Mendelian characters. (2) Wider crosses, involving a considerable number 
of Mendelian factors, in many plants will produce a first hybrid generation 
which is characterized by a certain amount of hybrid vigor, the amount 
increasing with the width of the cross. (3) A point is reached where the fer- 
tility of the hybrid falls off, and at a certain width of cross perfectly sterile 
hybrids may be expected. It is interesting that hybrid vigor commonly 
continues to increase even though fertility decreases. Here loss in “efficiency” 
in the reproductive system is distinctly not accompanied by loss in efficiency in 
vegetative development. This peculiarity is clarified by the following idea. 
Wide crosses involve the fusion of relatively ‘‘inharmonious” gametes, which 
might be expected to produce disturbances in the ontogeny of the resulting 
individual. The grosser mechanism which regulates vegetative development 
can evidently weather such disturbances, while the more finely balanced 
mechanism of gamete formation is upset. (4) Finally, in the widest crosses, 
even vegetative development is impaired, and poorly developed hybrids result, 
sterility is pitires 4 in the os , Sar was proposed by BaBcocK 
and CLAUSEN, ed by East. Two Nicotiana 
species, A and B, each having a haploid chromosome number of 24, are crossed 
3 East, E. M., and Hayes, H. K., Heterozygosis in evolution and in plant breed- 
ing. USS. mide Bur. Plant Ind. Bull. 243. pp. 58. 1912. 
4GoopspeeD, T. H., and Crausen, R. . Mendelian factor differences versus 
Partisan, ae in mene Amer. ‘Nat. = bt Q2-I0I. 1916. 
5 East, E. M.,A study fr ility i y bri Genetics 6:311-365. 
6See Bascock, E. B., and Crausen, R. E., Genetics in relation to agriculture 
New York. 1918 (p. 238). 
