196 Varietäten, Descendenz, Hybriden. 



are the relations of chromosomes to (internal and external) somatic 

 characters? 2. What are the relations of normal chromosome beha- 

 vior to the transmission of characters? 3, What are the relations of 

 peculiar or unusual chromosome behavior to the transmission of 

 characters? Answering the second of these questions, the writer 

 submits this striking thesis: "The maximum possible difficulty in 

 the improvement of animals and plants by hybridization usually 

 depends directly upon the chromosome number." It was his Obser- 

 vation of the extreme difificulty in the experiments with cotton 

 (chromosomenumber ln=3 10, 2n = 56) and tobacco (n = 24, 2n = 48) 

 as compared with corn (n=ilO, 2n = 20) and wheat (n =:8, 2n = 16) 

 that led to this theory of the cause. M. J. Sirks (Bunnik). 



East, E. M., The phenomenon of sei f- st er ility. (American 



Naturalist. IL. p. 76—86. 1915.) 

 East, E. M., The phenomenon of self-sterili ty. A correc- 



tion. (Am. Nat. IL. p. 712. 1915.) 



A cross made by the writer between a small red-flowered Nico- 

 tiana forgetiana (Hort.) Sand, and the large white-flowered N. alata 

 Lk. and Otto var grandiflora Comes led to a F^, all of the plants 

 of which appeared to be self-sterile, as could be expected, both the 

 parents being self-sterile. Several experiments were made in which 

 crossing and seifing was done on a large scale, using plants of the 

 F2, F3 and F4-generations. Between 20 plants of the F.2-generation 

 131 intercrosses were made; the results were: 1. Fach plant was 

 absolutely self-sterile. 2. Leaving out of consideration one plant 

 with shrunken imperfect pollen, only two crosses failed. 3, Of the 

 129 successful intercrosses, 4 produced capsules with less than 50 

 percent of the ovules fertilized, the remaining crosses produced füll 

 capsules. Other crossing experiments (120, 100, 58, 85 in number 

 from which 3, 6, 5 and 5 failed) corroborated these results. Self- 

 fertile plants were never obtained. The difference between the 

 development of the tubes in the selfed and the crossed styles is 

 whoUy one of rate in growth. The tubes in the selfed pistils develop 

 steadily at a rate of growth of about 3 milUmeters per twenty-four 

 hours. Since the maximum life of the flower is about 11 days, 

 hower, the tubes in selfed pistils never traverse over one half of 

 the distance to the ovary. 



From these facts it seems reasonable to conclude that the secre- 

 tions in the style offer a Stimulus to pollen tubes from other plants 

 rather than an impediment to the development of tubes from the 

 same plant. 



The author proposes as Solution the assumption that different 

 hereditary complexes stimulate pollen tube growth and in all like- 

 lihood promote fertilization , and that like hereditary complexes are 

 without effect. Gametes forthcoming from a certain plant are all 

 parts of the mother cell and contain nothing that that cell did not 

 contain. These gametic cells can not reach the ovaries of flowers 

 on the same plant because they can not provoke the secretion of 

 direct stimulant from the somatic cells of that plant. All gam.etes 

 having in their hereditary Constitution something different from 

 that of the cells of a mother plant, however, can provoke the proper 

 secretion to stimulate poUentube growth, and reach the ovary be- 

 fore the flower wilts and produce seeds. It must be granted, says 

 the writer, that this hypothesis satisfies the facts; it is admittedly 



