Self-Sterility 273 



tion, the pollen tubes grow at a continually accelerated rate. In 

 a sterile mating, on the other hand, this acceleration does not 

 occur. The pollen tubes in both types of mating grow at the 

 same rate during the earlier stages of growth, and the subsequent 

 failure of acceleration in an incompatible combination prevents 

 the pollen tubes from reaching the ovules before the flowers 

 wither and drop off. 



The genetic basis for the different behavior of the pollen tubes 

 was worked out by East and Mangelsdorf for the genus Kico- 

 tiana but is similar in most other plants. Let us suppose that 

 a plant has two different alleles for self -sterility, which can 

 be designated s^ and s^. (The original notation of East and 

 Mangelsdorf used subscripts, as Si and So. Since multiple al- 

 leles are generally denoted by superscripts, this notation should 

 be observed for the self-sterility alleles.) These oppositional 

 genes behave in such a way that if the tissues of the style contain 

 the s^ gene, pollen tubes containing the s^ gene will not be ac- 

 celerated in their growth through the style. Similarly, s^ pollen 

 tubes will not be accelerated if an s^ gene is present in the female 

 plant. Therefore, if an s'^s^ plant is self-fertilized, neither kind 

 of pollen tube will be accelerated and no fertilization will occur. 

 The same results will be obtained if two s^s^ plants are mated. 



In some plants, a third allele, s^, may be found. If an s^s^ 

 plant is crossed with one whose genetic constitution is s'^s^, the 

 s^ pollen will not be stimulated as before, but the s^ pollen will 

 grow at an accelerated rate since there is no s^ gene in the female 

 plant to '^oppose" it. The cross s'^s^ X s'^s^ will be fertile. In 

 the reciprocal cross, s^s^ X -s^s", the s- pollen will grow at an 

 accelerated rate because no s^ gene occurs in the female plant, 

 and fertility again will be the result. In a similar manner, plants 

 of the genotype sV will be fertile reciprocally with both s^6- 

 and sV plants. If a fourth allele is present, plants of the con- 

 stitution s^s*, sh"^, and sV might be found. They would all be 

 fertile together reciprocally and would be fertile with all the 

 three types previously mentioned. It will be noted that no nor- 

 mal diploid plant has more than two members of the series of 

 alleles. Since an s^ gene in the style prevents the s^ gene in the 

 pollen from fertilizing *the eggs, homozygous plants, such as s'^s''-, 

 are not normally produced. In some plants, however, includ- 



