94 R. A. BRINK 



intrinsic vigor of the embryo. This statement is not intended to imply that 

 the two phenomena are unrelated, but rather that they vary independently 

 of each other to a significant degree. Many interspecific matings, for example, 

 yield poorly developed seeds. Often the embryos in these seeds give rise to 

 relatively weak plants. Sometimes, however, the embryos within such seeds 

 are capable of forming plants of great vegetative vigor. In other words, the 

 fact that development of the seed is impaired, even to a degree that calls for 

 special methods of propagation, does not necessarily mean that the embryo is 

 intrinsically weak. The hybrid during the seed stage may merely be the 

 victim of a faulty endosperm. Only when released from this stricture can the 

 inherent potentialities of the new individual be expressed. 



Two examples of such intrinsically vigorous hybrids in which the condi- 

 tions of seed development have been explored will be briefly mentioned. They 

 differ in the grade of seed development attained. Small but nevertheless 

 germinable seeds are formed in the one case, whereas in the other the embryo 

 egularly dies unless special precautions are taken to save it. 



Cooper and I found that when the diploid (2» = 24) Red Currant tomato, 

 Lycopersicon pimpinelUfolium, is pollinated with a particular strain of L. 

 peruvianium., likewise a diploid, fertilization occurs with high frequency but 

 all the seeds collapse before the fruit is ripe. Seed development follows a 

 familiar pattern. The endosperm grows less vigorously than in normal L. 

 pimpinelUfolium seeds, and the endothelium enclosing it tends to become 

 hyperplastic. Endosperm cells become highly vacuolate and starved in ap- 

 pearance. Densely staining granules of unknown composition accumulate in 

 the chalazal region just outside the endosperm, suggesting that the latter 

 tissue is incapable of absorbing the available supply of nutrients. All the 

 seeds in the ripe fruit are shrivelled and incapable of germination. 



Following the application of pollen from the same diploid strain of L. 

 peruvianium to a tetraploid (2w = 48) race of L. pimpinelUfolium, about 

 one-half the fertile ovules develop into small but germinable seeds containing 

 triploid embryos. The other seeds collapse at various stages of growth. Histo- 

 logical examination of the 4n L. pimpinelUfolium X 2n L. peruvianium seeds 

 shows retarded embryo development and a less rapid endosperm growth 

 than occurs in the normally pollinated tetraploid parent. The endosperm in 

 sixteen-day-old hybrid seeds lacks the rather densely packed starch reserves 

 characteristic of tomato seeds at this stage. The peripheral layers of endo- 

 sperm cells adjacent to the endothelium break down. An unusually large 

 cavity is formed in the interior of the tissue as a result of digestion of the 

 cells by the slowly differentiating embryo. Endosperm function is markedly 

 impaired in this cross, but in many seeds remains somewhat above the 

 threshold at which complete failure occurs. 



The triploid plants resulting from germinable 4» L. pimpinelUfolium X 

 2w L. peruvianium seeds are extraordinarily vigorous. Although partially 



