ALBERT F. BLAKESLEE 63 



times. There are a series of problems the plants have to solve or barriers 

 that have to be overcome before a hybrid between species is possible, 

 assuming pollination has taken place. The first problem is that of ger- 

 mination on the stigma of the female parent. Pollen germination may 

 be induced by manipulation of the osmotic pressure and the chemical 

 constituents of artificial media. The function of a pollen grain is by 

 means of the pollen tube to carry the two sperm cells down the style 

 and discharge them into the ovule. As Buchholz had shown, in many 

 species' combinations the pollen tubes burst in the foreign style and are 

 put out of commission before they reach the ovary. In one case we 

 were able to overcome this bursting. One of our students, Miss Carmen 

 Sanz, found that most of the tubes burst when pollen of tomato was 

 used on diploid styles of Datura stramonium but that most of them grew 

 without bursting when used on tetraploid styles. Some of our students 

 have had a measure of success in getting slow-growing pollen tubes 

 down to the ovary by shortening and splicing the styles. By these two 

 methods, therefore, the barriers of slow-growing and bursting pollen 

 tubes have been partially overcome. It is surprising how many species 

 are able to overcome the earliest barriers to crossability. Miss Sanz 

 tested the pollen of over 60 species on the pistil of Datura stramonium. 

 She found that in nearly half the cases the pollen germinated. It was 

 especially surprising that several of the monocots such as Freesia and 

 Tulip showed relatively good germination though their pollen tubes 

 grew only a short distance in the stramonium styles. What is it that slows 

 up the growth of pollen tubes in such wide pollinations or causes their 

 tubes to burst? Perhaps the difficulty is related to the cause of bursting 

 of tubes from a ^n parent in the in style of a female, the cause of which 

 is still unexplained. 



A special study has been made of crossability in the 90 combinations 

 among the 10 species oi Datura. We had anticipated that failure of egg 

 and sperm to unite might be an important barrier to crossability. There 

 is no evidence that this is the case in Datura. Apparently whenever the 

 egg and sperm are brought into contact they fuse. Barriers come later 

 which may prevent their further development. In some hybrid com- 

 binations no division of the fusion cell takes place. In other combinations 

 cessation of growth occurs after 6 or 8 cells at most are formed. In slill 

 other combinations young hybrid embryos of various stages of develop- 

 ment are produced. In only 19 species' combinations were viable seeds 



