FERTILIZATION 193 



matings, on the other hand, the tubes grow very slowly, if at all, 

 and the flower withers away before they reach the embryo sac. 



In Brassica pekinensis, which has been thoroughly studied by 

 Stout (1931), the pollen tubes grow rapidly in cross-pollinated 

 flowers. Self-pollinated flowers, on the other hand, exhibit the 

 following incompatibility reactions: (1) low percentage of germina- 

 tion of pollen on "own" stigma, (2) coiling of pollen tubes on the 

 stigmatic papillae, (3) feeble or limited growth of the tubes through 

 the style, and (4) coiling of tips of the tubes in the ovary or ovules. 

 In Petunia violacea (Yasuda, 1930) also the pollen tubes grow 

 rapidly and reach the base of the pistil in about 36 hours after cross- 

 pollination. In self-pollinated flowers, on the other hand, not only 

 is the initial growth rate much lower, but also it continues to decrease 

 and the tubes reach only about one-fifth of the length of the style, 

 forming irregular swellings at their tips. In sugar solutions to which 

 an extract of "own" stigma is added, the growth of the tubes is also 

 extremely slow, but on the addition of extract from a different strain 

 of the species the tubes grow normally. Anderson and Sax (1934) 

 report that in Tradescantia the growth of the incompatible pollen 

 tubes is much slower and the generative cell does not enter the tube 

 even after 24 hours, while in compatible matings this takes place 

 in only 40 minutes. In Linaria reticulata (Sears, 1937) compatible 

 tubes reach the base of the style in less than 25 hours, while in- 

 compatible tubes grow only about one-fourth of the distance even 

 in four days' time. In Nemesia strumosa (Sears, 1937) incompatible 

 tubes grow at approximately the same rate as compatible ones 

 through the first three-fourths or four-fifths of the style, but slow 

 down rather suddenly and finally come to a stop at the base of the 

 style. In Trifolium repens (Atwood, 1941) incompatible matings 

 are characterized by two interference zones, one on the stigma and 

 the other in the style. Germination of pollen is poor, and the few 

 pollen tubes which happen to be formed seldom travel more than 

 three-fourths of the way down the style. Those which do proceed 

 further grow so slowly that the flowers wither and fall off before 

 fertilization can take place. 



An effect more or less similar to the above is seen in the so-called 

 "illegitimate pollinations" between plants showing heterostyly. 

 Working on Fagopyrum esculentum, Stevens (1912) found that in 

 legitimate pollinations, fertilization takes place in 18 hours, but in 



