EXPERIMENTAL EMBRYOLOGY 381 



without greatly affecting the length of the pollen tubes, while gua- 

 nine increased the length of the tubes up to 157 per cent over that 

 of the controls without significantly affecting the percentage of 

 germination. Two other substances, paraaminobenzoic acid and 

 acenaphthene, were found to affect both processes. 



Eyster (1941) has recently reported that the self -incompatibility 

 in Petunia, Tagetes, Trifolium repens, and Brassica oleracea can be 

 counteracted by spraying the plants with a solution of a-naphtha- 

 leneacetamide. This chemical is said to "neutralize the effects of 

 an ovarian secretion which diffuses into the style and inhibits or 

 greatly retards the growth of pollen tubes." Lewis (1942), who 

 used it with Prunus avium, found it ineffective in increasing or 

 decreasing the rate of pollen tube growth, but it delayed the forma- 

 tion of an abscission layer at the base of the style and thereby 

 allowed a longer time for the incompatible tubes to reach the ovary. 

 In his opinion, therefore, the use of a-naphthaleneacetamide may 

 not be confined to the counteraction of self -sterility but may also 

 be used to combat interspecific sterility in certain plants. 



That a change in chromosome number can also be of service in 

 the inactivation of incompatibilities has been shown recently by 

 the work of Stout and Chandler (1941) and Stout (1944). As the 

 result of a series of controlled pollinations they found that the po- 

 tentially fertile flowers of Petunia axillaris are entirely self -incom- 

 patible and produce no seeds or capsules on self-pollination. How- 

 ever, the self -pollinated flowers of tetraploid branches on the same 

 plants (produced by colchicine treatment) produced large well-filled 

 capsules. Flowers on the self -incompatible diploid branches also 

 produced capsules when pollinated from flowers of tetraploid 

 branches on the same plant, but tetraploid 9 X diploid cf combina- 

 tions on the same plant failed to yield any seeds. 



Of interest in this connection are also some observations of Buch- 

 holz and Blakeslee (1929), Satina (1944), and Blakeslee (1945) 

 on Datura. When pollen from a 2n plant is applied to the stigma 

 of a 4n plant, fertilization takes place freely and some viable seeds 

 are also produced ; but in the reciprocal cross when pollen is applied 

 from a 4??. plant to a 2n stigma, seed formation is extremely rare. 

 Histological studies have shown that the pollen tubes derived from 

 the 4n male parent burst in the 2n styles and never reach the ovary. 

 It was possible, however, to overcome this difficulty and change 



