276 University of California Publications in Botany [Vol. 5 



investigation of the response of the hybrid and parental pollen to 

 various natural and artificial germinating tluids; and (c) a nutrition 

 experiment carried on in the hope that the formation of the absciss- 

 layer might be inhibited or retarded to favor the production of a 

 larger proportion of normal pollen. 



So far as possible we have attempted to attack simultaneously all 

 these four main aspects of the general problem. The present paper 

 aims to report such progress as has been made with reference to certain 

 of the points mentioned in (4) above. The various points of attack 

 enumerated above are by no means assumed to be the only ones or 

 the only important ones ; they simply represent those which at present 

 our facilities and the time at our disposal will permit us to undertake. 



II. Response of the Hybrids and Parental Pollen to Natural 

 AND Artificial Germinating Fluids 



As mentioned above, the pollen of the F^ species hybrids contains 

 a few grains normal in appearance among a great mass of abnormal, 

 evidently functionless grains. We have made hundreds of attempts 

 to secure self-fertilized seed. Protected flowers in numbers or trimmed 

 to a single bud have been allowed to self-pollinate. Great numbers 

 of protected flowers have by hand been close-pollinated or pollinated 

 from other flowers on the same plant or from protected flowers o]i 

 other plants in the same row. Pollen has been collected and put on 

 the stigmas in great amount, either as a single application or in 

 successive applications. A variety of experiments in the field have 

 been attempted in the effort to vary the moisture conditions and the 

 temperature surrounding individual flowers at and immediately 

 following pollination. Not a single seed has resulted from any of 

 these efforts and rarely has a shrunken, empty seed-capsule matured. 



As has been said, only a few ovules are capable of fertilization 

 and the production of viable seeds. In spite of the numerous efforts 

 mentioned above we have been unable to cause the few apparently 

 normal i^ollen-grains to fertilize them. This was assumed to be due 

 either to the slight probability that the pollen tube of a chance pollen- 

 grain would come in contact with one of the few normally matured 

 o\ailes, or to the fact that such pollen-grains were unable to germinate 

 in their own stigmatic fluid, or lastly to structural or physiological 

 blocks to the proper penetration of the stylar tissue by the pollen 

 tube. The presumption stood in favor of one of the latter two alterna- 



