94 MAINE AGRICULTURAL, EXPERIMENT STATION. I904. 



The germination of golden custard pollen on pistils of crook- 

 neck, is not a rapid process. On an average the tubes have to 

 pass through one and three-eighths inches of tissue, and it gen- 

 erally takes about 8 days for the generative nucleus to reach the 

 embryo-sac. In other words the tubes descend at the rate of 

 four and one-fourth millimeters per day. Germination of pollen 

 begins very soon after the deposit of the grains on the stigma, for 

 at the end of 24 hours many of the tubes have attained a length 

 equal to twice and in some cases three times the diameter of the 

 grain. Before the end of the second day the vegetative nucleus 

 has passed into the tube, and soon after this the generative 

 nucleus has also started on its downward course. During the 

 two following days the tubes continue to descend, and often- 

 times they branch to some extent. From this time on nothing 

 important happens till the male protoplast reaches the ovarian 

 region and comes in the vicinity of the micropyle. This occurs 

 about the sixth day, or sometimes between the fifth and seventh 

 days. Before the end of the eighth day the generative nucleus 

 has passed the vegetative, enters the embryo-sac, and finds a 

 resting place near the germ cell. This seems to be the final 

 action of the pollen spore, for tissue nearly ten days old shows 

 the same condition as that which has been pollinated eight days. 

 This seems to give a key to the situation. Pollen grains germi- 

 nate, pollen tubes descend, the pollen spore enters the embryo- 

 sac; but the nuclei do not fuse. Fertilization does not occur. 

 The reason for failure is, apparently, a refusal of the embryo of 

 crookneck to be impregnated by pollen spores of the other vari- 

 ety. 



It now becomes evident that where there is no incompatability 

 in the structure of the flowers, where no mechanical difficulties 

 exist, and when the pollen tubes descend and enter the ovarian 

 region, the reason that certain reciprocal crosses cannot be made, 

 is non-fusion of male and female nuclei. The elements do not 

 come together, although the protoplast of the pollen grain 

 arrives, that is, in many cases it enters the embryo-sac. 



In some cases the nuclei are some distance apart in the 

 embryo-sac while in others the distance which separates them is 

 only twice their diameter. Such cases are quite common. Now, 

 with these facts before us, we are confronted with the question 



