248 



The Jovirnal of Heredity 



DEGENERATION OF GENERATIVE NUCLEUS 



(a) Pollen grain of Brighton, in cross-section, showing 

 commencement of degeneration in the generative 

 nucleus (at the left), (b) Grain of same variety, 

 in which the generative nucleus has wholly degen- 

 erated, and the vegetative nucleus is still functional, 

 (c) Normal pollen grain of the wild grape, Vitis 

 Vulpina, shown for contrast. Both nuclei here 

 are still functional. Photomicrographs. (Fig. 5.) 



liberation of the microspores from the 

 common mother cell. It is first notice- 

 able during the early growth period 

 of the free microspore and shows various 

 degrees of arrested growth combined 

 with loss of cytoplasm. 



This aborted pollen, however, docs 

 not seem to be important from the 

 standpoint of the setting of the fruit, 

 since there is an abundance of pollen 

 produced and even in the instance cited, 

 where 69% of the pollen produced was 

 of the aborted type, still there would be 

 enough normal pollen, if potent, to 

 insure a good setting of fruit. 



The Degeneration of the Nuclei: In 

 the mature ]jollen grain we have two 

 nuclei, the generative and the vegeta- 

 tive. The generative nucleus, as has 

 been stated, divides again to form the 

 gametes, and the vegetative nucleus 

 functions in the growth of the pollen 

 tube. These two nuclei are shown in 

 figure 4, the smaller one being the 

 generative nucleus. A careful study 

 of the i^ollen, ]jroduccd by those varie- 

 ties which bagging tests have shown to 

 be more or less self-sterile, shows that 

 the generative nucleus, and in some 

 cases, also the vegetative, degenerate, 

 as shown in figures 4, 5, and 6. Such 

 degeneration i)recludes the ]KJSsibJlity 

 of normal functioning in any pollen 

 where it occurs. These studies show 

 that degeneration in both nuclei occurs 

 in a large i)er cent, of the pollen in 

 Brighton, while in other varieties which 

 are self fertile, normal jjollen (fig. 5c) 

 is produced. Sterile pollen in the 



grape, then, is due to degeneration in 

 the generative nucleus. 



The Germ Pores: Another fact which 

 is interesting in this connection is that 

 the germ pores (Fig. 3) are not formed 

 in pollen borne by the reflexed type of 

 stamen. There is an interesting correla- 

 tion, then, in the absence of the germ 

 pore, sterile pollen, the reflexed ty]X> of 

 stamen, and the tendency toward dioeci- 

 ousness. 



The general question of sterility 

 in plants at the present time is being 

 investigated from a number of stand- 

 points. It has been known for a long 

 time that a good many hyl^rids are 

 sterile. Some workers, particularly Jef- 

 fries, have emphasized the relation 

 between aborted pollen and hybridity. 

 A number of cases arc recorded where 

 an unequal number of chromosomes are 

 brought into the zygote from each 

 parent. This is not the case, however, 

 in such varieties as Brighton and Barry, 

 which, like Concord and \'. vulpina, 

 have twenty chromosomes in the re- 

 duced number. From the heredity 

 standpoint some workers hold that 

 sterility results from the presence of a 

 factor for sterility. The i)hysiology 

 of jjollen growth and fertilization is now 

 being inx'cstigated and it is probable 

 that some interesting things will be 

 discovered from this standi)oint. All 

 told, the influences leading to sterility 

 are not well understood and we shall 

 have to reserve judgment on a number 

 of points until further work has Ix^en 

 done. 



