414 University of California Puhlications tn Agricultural Sciences [Vol. 2 



These counts are all from fixed and sectioned material. The deviation 

 from the 4/4 distribution in several cases is caused by a detachment 

 of one or two chromosomes (pi. 60, figs. 31, 32). 



But the homotypic division and the following "tetrad" formation 

 cause many irregularities as plate 60, figures 35-41, shows. Table 2 

 tells something about the nature of these irregularities. Usually the 

 tetrad consists of 4 cells, but these may contain more than one nucleus. 

 In such eases the extra nucleus or the extra nuclei are very small, 

 apparently consisting of only one chromosome (pi. 60, figs. 37, 39). 

 Sometimes these 4 cells are of very unequal size as figure 40 shows, 

 the tetrad containing 4 cells but 8 nuclei. A pentad is shown in 

 figure 38 and a triad with one large and 2 smaller nuclei in figure 41. 

 Two pollen mother cells with a tendency to form diads are shown in 

 figures 35 and 36. They have numerous nuclei. The "tetrad" stage 

 as a whole gives a very irregular impression. It seems as if the walls 

 circumscribing the cells that later become pollen cells are formed 

 regardless of the nuclei, including 1, 2, 3, or perhaps 4 nuclei as the 

 chance may be. In this w'ay triads and diads may be formed. Prob- 

 ably most of the micronuclei degenerate. 



There is an increase in amount of irregularity from the end of 

 meiosis I, giving 31.4 per cent of pollen mother cells that have one 

 or more chromosomes detached, to the end of meiosis II where 54.1 

 per cent of irregularities of difi^erent kinds are found (tables 1 and 2). 

 To this percentage must be added the cases in which the irregularity 

 does not manifest itself in the form of extra nuclei or in other ways 

 but consists only in an unequal distribution of the chromosomes 

 to four daughter nuclei as for instance 5 -|- 5 -|- 3 -|- 3 instead of 

 4 + 4 + 44-4. 



It is hard to account for sterility in such a case as this. Even if 

 irregularities are very common, still about 46 per cent of the pollen 

 mother cells consist of only 4 nuclei, and a fairly large percentage 

 of these ought to have 4 chromosomes. And also some of the irregular 

 pollen mother cells ought to give gametes with 4 chromosomes. If 

 only gametes containing the same sets of chromosomes as the pure 

 parental species are viable, then the sterility can be accounted for 

 satisfactorily. Out of 16 gametes with 4 chromosomes one gamete 

 ought to give a pure set of aspera and one a pure set of hursifolia 

 chromosomes. Each floret has only one ovule and if the irregularities 

 in the formation of the eggs are simihir to those for formation of 

 pollen there should be expected one ovule with a pure set of aspera 



