416 TJniversity of California Publications i7i Agricultural Sciences [Vol.2 



be in each of these nine nuclei ; five of them ought to have 1 chromo- 

 some each, one would have 2 chromosomes and three ought to have 

 3 chromosomes each. Figure 52 shows an unquestionable diad and 

 figures 53 and 54, triads, figure 54 having 4 nuclei but only 3 cells; 

 figure 51 shows a tetrad with cells unequal in size. The amount of 

 fertility is about the same in this hybrid as in the preceding one, 

 according to Haney. Table 5 gives an analysis of 13 pollen mother 

 cells in homotypic anaphase by direct count of chromo.some number. 

 Actually a regular distribution of the chromosomes seems to be fairly 

 common. Table 6 shows that 66.6 per cent of the nuclei observed in 

 these 13 pollen mother cells have 4 chromosomes. One fact must 

 here be borne in mind, namely, that pollen mother cells with a regular 

 distribution of the chromosomes will show up best and are more 

 likely to be countable than those with fairly irregular distribution of 

 chromosomes. If no crossing over took place and if 66.6 per cent 

 of the gametes have 4 chromosomes, then 8.3 per cent of the gametes 

 ought to have either a pure taraxacifoHa or a pure tectoruni group 

 of chromo.somes. The plants exhibited exceedingly low fertility but 

 this may have been partly caused by the unfavorable season at which 

 the}' reached maturity. 



In this hybrid no chromosomes can be distinguished during the 

 meiotic divisions as belonging to either of the parental species. They 

 are all of practically the same length. 



3. Crepis aspera X aculeata. — This hybrid is conspicuously diifer- 

 ent from the first two as regards the number of irregularities during 

 meiosis, being much more regular. As in taraxacifoJia X tectonitii, 

 the chromosomes of the two parental species cannot be distinguished 

 during meiosis. Four bivalent chromosomes are most commonly 

 observed iu the heterotypic metaphase ; only 38.9 per cent of the 

 pollen mother cells show irregularities during this phase; and only 

 4.1 per cent of the pollen mother cells have chromosomes detached 

 through meiosis I (table 1, page 412). From table 2 it will be seen 

 that only 11.6 per cent of the pollen mother cells in the tetrad phase 

 have irregularities. The hybrid is fairly fertile; it is estimated that 

 35 to 40 per cent of the florets set seed. 



Plate 61, figures 55-58, shows some of the division figures, mainly 

 of the relatively few irregular ones. Figure 57 shows a heterotypic 

 anaphase with the distribution of 4 chromosomes to one pole, 3 to 

 the other, and 1 chromosome between the plates, splitting. Figure 58 

 is a tetrad with a micronucleus in each of two of the cells. 



