Forms Having an Extra Chromosome. 33 



The determinations of chromosome numbers from root tips by Miss 

 Lutz are therefore probably correct, although it is certainly 

 desirable that they should be confirmed by study of the germ cells 

 Although Hance regards the diminutive extra chromosomes of 

 Miss Lutz in (E. rubrinervis De Vries and (E. mut. aberrans as 

 mere temporary fragments, yet the evidence seems to indicate 

 that they are permanent products of a fragmented meiotic 

 chromosome dividing by mitosis, and therefore present in every 

 nucleus. Their constant difference in size in the two forms also 

 supports this view. 



Summing up the results regarding 15-chromosome forms of 

 (Enothera, we may conclude that while several of the types have 

 probably arisen from lamarckiana through having different 

 chromosomes as the extra one, yet several others, which have 

 appeared only in the offspring of lata x lamarckiana or of lata 

 selfed, evidently have a closer relationship to lata than to any 

 other form, and semilata Gates at least probably has the same 

 extra chromosome. The differences between the other forms may 

 be accounted for partly by the presence of other factors or mutant 

 characters in addition to the extra chromosome, and partly by 

 fresh rearrangements of the 15 chromosomes. 



The same irregular meiotic division, or non-disjunction of a 

 pair of chromosomes, also takes place no doubt in other forms, and 

 may be expected to lead to the appearance of occasional aberrant 

 members in other species. Winge (1917) apparently found such 

 a case in Staphylea pinnata, which has 12 pairs of chromosomes, but 

 one haploid group with 13 was found in pollen formation. Also in 

 Cornus glabrata, anaphase groups of 11 and 12 were observed. 

 Nawaschin (1911) found in the meiotic divisions, in the pollen 

 mother cells of Tradescantia virginica that one pair of chromosomes 

 in the heterotype division is left near the equator, where it forms a 

 vacuole which later becomes included in one of the daughter 

 cells. He finds it dividing later, so that two cells of a tetrad 

 usually receive 11 chromosomes and two 11 -f this body, which is 

 called a chromatin nucleolus or heterochromosome. But the brief 

 account leaves many points undetermined. In the pollen formation 

 of Callitriche verna, Winge (1917) has described somewhat 

 similar conditions. The diploid number of chromosomes is 16, and 

 in the heterotype division 7 pairs are usually distributed regularly, 

 while the other two frequently pass to positions in line with the 

 equatorial plate but far from the spindle, where they form nuclei. 



c 



