30 Mutations and Evolution. 



about this conclusion if control counts of chromosomes had been 

 made in the germ cells. 



At least two of these reputed 15-chromosome forms, rubrinervis 

 de Vries and albida, " breed true." Miss Lutz points out that this 

 can only take place (1) by apogamy, of which there is practically no 

 evidence, or (2) when two gametes having different chromosome 

 numbers only (one odd and one even) unite to produce viable seeds. 

 Moreover, it is possible as regards rubrinervis that this individual 

 derived from lata X lamarckiana and having a small extra 

 chromosome (Table I) may differ in some respects as yet unobserved 

 from other rubrinervis mutants derived from lamarckiana direct and 

 possibly not having the extra fragment. More critical study of 

 albida may lead to a similar result. 



A number of other mutants recently described, such as cana t 

 pallescens, lactuca, and liquida from (E. lamarckiana (De Vries 

 1916b), which split in their offspring and are closely related to each 

 other in several features, may contain an extra chromosome. The 

 same may be true of (E. stenomeres mut. lasiopetala (Bartlett 1915c). 

 These forms will be considered later. 



One of the most interesting results obtained by Miss Lutz 

 (1916), is the existence of two types with a small extra chro- 

 mosome a fragment. In rubrinervis (one plant) 125 mitotic 

 figures showed the small chromosome, and in 52 of them all 

 the chromosomes, 14 +1 , could be counted. The small chromosome 

 is constantly larger than in the new type aberrans, of which there 

 were two individuals. In one plant 30 figures and in the other 

 8 figures all showed 14 +1 chromosomes. 



In a former paper (Gates and Thomas, 1914) it was shown that 

 a variety of irregularities occur in the meiotic divisions in pollen 

 formation of (E. lata, semilata and other forms which are more or 

 less sterile in pollen. This included cases of fragmentation or 

 pulling apart of chromosomes, particularly the odd one, on the 

 heterotypic spindle. It might be expected that such fragments 

 would occasionally be included in the daughter nuclei, and if they 

 afterwards persisted they might be perpetuated by the mitotic 

 mechanism. This is apparently what has happened in rubrinervis 

 de Vries and aberrans, and it is significant that these three 

 individuals all appeared in the offspring of lata x lamarckiana. 

 The writer does not agree with Hance (1918) that they probably 

 represent merely a temporary fragmentation of certain somatic 

 chromosomes. Obviously if new forms can arise having gained a 



