264 



SCIENCE 



[N. S. Vol. XXIX. No. 737 



Experimental Evolution and observations upon 

 the somatic chromosomes of these plants vrere 

 embodied in a report read at the Seventh In- 

 ternational Zoological Congress. 



The peculiar combination of parental vege- 

 tative characters in each of these hybrids, as- 

 sociated vifith a certain fixed number of 

 somatic chromosomes, suggested the desira- 

 bility of repeating the cross upon a much 

 larger scale in hopes of throvcing some light 

 upon the behavior of chromosomes in in- 

 heritance and of determining, if possible, 

 vehether number, size and shape of chromo- 

 somes are regularly associated with the in- 

 heritance of certain definite external char- 

 acters. 



In a previous note 0. gigas was reported to 

 have double the number of chromosomes found 

 in 0. lamarcTciana, with evidence to suggest 

 that the number may vary within a limited 

 range among individuals of the species.' 

 Gates^ later sustained the count of 28 for 

 gigas from observations of the germ-cells. In 

 an earlier paper he reported 14 chromosomes 

 for lata.' The few individuals of this species 

 which I have examined have given 14 and 15 

 chromosomes with considerable, though not 

 conclusive, evidence that a single plant had 

 16.* The number for each individual however 

 remained constant. 



The striking differences in the number of 

 chromosomes of lata and gigas, associated with 

 marked differences in external characters, 

 caused the progeny of a cross between in- 

 dividuals of these two species to become a 

 subject of particular interest. Seeds for the 

 first generation offspring of 0. lata ? X 0. 

 gigas c? were secured from guarded pollina- 



' " A Preliminary Note on tlie Chromosomes of 

 (Enothera lamarckiana and One of its Mutants, 

 0. gigas," Science, N. S., 26: 151-2, August 2, 

 1907. 



' " The Chromosomes of (Enothera," ScnsNCE, 

 N. S., 27 : 193-5, January 31, 1908. 



" " Pollen Development in Hybrids of (Enothera 

 lata X 0. lamarckiana and its Relation to Muta- 

 tion," Bot. Gazette, 43: 81-115, 1907. 



■* " Chromosomes of the Somatic Cells of the 

 (Enothera," Science, N. S., 27: 335, February 28, 

 1908. 



tions through the kindness of Dr. G. H. Shull. 

 The hybrids arising therefrom were the 

 progeny of a single pair of parents; the pistil 

 parent in this case being a mutant which 

 arose from a culture of pure-bred lamarckiana 

 and the pollen parent a pure-bred gigas. 

 Seventy^seven young rosettes were transferred 

 to the experimental garden early in May, fixa- 

 tions having previously been made of the root 

 tips of 50 of this number for the study of 

 somatic chromosomes. 



By sowing seeds in the greenhouse early in 

 February and transplanting rosettes to the 

 garden as soon as danger of frost is past, lata, 

 like lamarckiana and others, may readily he 

 brought to flower the first season. Gigas, on 

 the contrary, is strongly biennial and even 

 when subjected to the same conditions as lata 

 from seed-sowing to maturity only a small 

 percentage can be expected to flower and 

 ripen seeds the first season. Consequently, 

 the fact that 44 of these hybrids were annuals 

 is of considerable apparent significance. It is, 

 however, unsafe to conclude that this is a 

 manifestation of an hereditary character, inas- 

 much as the majority of those remaining in 

 the rosette stage were subjected to slightly 

 different environmental conditions, having 

 been transplanted to the garden 9 days later 

 than the majority of those which proved to be 

 annuals, and transferred to somewhat richer 

 soil. Designating the two plots as east and 

 west gardens, 4Y young rosettes were trans- 

 planted to the former May 1 and 3 on May 10. 

 Of the first lot, 37 came to flower during the 

 season, 3 died and 7 have gone into the winter 

 as rosettes. 



Twenty-seven rosettes were transferred to 

 the west garden May 10, of which 5 came to 

 flower during the season, though very much 

 later than the majority of those of the same 

 class in the east garden. 



The somatic chromosomes of 40 of these 

 hybrids (25 of which were annuals) have 

 been carefully studied and the remaining 10 

 will be included in the final publication to 

 appear shortly. To insure accuracy of 

 chromosome counts, none was considered con- 

 clusive unless sustained by 10 or more per- 

 fectly clear figures distributed through 2 or 



