A CONSIDERABLE number of organisms exhibit, as well as the normal chromosomes of the diploid chromosome set, B-chromosomes which behave in an irregular manner partly from a morphological point of view, but more often from the physiological. Although it is usually easy to distinguish between the B-and the ordinary or A-chromosomes, there are many difficulties in the way of finding features which are characteristic of all known B-chromosomes.Oestergren (i4) tried to find a universally valid definition by grouping together all known characteristics of B-chromosomes. But he himself points out that no single B-chromosome possesses all the characteristics mentioned.B-chromosomes clearly represent an extraordinary divergent group of chromosome bodies, and at present it is not easy to see how, for example, the big gap between the accessory chromosomes of Godetia viminea (Hâkansson, 1949(Hâkansson, , 1950) and the B-chromosomes of ,ea Mas (Randolph, 1941) can be bridged. Further extensive research is necessary to clarify the origin and function of these interesting chromosomes.The present work is a report on several experiments which were made with the particular object of investigating the distribution of fragment chromosomes of Trillium grandflorum. The results obtained throw some light on their distribution .during the heterotypic division of the EMC. Because of the great difficulties of breeding from Trillium, the investigations into fragment chromosomes were undertaken with hybrid endosperms. Because one of the ordinary chromosomes of T. grandflorum is termed a "B-chromosome ", we are forced to use the expression "fragment" for the supernumerary chromosomes.
MATERIAL AND METHODSThe test-plants, 75 specimens of T. gran4florum and 3 of T. luteum were taken from three nurseries-A. Vogt, Erlenbach; C. Frickart Söhne, Stafa/ZUrich (both classified G) and B. Ruys, Dedemsvaart/Holland (GH). The plants from all three sources proved to be highly self-sterile.To obtain information about the heterochromatin content, all plants were exposed to a temperature of o3° tO i .00 C. for four days. This was before fixation.five or six weeks after pollination. My experiments show that this treatment is sufficient to obtain maximum starvation effect with normal chromosomes of root tip and endosperm mitosis.After the exposure to cold the seeds are taken out of the fruit, fixed in Carnoy and then transferred to 8o or 90 per cent. alcohol. In this fluid, the seeds can be '95
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