Showing papers by "G. Ledyard Stebbins published in 1938"

Cytological characteristics associated with the different growth habits in the dicotyledons

Abstract: IN REVIEWING the cytology of a number of different families of dicotyledonous angiosperms, the writer noted that although there were no characteristic differences between the chromosomes of the various larger systematic divisions of this subclass, there appeared to be, on the other hand, a definite set of characteristics associated with the different growth habits, particularly the woody plants as opposed to the herbs, and the annual or biennial as distinguished from the perennial species. In order to test this apparent phenomenon, he undertook a systematic review of the chromosomal characteristics of all dicotyledonous genera sufficiently recorded in the literature for this purpose. The results of this survey are recorded in the present paper. MATERIAL AND METHODS.-Three chromosomal characteristics were selected as the easiest to treat in the manner contemplated: (1) the basic chromosome number of a genus, (2) the percentage of polyploid species within a genus, and (3) the absolute size of the chromosomes within the genus. The data for the first two characteristics were obtained chiefly from the lists of Tischler (1931, 1936), while additional information on certain genera was obtained from more recent partial lists and from a few treatments of particular genera. In general, genera included in the classification by basic number and by absolute size are those of which at least five species are known, but unless the known species were fairly uniform in character, they were not admitted to the lists unless a larger number of species was recorded. For classification as to percentage of polyploidy, only those genera were selected of which 30 per cent or more of the species are known cvtologically, but an additional list was compiled including all the genera classified as to basic number. The basic number was considered that which was found, either in itself or in multiples of it, in fourfifths or more of the species of the genus. If a second number occurred in a considerable proportion of the species, the genus was given one-half value under each number, while if there were three, it was given onethird value under each of them. If there were more than 3 numbers in the genus, only the 3 most common ones were recorded. Numbers near to multiples of the lowest basic number were considered to be derived by polyploidy--e.g., if a genus contained the gametic numbers 8 and 17, the latter was considered as a hypertetraploid based on the former. Only those basic haploid numbers actually found in the genus were recorded; the evidence from secondary pairing was disregarded as being not firmly enough established and not recorded in a sufficient number of genera to be of use in a comprehensive survey such