Cytological Studies of Triploid Progenies of Populus Tremula

WO years ago the writer submitted some data concerning the T occurrence of triploid races of Populus tremula, crosses between diploid and triploid races being also dealt with (JOHNSSON, 1940).It was then shown that nine different clones of triploid aspen are known in Sweden.Both male and female clones occur.The chromosome number of P. tremula is x = 19.Meiosis in the triploids *shows at IM trivalents, bivalents and univalents and gives rise to gametes with chromosome numbers varying between 19 and 38.Further, unreduced gametes with n = 5 7 are formed.Crosses between diploid and triploid races are noteworthy in that individuals with aneuploid chromosome numbers are realized in high frequency, in which respect Populus tremula differs markedly from nearly all species in which 2x-3x crosses have been investigated.Crosses in both directions gave the same results, thus showing that the pollen grains are not more susceptible to numerically unbalanced chromosome numbers than the embryo-sacs.These studies have, however, been extended to embrace also crosses triploid X triploid, the results of which are submitted as a supplement to the earlier account of crosses diploid X triploid. Diploid X triploid crosses.-As there is now available a far larger material of the combination diploid X triploid than that published in