Resumo em inglês 1. New chromosome numbers have been determined tor the following species from material collected in various counties of North Carolina, U . S . A. Thalictrum clavatum n = 7 Th. dioicum n = 14 Th. polygamum n = 42 Th. coriaceum n = 70 Ilex decidua n = 20 I. monticola n = 20 I. vomitoria n = 20 I. opaca n = 18 I. verticillata n = 11. New chromosome numbers have been determined tor the following species from material collected in various counties of North Carolina, U.S.A. Th (mais) alictrum clavatum n = 7 Th. dioicum n = 14 Th. polygamum n = 42 Th. coriaceum n = 70 Ilex decidua n = 20 I. monticola n = 20 I. vomitoria n = 20 I. opaca n = 18 I. verticillata n = 18 2. The genus Thalictrum has been rather well investigated cytologically but the genus Ilex has not yet received the attention it merits, and offers cytologists in North, Central, and South America an opportunity ro study an extensive genus which has been, and probably is, undergoing interespecific hybridization while a shift from the hermaphroditic to the unisexual habit is also well established. These two great evolutionary factors, hybridization and the advent of the dioecious condition, appear to stimulate divergent lines of meiotic behavior depending upon whether or not they appear in the phylogeny of a race simultaneously or successively, and in the case of the latter, which came first. The genus Thalictrum was used as a basis for comparison with the situation as found in the eastern North American forms of Hex with the following conclusions. a. The genus Thalictrum is vastly polyploid and meiotically regular. This condition is thought to be the result of previous hybridization of the species before the advent of the dioecious habit. b. The genus Ilex is, as far as research has gone, negatively aneuploid and frequently irregular meiotically. This condition is thought to be the result of previous hybridization of the species after , or no more than simultaneously with, the advent oí the dioecious habit. Heterochromosome Complex Division in first seen Choromatin ejected Other diagnostic hybrid characteristics Species Type of Chromosome I.. decidua Medium Homeotypic One choromatid during second anaphase Laggards anaphase bridges I. monticola. Smallest member of complement Heterotypic One chormatid during first anaphase. Left in cytoplasm Lagging Precocious chorm I. opaca Medium Heterotypic Whole chorm. One chromatid sometimes accepted by complement in second div. Lagging chrom I. verticillata Two types. One tripartite X-- X1Y2 small Other medium Heterotypic Yes. Degenerates during first telophase. Yes Degenerates during first telophase. I. vomitoria Medium sized tripartite complex Heterotypic None found A number of laggards dur-first division 3. Heterochromosomes were found in microsporogenesis of all five species of Ilex, along with other meiotic peculiarities of suggested hybrid origin, but no two were alike. Behavior of the heterochromosomes was not found to be consistent and in approximately 5 0% of the PMCs. meiosis was normal. Findings are summarized in following table.8 4. In view of the generally ¡rabie chromosome number for the five species of Ilex investigated, only such pollen as arises from PMCs. which possess the normal meiosis may produce the gametes which effect fertilization. This presents the problem of explaining how the ability to produce meiotic abnormalities is inherited through cells which dcnot exhibit it or through the female side and its significance on the whole question of the evolution of sex chromosomes from various types of heterochromosomes. 5. A comparison of the cytology or species of Thalictrum and Ilex adds support to the author's earlier contention that so-called sex chromosomes in higher plants have their origin in special types of heterochromosomes which in turn owe their appearance to meiotic disturbances induced by previous hybridization of the species and preserved through the more or less simultaneous advent of the dioecious habit. In the author'- opinion, the genus Ilex offers an opportunity to increase our knowledge of what occur? during the stage intermediate between autosome and the specialized types of sex-linked heterochromosomes.