Pollen Development in Hybrids of Oenothera lata×O. Lamarckiana, and Its Relation to Mutation

Failure of pollen development in O. lata is not due to ingrowth of the tapetum to fill the loculus, as described by POHL, but to some other agency at work in the hybrid, the nature of which is not fully explained. 2. Pollen development in O. lata may proceed to the formation of tetrads, but degeneration of both mother cells and tapetum frequently begins in the resting stage or during the prophase of the first mitosis. 3. Heterochromosomes arise in O. lata in the prophase after synapsis. There may be one or two such bodies, which are formed as large rings by the cutting off of a portion of the spirem thread before the remainder breaks up into chromosomes. These bodies later are found in metaphase of the heterotypic mitosis, usually in the cytoplasm near the spindle. They do not divide, but become smaller and probably disappear at the end of the first mitosis. These bodies are also found in the O. Lamarckiana hybrid, in which they doubtless have the same origin. They probably represent discarded chromosomes, and this is perhaps a means of lessening the number of chromosomes in certain germ cells of the species. Some mother cells do not contain them. In such cells the (sporophyte) count of chromosomes in O. lata is fourteen and in the O. Lamarckiana hybrid probably twenty. 4. The occurrence of extra nuclei formed by chromosomes left behind in the cytoplasm in the tetrad divisions of the hybrid O. lata, as well as in a number of other hybrids recently described, puts under suspicion the purity of any other plant, such as Hemerocallis fulva, in which such extra nuclei are known to occur. 5. The inference seems justifiable that the mutations of O. Lamarckiana arise during the reduction divisions and that pollen grains which will give rise to mutants differ in their potentialities and probably also in chromatin morphology from the ordinary pollen grains of the plant. These facts lead to the conclusion that a cytological basis may be found for the phenomena of mutation as observed in the Oenotheras.