Smaller Cotyledons Research Articles

ECOTYPE DIFFERENTIATION IN TEUCRIUM SCORODONIA WITH RESPECT TO SUSCEPTIBILITY TO LIME‐INDUCED CHLOROSIS AND TO SHADE FACTORS

SummaryA study has been made of populations of Teucrium scorodonia from Britain and Europe. Edaphic ecotypes have been found to occur with respect to lime‐induced chlorosis. Chlorosis‐susceptible populations were found to be from acidic and non‐calcareous sites whilst chlorosis‐resistant populations were invariably from calcareous sites. The severity of the chlorosis and the retardation of growth in the laboratory suggests that in the field chlorosis‐susceptible populations would be at a great disadvantage compared with resistant ones, on many calcareous soils. In limited field‐experiments, however, seedling failure was found to be due to catastrophic effects such as drought, frost and animal depredation which eliminated both green and chlorotic seedlings. It was found, however, that seedlings of populations from shaded habitats had considerably larger cotyledons than had those from unshaded habitats. This was also true under controlled conditions, irrespective of the shade and nutritional regime given. The seedlings with large cotyledons formed their leaves more rapidly than did those with small cotyledons. Other morphological differences occurred which also appear to be of an adaptational nature. These differences were independent of the soil types from which the populations originated. A study of populations from within a very small area indicated that ecotypic differentiation can take place rapidly and precisely within such an area.

Observações Citológicas em coffea: VI — Desenvolvimento do embrião e do endosferma em Coffea Arabica l.

The ovule of C. arabica L. consists of a single integument and a small nucellus which disappears as the ovule matures. Three of the four macrospores resulting from the'division of the macrosporocyte, degenerate. The remaining chalazal cell gives rise to a normal'' embryo sac, which is ready for fertilization at the time of the flower opening. Double fertilization occurs, as a rule, the day the flower opens. The embryo sac then increases in volume and compresses the inner integument cells. The outer cells of the integument, however, multiply actively, giving rise to the perisperm. After degeneration of the synergids and antipodals, the zygote stays near the micro-pyle in a resting stage, while the primary endosperm nucleus divides. This first division of the endosperm occurs from 21 to 27 days after flower opening. The cytoplasm condenses around the newly formed nuclei, permitting the adjacent tissues to sink into the embryo sac. Since the separating walls were not seen at the binueleate stage and were present at the four-nucleate stage, it seems that the endosperm belongs to the' nuclear type. As the number of endosperm cells increases, the perisperm cells are again compressed and give more and more room to the new tissue. The first division in the zygote occurs from sixty to seventy days after flower opening, when the endosperm is already multinucleate. A differentiated embryo develops, with a hypocotyl and two small cotyledons in the ripe seed. In the ripe seed the perisperm disappears almost completely: its remains form the thin silver skin which envelops the endosperm. The parchment layer which envelops the seed is the endocarp.