The Development of the Gametophytes and Fertilization in Juniperus communis and Juniperus virginiana

Abstract

The staminate cones consist of many sporophylls, each bearing microsporangia on the under side near the axis. The pistillate cones contain three ovules, each subtended by a sporophyll. The mature pollen grain contains but one cell, no prothallial cells being present. The macrospore mother cell first appears in J. communis one year after pollination. The first mitosis taking place within it shows clearly the stages characteristic of heterotypic division. The contents of the pollen grain divide into antheridial cell and tube cell soon after pollination. In J. communis the antheridial cell remains in the pollen grain and does not divide until April of the next spring. In J. virginiana the antheridial cell divides in the early summer of the same year in which pollination takes place, and fertilization occurs in the latter part of June or first of July. The generative cell and the stalk cell migrate immediately into the tube. The stalk nucleus passes the generative cell and lies near the tube nucleus. The generative cell does not divide until after the pollen tube has reached the archegonia and just before fertilization. Two similar sperm cells are formed, each hemispherical in shape. The female prothallium develops, as in the other gymnosperms, by free cell formation in a hollow sphere of cytoplasm. After many nuclei are formed, cell walls are laid down between them. These cells grow toward the center, but before they meet, cross-walls are laid down. The archegonia develop from superficial cells at the micropylar end of the prothallium. They are arranged in a group, and the entire complex is surrounded by a single layer of sheath cells. During the same day that the generative cell divides, and presumably before its division, the central cell of the archegonium divides. A ventral canal nucleus is formed and remains throughout its life free in the cytoplasm of the egg, that is, no true ventral canal cell is ever cut off. This nucleus remains at the tip of the egg and, as a rule, soon disintegrates. After fertilization the first division occurs before the fusion nucleus has passed to the organic apex of the archegonium.