Insemination of the archegonium and fertilization in Taxus baccata L

A study of fertilization in Taxus baccata in the electron microscope has revealed novel features. Insemination of the archegonium is facilitated by local perforation of the wall of the young pollen tube. Digestion of the wall begins before the pollen tube pierces the megaspore membrane but is not completed until its tip makes contact with the neck cells of the archegonium. As soon as a pore is formed a single sperm nucleus and some cytoplasm of the male gametophyte enter the archegonium. Which of the paired sperm nuclei move from the pollen tube into the archegonium appears to be a matter of chance. Close apposition of sperm nucleus and egg nucleus is followed by the formation of numerous points of contact between the two. The membranes fuse at these points and pores are rapidly formed. The progressive enlargement of these pores ultimately eliminates any partitions and yields the zygotic nucleus. There is a possibility that, as in some other gymnosperms, the plastids and mitochondria of the zygote come in part from the male gametophyte, but whether from the remains of the spermatogenous cell cytoplasm or from the. pollen tube lumen is not clear.

The development of the male gametophyte and spermatogenesis in Taxus baccata L

The development of the male gametophyte of Taxus baccata has been studied over a period of 20 weeks, from germination of the microspore in February to spermatogenesis in July. A few days after germination the microspore nucleus divides and a transverse wall forms at the equator cutting off the small generative cell and a large tube cell. The latter immediately begins to expand to form the pollen tube. The first division thus establishes the polarity of the gametophyte and the generative cell is regarded as proximal. The transverse wall is ephemeral, and within six weeks it has disappeared. The nucleus of the generative cell divides while still at the proximal pole. The two daughter nuclei are unequal in size, but they remain associated and together move distally. The larger nucleus eventually becomes the nucleus of the spermatogenous cell, and the smaller the sterile nucleus. The spermatogenous cell acquires a distinctive cytoplasm and becomes surrounded by a wall which arises de novo . The nucleus of the spermatogenous cell enlarges, but always remains towards one side of the cell so that at mitosis the spindle is contained within one hemisphere. After division the wall of the spermatogenous cell is ruptured and the two sperms are released as naked nuclei of equal size. The cytoplasm of the spermatogenous cell degenerates as it enters the tube, but remains recognizable until fertilization.

ON THE ANDROCYTE OF POLYTRICHUM, WITH SPECIAL REFERENCE TO THE DREIERGRUPPE AND THE LIMOSPHERE (NEBENKERN)

The limosphere (nebenkern) of the Polytrichum androcyte is a complex structure composed of a plastid that is partially enclosed in a mitochondrial shell. This shell forms by the coalescence of the mitochondria found in the young spermatogenous cell. During or subsequent to the fusion of the mitochondria, a portion of the mitochondrial mass separates from the aggregate and becomes the apical body that is later attached to the anterior of the androcyte nucleus. A three-parted structure, the Dreiergruppe (DG), is found associated with the apical body and the flagellar bases. The element of the DG nearest the flagellar bases is composed of a band of parallel tubules, ca. 150 Å in diameter. These tubules extend over the surface of the nucleus for some distance and are thought to be the so-called filamentous appendage reported for bryophyte sperms, and the analog of flagellar "roots" found in certain algal cells.