Spermatogenesis and Oogenesis in Ephedra trifurca

Abstract

Ephedra trifurca is monosporangiate, but bisporangiate strobili occasionally occur. The beginnings of the staminate flower were clearly apparent in December, and the pollen was shed about the middle of April, the interval being thus a little over four months. The anthers develop in acropetal succession on a strobilus, and are surrounded by a perianth. Microspore mother-cells were observed about the middle of February, and the reduction division occurs about one month later. The gametophyte number of chromosomes is twelve. There are two persistent prothallial cells; the first is cut off by a wall; the second is not cut off by a wall. The primary spermatogenous cell surrounds itself by a membrane (Hautschicht?), and on the division of the primary spermatogenous cell, the stalk cell and body cell continue to be surrounded by this membrane, and are not separated from each other by a wall. The only wall formed in the pollen grain is the one which cuts off the first prothallial cell. The male gametophyte at the time of shedding consists of two prothallial cells, stalk cell, body cell, and tube nucleus. The megasporangium is surrounded by two integuments, the outer of which consists of four fused leaves; the inner of two fused leaves. The megaspore mother-cell is deeply placed within the nucellus and gives rise to either three or four megaspores arranged in a row, the most deeply placed megaspore being functional. The nuclei resulting from the division of the megaspore show polarity in that they are definitely oriented with respect to the axis Of the megasporangium. A vacuole appears between the nuclei resulting from the division of the megaspore before the spindle has disappeared, and soon becomes filled with delicate cytoplasmic structures which increase in density until walls appear. The free nuclei are parietally placed from the beginning, divide simultaneously, and are presumably 256 in number before walls appear. The female gametophyte is separated into two regions: a loosely formed archegonial region, and a more compact antipodal region, the latter being composed of a haustorial and a storage region. The archegonia vary from one to three, two being the usual number; the neck is composed usually of eight tiers of cells; and there are no archegonial chambers. The apex of the nucellus breaks down, and a conspicuous pollenchamber is formed. The necks of the archegonia are thus exposed to the air, and the microspores are brought directly into contact with the female gametophyte. No wall is formed between the ventral nucleus and the egg; the former becomes quite large and takes a position a short distance below the neck of the archegonium. The egg takes a position midway in the cytoplasm of the archegonium, surrounds itself with a membrane comparable to the one which invests the eggs of angiosperms, and in this position awaits fertilization, At the time of fertilization the cytoplasm in the archegonium has become almost homogeneous and very dense, except in the region immediately below the neck of the archegonium, where it is loosely vacuolate.