In this fructification the organization is at first altogether that of a Lepidostrobus; in each megasporangium, however, only a single megaspore came to maturity, occupying almost the whole of the sporangial cavity (see fig.
Denticulata, young plant attached to the megaspore (enlarged).
In Welwitschia also the megaspore is filled with prothallus-tissue, but single egg-cells take the place of archegonia.
Each sorus includes both microsporangia, with numerous spores, and megasporangia, each of which contains a single megaspore with a complicated wall.
In certain species of Gnetum described by Karsten the megaspore contains a peripheral layer of protoplasm, in which scattered nuclei represent the female reproductive cells; in Gnetum Gnemon a similar state of things exists in the upper half of the megaspore, while the lower half agrees with the megaspore of Welwitschia in being full of prothallus-tissue, which serves merely as a reservoir of food.
From the microspore an extremely reduced male prothallus and from the megaspore the female prothallus, develops (cf.
In some of the heterosporous forms (Lepidocarpon, Miadesmia) the sporangia were sometimes surrounded by an integument; and since only a single megaspore attained maturity, the structure of the megasporangium suggests a comparison with an ovule.
In each ovule one megaspore comes to maturity, but, exceptionally, two may be present (e.g.
The megaspore-nucleus divides repeatedly, and cells are produced from the peripheral region inwards, which eventually fill the sporecavity with a homogeneous tissue (prothallus); some of the superficial cells at the micropylar end of the megaspore increase in size and divide by a tangential wall into two, an upper cell which gives rise to the short two-celled neck of the archegonium, and a lower cell which develops into a large egg-cell.
A single cell of the nucellus enlarges greatly to form the embryo-sac or megaspore (fig.