A, Carpogonium, c, with its trichogyne t (X405).
This is the carpogonium; it consists of a ventral portion which contains a nucleus, but in which no oosphere is differentiated, and an elongated tubular portion known as the trichogyne, into which the cytoplasm extends.
The ventral portion of the carpogonium may be imbedded deep in the thallus in the massive species; the trichogyne, however, always reaches the surface.
The carpogonium germinates forthwith, drawing its nourishment almost wholly from the parent plant.
In Batrachospermum filaments arise from the carpogonium on all sides; in Chantransia and Scinaia on one side only; in Helminthora the filaments are enclosed in a dense mucilage; in Nemalion, prior to the formation of the filaments, a sterile segment is cut off below.
In all these cases, however, the end-cells of the filaments each give rise to a carpospore, and the aggregate of such sporiferous filaments is a cystocarp. Again, in the family of the Gelidiaceae, the single filament arising from the carpogonium grows back into the tissue and preys upon the cells of the axis and larger branches, after which the end-cells give rise to carpospores and a diffused cystocarp is formed.
In Naccaria, one of the Gelidiaceae, it is observable that the ooblastema filament, as the tube arising from the fertilized carpogonium has been called, fuses completely with a cell contiguous to the carpogonium before giving rise to the foraging filaments already refered to.
In a whole series of Red Algae, the existence of a highly specialized auxiliary cell in the neighbourhood of the carpogonium is a characteristic feature.