PYRONEMA. 113 



between the trichogyne and the antheridium, these nuclei collapse and 

 break clown into dense strands or shreds, which are frequently so 

 connected as to form a coarse and much bi'oken network in the cyto- 

 plasm (Fig. 42, D). Thestructure of the mature sexual organs, which 

 are aggregated in rosette-like clusters, is summarized by Harper as 

 follows (1900, p. 344) : 



The oogonium is a spherical or flask-shaped cell filled with dense protoplasm 

 and many nuclei, which are very much larger than those of the ordinary vege- 

 tative cells. Its stalk consists of two or three broad disk-shaped cells, of which 

 the basal one is a part of the mass of thickened, swollen cells forming the base 

 of the rosette. The apex of the oogonium is continued into the narrow conju- 

 gating tube which curves upward to unite with the end of the antheridium. The 

 antheridium is a curved, club-shaped cell, thickest near its upper end, and taper- 

 ing gradually to its base, where it is continued into a stalk of one or more cells. 

 The basal wall of the antheridium is, as a rule, somewhat higher up than that 

 of the oogonium. It follows a somewhat oblique path upward, conforming 

 rather closely to the surface of the oogonium, and its apex is even with, or 

 reaches somewhat past, that of the latter. 



The mutual relation of the sexual organs will be best understood 

 from Fig. 44. 



The changes taking place in the mature sexual apparatus, and which 

 lead up to fecundation, are of much interest, especially when compared 

 with the same process in other plants exhibiting similar phenomena. 

 First among these are what may be termed the receptive spots of both 

 the antheridium and the trichogyne. In that part of the antheridium 

 near which the tip of the trichogyne presses against its wall and where 

 the fusion-pore is formed, an area of protoplasm is differentiated as a 

 finely granular and irregularly lens-shaped disk from which the nuclei 

 have withdrawn. This granular area, although situated in the anthe- 

 ridium, Harper very fittingly compares to the so-called mouth-piece, or 

 receptive spot, of the egg in such algae as CEdogonium and Vaucheria, 

 The beak-like prolongation of the trichogyne reveals also a similar, 

 though less conspicuous, cytoplasmic differentiation (Fig. 41, C; Fig. 

 42, D). These areas seem to exercise a chemotactic influence which 

 tends to bring together the tube and the antheridium, and also to secrete 

 an enzyme by which the walls are dissolved in the formation of the 

 conjugation-pore. The presence of a similar differentiation in both 

 the tube and the antheridium would seem to indicate also that the 

 influence is mutual. 



At the point where the beak of the trichogyne is closely pressed 

 against the antheridium the walls are dissolved and a pore is formed 

 by which the cytoplasm of these two cells is made continuous (Fig. 



