THE GAMETOPHYTE GENERATION OF THE PSILOTACE^E. 
109 
The surface of the prothallus bears numerous long rhizoids which grow out from 
all sides. 
The antheridia and archegonia are always borne upon the same prothallus, and are 
not localised in their distribution. The male gametes are coiled multiciliate bodies. 
The antheridia develop from superficial cells, and in their mature state appear 
as spherical-shaped bodies that extend out beyond the surface of the prothallus. 
They are very numerous, and develop from all surfaces. 
The archegonium consists of a venter which lies below the surface, and a straight 
neck which projects as a short tube beyond the surface. The neck consists of four 
rows of cells arranged in several tiers surrounding a neck canal. 
Like the antheridia, 'the archegonia develop from all surfaces. 
Both in their vegetative and reproductive characters the gametophytes of 
Tmesipteris and Psilotum bear a remarkable resemblance to one another. They 
differ from one another in the following features : — 
In Tmesipteris the archegonia are much more numerous than the antheridia. 
In Psilotum the antheridia are much more numerous than the archegonia. 
In Tmesipteris the archegonia appear in dense crowded groups. In Psilotum 
these organs are widely separated and more scattered over the surface of the 
prothallus. 
There is a marked difference in the size of the sexual organs of the two plants. 
The antheridia and archegonia of Tmesipteris are just about twice the size of those 
of Psilotum. 
From this inquiry it seems quite clear that the gametophyte generation of the 
Psilotacese bears no structural resemblance to the prothallus of either Lycopodium 
or Equisetum. 
While the evidence shows quite conclusively that Tmesipteris and Psilotum are 
very closely related, it also proves that the Psilotacese are very remote indeed, in 
their phylogenetic relationship, from either the Lycopodiales or the Equisetales. 
There were no new facts revealed that would tend to discount the view, now 
generally held, that the Psilotacese are more nearly related to the extinct Spheno- 
phyllales than to any other known group of the Pteridophyta. 
EXPLANATION OF FIGURES. 
Figs. 1-17 are of Tmesipteris tannensis. 
Fig. 1. A representation of an entire prothallus of Tmesipteris. The curious irregular form suggests 
that its configuration is determined by the particles of sand and other obstructions met with in the soil in 
the course of its development. The figure shows, a distinct organic apex ; at least two antheridia and several 
archegonia. The numerous long rhizoids are also indicated. x 55, 
Fig. 2. A portion of a prothallus drawn from the surface view. Eleven archegonia are to be observed 
in a small area, and one antheridium. The majority of the archegonia are drawn from the view looking 
down into the neck cells. One of them, howevei', is seen' from the side view, and shows how the necks of 
TRANS. ROY. SOC. EDIN., YOL. LII, PART I (NO. 4). 17 
