108 
PROFESSOR A. ANSTRUTHER LAWSON ON 
the case of Psilotum , where the leaves lend themselves readily to an interpretation 
as reduced structures.” 
The main point of interest of the Psilotacese, however, is the question of their 
affinity to one or other of the known groups of the Pteridophyta. Attempts have 
been made to classify them with the Lycopodiales, with the Equisetales, and finally 
with the Spenophyllales. Professor Seward, # on the other hand, believes that the 
position of Tmesipteris and Psilotum in the plant kingdom will be probably best 
expressed by adopting the group-name Psilotales, rather than by transferring the 
recent genera to the Sphenophyllales. This view is also held by Miss Sykes. t 
Dr Scott \ was one of the first to emphasise the importance of recognising the 
possible affinity of the recent Psilotacese with the extinct fossil Sphenophylls. Since 
his views were published other investigators have confirmed his results. These 
more recent inquiries, especially those of Boodle § ( 1904 ), Bower || ( 1908 ), Ford *1 
( 1904 ), and Thomas** ( 1902 ), based upon anatomical, soral, and fossil evidence, have 
done much to direct current opinion to the view that the Psilotacese are more nearly 
related to the fossil Sphenophylls than to any other known groups of the 
Pteridophyta. 
It is obvious, of course, that the gametophyte generation of Tmesipteris and 
Psilotum can offer no positive evidence in support of this view, because the corre- 
sponding phase of the Sphenophylls is entirely unknown. The prothalli of these two 
recent types, however, prove quite conclusively that the Psilotacese are related to 
neither the Lycopodiales nor the Equisetales. This inquiry reveals no features 
that would conflict with the view that the Psilotacese should be classed with the 
Sphenophyllales. 
Summary. 
The results of this investigation may be briefly summarised as follows : — Ip 
The gametophyte of the Psilotacese is a subterranean prothallus of a light- 
brown colour. 
It has a cylindrical branching form of body, each branch terminating in an 
apical meristem. 
The prothallial tissue is uniform, there being no differentiation into vegetative 
and reproductive regions. 
■ The gametophyte is devoid of chlorophyll. It is completely saprophytic, and 
for its nutrition is dependent upon the co-operation of a mycorrhizal fungus. 
This fungus is endophytic and not localised in its distribution in the body of the 
host. It may infect any cells of the prothallus except those at the merismatic apex. 
* Seward, A. C., 1910. + Skies, M. G-., 1908. 
| Scott, D. H., 1897-1900. . § Boodle, L. A., 1904. 
|| Bower, F. O., 1908. H Ford, S. O., 1904 
** Thomas, A. W. P., 1902. 
