THE GAMETOPHYTE GENERATION OF THE PSILOTACEHi. 
107 
both these generations. Such a knowledge is necessary in assisting us to arrive at 
conclusions in regard to the problems of their ecology, their geographical distribution, 
and, above all, their phylogenetic relationship. 
Our knowledge of both generations of existing Pteridophyta is fairly complete. 
The gametophyte generation of extinct types, however, will probably never be 
known. If the prothalli of these latter types were of the same nature as the 
Lycopodiales or the Equisetales — races that are practically extinct — it is not likely 
that they could ever be preserved. Their structure is too delicate and perishable to 
leave hope of their being found as fossils, or even impressions. Our knowledge of 
such extinct races is therefore dependent entirely upon the fossil remains of the 
sporophyte structures, and at most such remains are fragmentary. 
From the study of the geological history of plants, it seems quite certain that 
the Lycopodiales, Equisetales, Sphenophyllales, and Ophioglossiales are types repre- 
senting merely the vestiges of races of plants of great antiquity, which in early 
geological times constituted vast and important features of the earth’s vegetation. 
In the survival or extinction of such races it is obvious that the adaptations of the 
gametophyte, as well as the sporophyte, become a determining factor. It would 
seem that the production of gametes and fertilisation is essential to all Pteridophytes, 
and consequently the nutrition of the gametophyte generation is a matter of vital 
importance to such races. 
In this connection it is of prime importance to note that in all of the above- 
mentioned types of Pteridophyta, with the exception of the single genus 
Equisetum , the nutrition of the gametophyte is of a highly specialised nature. 
It is saprophytic, and dependent upon the co-operation of a mycorrhizal endophytic 
fungus. It is therefore not surprising to find this same highly specialised form 
of nutrition in the isolated, practically extinct race represented by Tmesipteris 
and Psilotum. 
To what extent this highly specialised form of nutrition in the most critical phase 
of the life-history has affected the powers of adaptation of the sporophyte it is 
difficult to. say. But if one may judge from a comparison of these types with the 
Eilicales, which have free-living, chlorophyll-bearing prothalli, the effect appears to 
be great. From this general comparison it would seem that where we find a sapro- 
phytic nutrition in the gametophyte there is a reduction in the structures of the 
sporophyte, and where the gametophyte is free and with the power of photosynthesis, 
as in the Filicales, the sporophyte shows a high state of organisation and reaches its 
maximum development. The most striking exception to this rule is Equisetum, but 
even here the sporophyte is a much more, highly organised structure than in any of 
" the existing types of the Lycopodiales, Sphenophyllales, or Ophioglossales. In regard 
to the Psilotacese themselves, in this connection, we might quote from Professor 
Bower in The Origin of a Land Flora : “ So far as expressed, current opinion seems 
to favour the probability of reduction in accordance with habit, and especially so in 
