Holloway. — Prothallus, <kc, of Tmesipteris. 421 



A third definite suggestion with regard to the origin of the leafy axis of 

 existing Pteridophytes from a strobiloid ancestor arises out of the facts 

 of the simply organized embryo of Tmesipteris. The only new feature to 

 be postulated here is the extension in length of the shoot from an apical 

 meristem instead of, as in Anthoceros, from an indefinite basal meristem, 

 and the initial cause of the continued shoot-elongation might be set 

 down as being the adoption of a subterranean mode of life by the gameto- 

 phyte. The differences between the ancestral strobilus and the derived 

 rootless shoot would then be referred largely to their different modes 

 of life. Further development in complexity of the sporophyte of this 

 " ])xo-Tmesvpteris " would take place by the continued growth of the shoot 

 and by its dichotomous and lateral branching. On this view, the sub- 

 terranean habit of the gametophyte would be regarded as of very early 

 origin in at least one line of descent of the higher plants, although in 

 other phyla, as, for example, that of Lycopodium, and possibly also that 

 of the Ophioglossaceae, it is probably a very much later development. 

 Bower in a general way regards the subterranean habit of the gametophyte 

 as being modified from the subaerial habit. He says (1, p. 710), "It 

 may accordingly be concluded as probable that the prothallus of early 

 Pteridophytes at large was a relatively massive green structure with 

 deeply-sunk sexual organs." If, as suggested above, the superficial position 

 of the sexual organs in the Psilotaceae is not a modified feature, the 

 gametophyte of this class stands apart from that of other Pteridophytes. 

 From Kidston and Lang's description of the asexual generation in the 

 Bhyniaceae one is tempted to conclude that the gametophyte of these 

 plants was subterranean rather than subaerial. The presence in the 

 gametophyte of an endophytic fungus is a widespread feature of existing 

 Pteridophyte prothalli, and I suggest that it may with as much reason 

 be considered to have played a part in leading to the development of the 

 rootless and leafless shoot of the Psilotaceae as to have been the cause 

 of reduction taking place in a more complex plant-body. Modern Pteri- 

 dophytes are so far removed in point of time from the hypothetical 

 primitive form or forms that deductions based on comparative embryology, 

 lacking as they do any support which might have been afforded by a 

 knowledge of the embryogeny of archaic vascular plants, might be con- 

 sidered as altogether undependable. On the other hand, the extreme 

 simplicity of the Tmesipteris embryo, wholly devoid as this is of appen- 

 dicular organs, is full of significance, and the demonstration of a rootless 

 and leafless condition in the earliest known land-plants strengthens the 

 belief that the Psilotaceae have preserved in the first stages of their 

 development primitive features. 



The lateral origin of branches in the young rhizome of Tmesipteris would 

 seem to be a more specialized character than branching by dichotomy of 

 the apex, and it is curious to find that the latter does not apparently take 

 place in the youngest rhizomes, whereas in those of a somewhat older age 

 it is present along with lateral branching. The initiation of the second apex 

 of growth may take place in the young embryo or be postponed till the shoot 

 is well advanced, and even then varies in its position. This is just such 

 a generalized character as might be expected in a primitive type of plant- 

 body. The distinction also between the subterranean and the subaerial 

 parts of the sporophyte would seem to be very indefinite, one or both of 

 the first apices of growth emerging and becoming leafy according as the 

 needs of the young plant direct. Sometimes the first-formed aerial shoot 



