HoiiLOWAY. — The Prothallus and Yowig Plant of Tmesipteris. 41 



theories, either as a reduction series or as a progressive series, the simpler 

 type of Lycopodium, such as L. Selago, being thus regarded either as very 

 much reduced or as primitive in form. Certain features of the embryo and 

 young plant, moreover, peculiar to a section of the Lycopodiaceae have 

 been interpreted as primitive, and primitive not only for the Lycopodiaceae 

 but for vascular plants generally. These are the protocorm and its sur- 

 mounting protophylls. According to this theory, the protocorm is regarded 

 as an indication of the way in which the primitive sporophvte first became 

 independent of the gametophyte, and in pursuance of this idea the peculiar 

 plant PhyUogJossiim has been spoken of as the most primitive form of 

 Lycopod. However, a simpler explanation of the protocorm, and one 

 widely accepted, is that it is merely a vegetative adaptation peculiar to 

 one or perhaps two sections of the Lycopodiaceae, and that Phylloglossum 

 has been derived from this particular section by reduction. Again, a third 

 interpretation has been suggested, that the protocorm is a modified form 

 of stem due to reduction, the basis of probability for the truth of this 

 theory being the very large size attained by the Carboniferous ancestors 

 of the Lycopodiums. These varying interpretations of the outstanding 

 features of the Equisetaceae and the Lycopodiaceae are so well known 

 that there is no need for me here to do more than merely indicate them 

 or to cite the authorities. They are mentioned to serve as an analogy to 

 the various interpretations which are possible in the case of the Psilotaceae. 

 It will be necessary for me to discuss briefly the evidence in favour of 

 regarding the Psilotaceae either as reduced forms or as retaining primitive 

 characters. 



Boodle (1901, p. 511) interprets the secondary tracheides found by 

 him in certain parts of the stem of Psilotum, as reduced secondary xylem, 

 and considers that this feature reinforces the similiarity which has been 

 traced between the Psilotaceae and the Sphenophyllales. He speaks of 

 Psilotum and Tmesipteris as being reduced from '" a common parent form, 

 in which the aerial stem had a rayed mesarch xylem mass " (ibid., p. 515) 

 and which also showed secondary thickening. Such a stem, he says, would 

 bear a strong resemblance to the axis of Gheirostrobus ; but at the same 

 time he is careful to point out that such a character as the presence of 

 secondary xylem is too adaptive to be taken by itself as evidence of affinity 

 (ibid., p. 513, note 1). However, the presence of secondary xylem in the 

 stem of Psilotum, he says, possesses certain significance in view of the fact 

 that the fertile organ of the Psilotaceae finds its nearest parallel in that 

 of the Sphenophyllales. 



There is no doubt that the saprophytic habit of both Psilotum and 

 Tmesipteris, the extreme reduction in the leaves of the former, and the 

 presence in the rhizomes of a mycorhiza, may be taken as suggesting that 

 their present form and structure is, at any rate partly, due to reduction. 

 And, of course, the absence of a root organ may be regarded in the same 

 way. Probably the most interesting point to be elucidated by a study 

 of the life-history of the two members of this class is whether or not there 

 is a rudimentary root organ to be traced in the embryo. Lawson (1917a, 

 p. 793), from his study of the one embryo found by him, concludes that 

 there is such a rudimentary root present. My own study of a number 

 of embryos and of a fairly complete series of young plants has convinced 

 me that there is not, but that the peculiar outgrowth of the absorbing 

 region of the embryo which Lawson speaks of as a rudimentary root is 

 only one of a large number of such outgrowths which are to be regarded 



