NATURE 



591 



THURSDAY, JUNE 3, 1909. 



THE EVOLUTION OF THE VASCULAR 

 SYSTEM IN FERNS. 

 Lectures on the Evolution of the Filicinean Vascular 

 Svstcm. By A. G. Tansley. Pp. viii+143. New 

 Phytologist Reprint, No. 2. (Cambridge : Botany 

 .School, 1908.) Price 3s. 6d. 



THIS is the second " New Phytologist Reprint " of 

 special courses of lectures in botany, delivered 

 under the auspices of the University of London. The 

 publication of these advanced lectures serves a very 

 useful purpose, and it is to be hoped will be continued. 

 The present reprint differs, as regards the intro- 

 ductory lecture, from the original report in the Nnv 

 Phytologist, of which Mr. Tansley is editor. This lec- 

 ture has been re-written, in the light of some friendly 

 criticisms published since its first appearance, so that 

 it is necessary to consult the reprint in order to learn 

 the author's mature views. The first lecture is of 

 wide interest, for it deals with the question of the 

 origin of the Pteridophyta, involving that of the 

 vascular plants generally. As the author says (p. 4), 

 there is much reason to believe that the true vascular 

 plants had a common origin — in other words, that the 

 Pteridophyta are a monophyletic group. We have, 

 however, no direct knowledge of any plants which 

 suggest " Pro-Pteridophyta," and are forced to take 

 refuge in speculation. The author first notices the 

 well-known " antithetic theory," which traces the 

 origin of the spore-bearing plant from a sporogonium 

 or fruit like that of the Bryophyta (see Nature, 

 November 5, igo8, pp. 1-4). The author points out 

 that this theory involves some tremendous morpho- 

 logical assumptions in the way of the origin of new 

 organs, particularly leaves. The position has changed 

 considerably since this book was published, and the 

 theory is no longer maintained by its chief advocate. 

 Prof. Bower, in its original form; ' we have, probably, 

 in the future to look rather for some e.Kplanation such 

 as is here suggested by Mr. Tansley. He starts from 

 a form like the seaweed Dictyota, " in which two 

 morphologically identical generations, the one bearing 

 sexual organs and the other bearing tetrasporangia, 

 follow one another in regular alternation." 



" If we imagine such a form to become sub- 

 terrestrial, its spores becoming adapted to aerial 

 distribution, its thallus-branches becoming specialised 

 into stem and leaf, while its sexual generation is 

 reduced in vegetative development, we have a prac- 

 ticable ancestor of the Pteridophyta " (p. 7). 



It is probably on such lines as these that the 

 problem of the origin of the plant in the Vasculares 

 will be solved, if a solution is ever attainable. 



The author, however, accepts the antithetic theory 

 for the moss phylum, and therefore concludes that 

 the alternation of generations in the two groups had 

 a distinct origin, and that the sporogonium of a 

 bryophyte is not homologous with the spore-bearing 

 plant of a fern. The author suggests an ingenious 

 view of the relation between the two forms of alterna- 



t the Linnean Socieiy, 



1 See Discussion on "Alternation of Generations ' 

 IVrc Phytotogtsi, March, 1909, pp. 104-16. 



NO. 2066, VOL. So] 



tion, but the reviewer inclines rather to the belief that 

 the sporogonium of the mosses and liverworts repre- 

 sents a reduction from some more plant-like type of 

 sporophyte. 



Lectures ii.-ix. treat of the main subject of the 

 course, the evolution of the vascular system in the 

 fern series. Lecture ii. is on the important Palaeozoic 

 group Botryopterideae, the most ancient family of 

 ferns of which the structure is known. In some of 

 these plants the leaf branched in more than one plane, 

 four series of pinnae, instead of two, springing from 

 the main rachis. In the opinion of the author, 



" This tendency to radial organisation of the frond 

 may perhaps be regarded as a relic of the time when, 

 according to our basal hypothesis, the structure of 

 the fronds of ferns was but little differentiated from 

 the structure of their stems " (p. 23). 



The radial branching of the frond, however, appears 

 to be characteristic of the more complex members of 

 the family, and may more probably be regarded as a 

 specialisation (perhaps peculiar to the fertile frond) 

 than as a vestige of primitive organisation. The 

 known Palaeozoic plants were, after all, a long way 

 removed from primeval simplicity. 



In lecture iii., the simple structure of the filmy 

 ferns, so similar in many respects to that of the 

 extinct Botryopterids, is considered. Lecture iv. J^s 

 concerned with the Gleicheniace^ and Lindsayeae, 

 families in which the solid vascular cylinder is be- 

 ginning to give rise to the tubular structure which 

 forms the transition to the more complex vascular 

 systems. The Lindsaya type, in particular, with an 

 internal strand of phloem running through an other- 

 wise solid woody axis, is of great evolutionary in- 

 terest, because the same structure recurs in the early 

 stages of development of more advanced ferns. 



Lecture v. treats of the evolution of the tubular 

 stele as showrf in the Schizaeaceae, a family of small 

 extent which presents a remarkable range of 

 anatomical structure. 



Lecture vi. !s devoted to the evolution of " dictyo- 

 stely " (the typical polystely of Van Tieghem), in 

 which the vascular system opens out into an elaborate 

 network of strands, each having in some degree the 

 structure of the entire vascular cylinder of the lower 

 forms. The author, however, in his glossary at the 

 end of the book, tells us that the use of the term 

 polystely and its variants should be discontinued alto- 

 gether, at least in the fern series, .^s a counsel of 

 perfection this judgment may be received with sub- 

 mission, but the terminology of the great French 

 anatomist still has a descriptive value, and its use 

 will probably not be wholly abandoned at present. 



The specially complex anatomical organisation de- 

 scribed as "polycycly," where the vascular system is 

 built up of two or more concentric cylinders, is con- 

 sidered in lectures vi. and vii. In the more extreme 

 forms of polycycly (Marattiaccae and tree-ferns), the 

 highest elaboration met with, either in the fern series 

 or elsewhere, is attained. 



In the next lecture (viii.) simpler types (those of the 

 Osmundaceae and Ophioglossales), which lie apart 

 from the main lines of descent, are taken up. The 



