532 



On the Lack of Adaptation in the TristichacecB and Podostemacece. 



By J. C. "Willis, M.A., Sc.D., etc., Director of the Botanic Gardens, 



Bio de Janeiro. 



(Communicated by Dr. D. H. Scott, F.B.S. Beceived February 24, — 

 Bead April 30, 1914.) 



With the exception of some of the parasitic orders, such as the Bala- 

 no r pJioracece, there are probably no families of flowering plants — one might 

 almost include flowerless — which are so completely transformed from the 

 average or mesophytic type of the phanerogams into types which are so 

 completely unique and peculiar, as the Tristichaceae and still more the 

 Podostemaceae. Nor are there any in which, with such very great uniformity in 

 the conditions of life, there is such remarkable variety in the morphological 

 structure. 



The structure of the orders, or rather of their members, being unique, and 

 the conditions under which they live being also unique, it has been taken for 

 granted that the former is in a high degree adapted to the latter, the flat 

 thallus-like expansions of stem or root being looked upon as admirably suited 

 to the rushing water in which they live. So long as we were almost completely 

 ignorant of the actual living plants, and content with dead material collected 

 mainly in the dry seasons, this was all very well, but now that for 17 years I 

 have devoted much attention to these plants* have studied them in the living 

 condition in their natural habitats in India, Ceylon, and Brazil, have followed 

 them from germination right through their life-history, and in other ways 

 become absolutely familiar with them, and as a result of all this have 

 arrived at diametrically opposite conclusions, it will repay us to examine into 

 the question in some detail. 



Evolution is usually supposed to have produced the extraordinary variety 

 of forms now existing by adapting members of very different families to very 



* Willis, "A Revision of the Podostemacese of India and Ceylon," ' Ann. Perad.,' 

 voL 1, p. 181 (1902) ; " Studies in the Morphology and Ecology of the Podostemacese of 

 Ceylon and India," loe. tit., p. 267. 



CONTENTS. 



Conditions of Life 



Morphological Structure .. 

 Geographical Distribution 



Absence of Adaptation 



Process of Evolution 



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