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CHAPTER XXVII 



WATER PLANTS AND THE THEORY OF NATU- 

 RAL SELECTION, WITH SPECIAL REFERENCE 

 TO THE PODOSTEMACEAE 1 



FROM a study of the Podostemaceae, Dr Willis 2 has 

 arrived at certain views as to their evolution which, if 

 accepted, have a peculiarly wide bearing. The great variety and 

 anomalous character of the features exhibited by this family 

 have been touched upon in Chapter ix. There is little doubt that 

 these plants have been derived from some terrestrial group, 

 since the structure of the flower and fruit is typically adapted to 

 land life. Willis suggests that a possible origin for the family 

 is from plants already growing on the banks of mountain 

 streams, with creeping adventitious roots, upon which secon- 

 dary shoots were regularly developed; these secondary shoots 

 might provide the opportunity for an entrance into aquatic life. 

 Most of the peculiarities of the group, as Willis points out, can 

 be traced to the remarkable plasticity of the skeletonless root, 

 and to the parallel dorsiventrality of the vegetative organs and 

 flowers. This dorsiventrality is associated with "their plagio- 

 tropic method of growth, forced upon them by the fact that they 

 live only upon an unyielding substratum; they have not, and 

 can never have had, primary roots going downwards into the 

 rock, and are thus, one might almost say, cut in half, or deprived 

 of one-half of their polarity 3 ." "No other family above the 

 liverworts shows so marked and far-reaching a dorsiventrality 

 in organisation 4 ." The dorsiventrality of the flowers, Willis 



1 For the sake of brevity the term Podostemaceae will be used in this 

 chapter in the old sense, to include both the Podostemaceae proper, and 

 the closely related Tristichaceae. 



2 Willis, J. C. (1902), (I9H 1 ), and (i 9 i 5 2 ). 



3 Willis, J. C. (I9I4 1 ). 4 Willis, J. C. (1902). 



