538 



Dr. J. C. Willis. On the Lack of 



only the two lower stamens. Now this species can live quite happily in 

 5-8 mm. of water, and again I may repeat that these three Tristichas 

 could perfectly well occupy all the localities of the Tristichaceae and 

 Podostemaceae, the conditions being identical throughout. In actual fact 

 they, with the other species of the genus — T. alternifolia — do occur in a 

 very great proportion of the places in which plants of these families have 

 been found, throughout the distribution zone of the genus Tristicha, which 

 is almost coterminous with that of the orders. 



Weddellina, a genus confined to the north of South America, has a 

 structure very similar to that of T. ramosissima, and a symmetrical 

 flower. 



Lawia, the remaining genus of this family, departs, so to speak, upon a side 

 line which was indicated by the secondary shoots of Tristicha sp. nov., but 

 here it is the primary axis which flattens out into a broad prostrate thallus, 

 which does not bear ramuli like the rest of the order and has symmetrical 

 flowers like T. ramosissima. The genus is confined to the region from Ceylon 

 to Bombay, and has only one species, L. zeylanica, which occurs in many 

 forms, differing slightly, so far as can be observed, in every locality. 



Before going on to deal with the Podostemaceae propel*, it may be well to 

 repeat that the Tristichaceae can, and do, live quite happily in every locality 

 occupied by the other family, and that the genus Tristicha, the typical genus 

 of the family, and the least modified (in one at least of its species), is the 

 most widely spread of all the genera of both families. The whole family 

 Podostemaceae, with its remarkable morphological constructions, is therefore 

 adaptationally unnecessary. All its extraordinary features are de luxe, and 

 cannot have arisen in response to any need for adaptation to different 

 conditions, for there are no different conditions to which to be adapted, 

 and none of the highly modified forms is so successfiil, or so common, as 

 T. hypnoides, which has only a very slight modification from the original type 

 from which the family is descended. We cannot suppose that all these 

 changes had to go on just to fit the family to live in running water, or 

 the first members of it would never have been able to survive, and the 

 Podostemons, etc., which are only slightly modified, would not be the most 

 widespread and common of all to-day. 



To pass on now to the Podostemaceae proper (tribe Achlamydatae of the 

 older writers), we may run very briefly over the striking morphological 

 differences to be found in the family. They are mainly of two kinds — 

 flattening of the secondary shoots and flattening of the primary roots. The 

 least modified types in many respects are the South American forms Ligea 

 (CEnone), Marathrum, and Mourera. All show, however, rather more dorsi- 



