i8o AQUATIC STEMS [ch. 



plants towards the condensation of the vascular system to a 

 single strand, devoid of secondary thickening, and in which 

 individual bundles cannot be distinguished, an interesting 

 suggestion, put forward some years ago by Scott ^, may be con- 

 sidered. Expressed very briefly, this suggestion was that the 

 cases of polystely^ occurring among the Angiosperms may be 

 due to descent from aquatic ancestors, from which a reduced 

 type of vascular system without cambium has been derived. 

 If plants with this heritage at any stage of their phyletic history 

 returned to terrestrial life, they probably experienced the need 

 for an increase of vascular tissue; but the production of normal 

 secondary thickening possibly presented difficulties, owing to 

 the condensed nature of the vascular system and the loss of the 

 cambial apparatus, and this may have led to the alternative 

 expedient of multiplying the existing steles. Scott refers to two 

 genera of flowering plants containing polystelic species 

 Auricula (Primulaceae) and Gunner a^ (Haloragaceae). Both 

 these genera include polystelic and monostelic species. The 

 single steles of the monostelic species are exactly like the indi- 

 vidual steles of the polystelic species; they have the vascular 

 bundles crowded together and are almost devoid of pith and 



1 Scott, D. H. (1891). 



^ The word 'polystely' is used in this connexion in a descriptive sense, 

 as a matter of convenience, irrespective of the possible validity of the 

 objections to its use as a morphological term raised by Jeffrey, E. C. 

 (1899). 



^ For the case of Gunnera a somewhat similar interpretation had been 

 proposed in 1875 by Russow, who however did not perceive that a return 

 from water to land life might be the factor initiating the polystelic con- 

 dition. He suggested that the Gunneras were descended from ancestors 

 whose vascular system had been condensed into a single central strand, 

 and that in the course of generations this form of stele might have become 

 so far stereotyped that it could no longer separate into its original con- 

 stituents (collateral vascular bundles) when a more elaborate conducting 

 system was required; it thus adopted the alternative of branching, and 

 reproducing its structural peculiarities in each branch. (Russow, E. 

 (1875}.) 



