the Genus Hottonia. 
265 
insertion of the branch traces differ. They are similar to the main stem, 
with which they unite at a very slight angle, so that they are at first mere 
bulges of the great central ring of vascular tissue (Text-fig. 5) which higher 
up get nipped off ; whereas in H . palustris the branches leave much larger 
gaps relatively to the size of the main stele, with which they unite almost 
at right angles. 
Whether the above is the true explanation of v its presence or not, we 
undoubtedly have for a centimetre or more below the inflorescence of 
H. palustris a siphonostele, which, by the union of the internal with the 
external endodermis at the branch gaps, together with the incurving of 
the arcs of vascular tissue, results in a transient phase of polystely. More 
than twenty years ago Scott (9) suggested that the origin of polystely in 
Dicotyledons might lie in their descent from aquatic ancestors, whose re- 
duced vascular system was not sufficient for a renewed terrestrial existence, 
and which supplied this deficiency by increasing the number of steles, rather 
than the size of the single central cylinder. In support of this view, he 
pointed out that the polystely of the ferns has undoubtedly arisen in this 
way, and that the two natural orders of Dicotyledons then known to exhibit 
the phenomenon (Halorraghidaceae and Primulaceae) both possessed aquatic 
representatives — Myriophyllum and Hottonia. It is interesting to note 
that most of the cases of polystely since described occur in aquatic and 
marsh plants, or in their near relatives (6) ( Nymphaea , Parnassia , Ranun- 
culus), and in Hottonia we seem to have the phenomenon in its very incep- 
tion, as in the individual we pass from the haplostele, through siphonostely 
to polystely. The fact that the haplostele is not primitive does not alter 
the value of the illustration afforded of the elaboration such a structure may 
undergo, and the significance of the nodal traces of the 4 endodermal pocket J 
is illumined by Boodle’s view (1) that ‘ advance in complexity probably 
begins at the nodes, and is afterwards continued through the internodes 
Followed upwards we have in all likelihood the evolutionary course of 
development ; traced downwards we may say, again to quote Boodle, there 
is taking place from the crowded nodes of the transitional region £ a down- 
ward progressive modification ’, which if continued would result in a poly- 
stelic form, such as we find in the Auricula section of the closely allied 
genus Primula. 
Probably the best test of the theory lies in experimental work, and 
cultures have been started with this end in view ; meanwhile, on the lines of 
comparative anatomy, the occurrence of a trace of polystely in the two 
geographically separated species of this water plant, taken in connexion 
with its variable habitat and plasticity of structure, seems to afford a link 
in the chain of evidence that an aquatic ancestral existence is the origin of 
at least some, if not all, cases of polystely in Dicotyledons. 
