Notes. 
5 1 7 
cylinder becomes narrowed and reduced ; the pith tends to disappear, 
the bundles are confused together, and the power of secondary growth 
in thickness, in the case of Dicotyledons, is nearly or wholly lost. 
Where the plants are entirely submerged this reduction may affect the 
whole axis ; where however the flowering stem rises above the surface 
of the water it may retain a typical vascular cylinder, as is conspi- 
cuously the case in Hottonia. 
Now supposing the descendants of a reduced aquatic Phanerogam 
to return to a terrestrial mode of life, they will evidently once more 
need that higher development of the vascular system which is proper 
to land-plants. The favourable variations bringing about this change 
may indeed take the direction of a renewed dilation and differentiation 
of the single cylinder, and if a Dicotyledon is in question, secondary 
thickening may again make its appearance. But it is also possible 
that the available variations may take a different line. Instead of the 
one vascular cylinder again becoming more complex, the necessary 
amount of conducting tissue may be provided in the form of many 
distinct cylinders, arising from the bifurcation of the original one. 
This is what undoubtedly happened in the case of most Ferns, which, 
when they originally adapted themselves to life on land, became 
as a rule polystelic 1 . It may well have happened also in the case of 
certain Dicotyledons, which, in having become aquatic, had sunk to 
the anatomical level of the simplest Pteridophyta. 
On this view then the polystely of Auricula and Gunnera may be 
regarded as the anatomical expression of the return of plants of aquatic 
habit to a terrestrial mode of life. 
We may conjecture that the Auriculas, which always retain a 
normal structure in their pedicels and leaves, may be descended from 
only partially submerged ancestors. Gunnera , which belongs to a 
more distinctly aquatic family, may well have had ancestors which had 
more completely lost the typical monostelic differentiation, and hence 
the renewed adoption of terrestrial habit may have been accompanied 
by polystelic modifications extending to all the subaerial organs. 
D. H. SCOTT, London. 
1 Anatomical complexity in Ferns nearly always takes the form of polystely. 
But in Botrychium we have an indication of the other alternative — indefinite de- 
velopment of the single stele. If Lyginodendron and its allies really had Filicine 
affinities, this monostelic differentiation must have gone much further in the 
Palaeozoic representatives of the class. 
