OP CEYLON AND INDIA. 
325 
The exceedingly dwarf habit of the species, and its 
wonderful capacity for rejuvenescence, are rather biological 
than physiological features. 
The classification of the Lawias is rendered exceedingly 
difficult by the variability of many of the characters, such as 
the length of leaves and pedicels, and the size or depth of 
the cupules. Seven species have been recognized. Of these, 
L. ramosissima is more allied to Tristicha, to which I have 
transferred it. Historical evidence and comparison of forms 
showed that L. pedunculosa was a synonym of L. longipes 
and L. Lawii of L. pulchella. The evidence on which the 
species L. foliosa was based has been demolished above. 
Tulasne’s two Indian species are based on differences in 
length of leaves and pedicels, but as I have found these 
characters to vary enormously on the same plant, with depth 
of water, age of plant, and other things, these two forms must 
be united, and as further investigation shows, that their 
differences from the Ceylon forms are probably not enough to 
be specific, we are thus reduced to a single species, L. 
zeylanica, Tul. 
In Lawia we have evidently a highly modified plant to deal 
with, and it is also evidently adapted to more peculiar con- 
ditions than is Tristicha. At the same time it represents 
distinctly a side line in the evolution of the Podostemaceæ, 
and is the only plant with a shoot thallus in the highest group, 
the Tristicheæ. 
The thallus, or at any rate its upper side, is here pretty 
evidently of “ shoot ” nature, and is without either roots or 
haptera ; it is of the most extreme dorsiventrality, and lies 
closely down upon the rock. It is thus well suited to con- 
ditions which, as pointed out above, must have had an 
important influence in the evolution of the Podostemaceæ ; 
no conceivable force of current, apart from scour, can detach 
it from the rock, and it can also, owing to its exceedingly 
dwarf habit, survive so long as there is the least trickle of 
