OF CEYLON AND INDIA. 
305 
With regard to the absence of intercellular spaces, Goebel 
(13, p. 354) has already given what I am inclined to think is 
the true explanation or near to it, that the Podostemaceæ, 
living in well aerated water, do not need, like plants whose 
lower parts are in mud or in stagnant water, large intercel- 
lular spaces to serve as oxygen carriers to the lower parts for 
their respiration. I do not think that the very common 
explanation of the large spaces, that they serve to float the 
plants up in the water, is a sufflcient one, though this may 
be part of their function. Tristicha floats freely enough 
with the current, though it has no such spaces, and is 
actually, like all the other Podostemaceæ that I have 
examined, so heavy that it sinks in still water. In general 
the development of large spaces seems to go with absence of 
oxygen in the lower part of the medium surrounding the 
plants, and with still water, both of which conditions are 
absent in the case of Tristicha. 
The great number of flowers produced, and their develop- 
ment all at one season, is another noteworthy feature as 
compared with plants of still waters ; the production of the 
flowers at the time when the water leaves the plant stranded 
may be compared with the development of the sexual organs 
in Riccia fluitans when it is stranded by the drying of the 
pool in which it has been floating. 
Lastly, the power of rejuvenescence of the thallus, which 
is well-marked here, and much more so in species to be 
considered later, is evidently a very great advantage to a 
plant exposed to the risks attending the temporary fall of 
water-level that are liable to occur. 
In Tristicha and Weddellina, then, we seem to have plants 
still very like ordinary water plants, but with certain new 
features correlated with their new conditions of life, such as 
the development of creeping roots and haptera as fastening 
organs, the multiplication of the shoots by the exaggeration 
of the not elsewhere unknown phenomenon of development 
of shoots from the roots, great powers of rejuvenescence. 
