452 WILLIS ; MOKPHOLOdY OP THE PODOSTEMACEÆ 
they usually affect without difficulty, and some of the dwarf 
forms, e.^., Hydrobryum olivaceum, only become completely 
dwarf as the water-level is falling, and have tall shoots 
at the season of deeper and swifter water. Some of the largest 
American forms inhabit very rapid water. 
With regard to the local distribution of the different genera, 
it may suffice to note that the richest region is the south- 
western corner of India, together with Ceylon ; here there 
occur three endemic genera, Lawia in the Tristicheæ, and 
Griffithella and Farmeria in the Eupodostemeæ, while there 
are endemic species in Tristicha, Podostemon, Dicræa, and 
Hydrobryum (and probably Willisia). Only the three last 
occur in the eastern peninsula. It will be noticed at once 
that the distribution of the Podostemaceæ in Ceylon and 
South-West India is just like that of the so-called Malayan 
element of the Ceylon and Malabar flora, rich in the south and 
fading out to the north. Only one species has as yet been 
recorded from the Malayan region (30), but this shows 
affinities to the Abyssinian Sphærothylax. Tristicha and 
Dicræa have their nearest relatives in Madagascar, while 
Hydrobryum, the most widely distributed of the Indian 
genera, is related to Sphærothylax. It would seem, therefore, 
not impossible that the order reached India and Malaya 
(including Burma and Assam) from the south-Avest, but if 
so, there must almost certainly have been, in the land forming 
the passage betAveen India and Madagascar,’^' some hilly or 
mountainous country, for even Tristicha and Dicræa require 
fairly rapid water Avith a rocky bottom. 
Th® Origin of the Podostemaoeæ. 
Tliis is a someAvhat difficult question, which must be left 
for detailed consideration in connection with the anatomical 
inAæstigations yet to be completed. The embryonic develop- 
ment and characters are so typically dicotyledonous, and 
Cf. Willis and G-ardiner. The Botany of the Maldives. Ann, Perad., !.. 
190L pp. 5, 141, HI 
