190 
WILLIS : PODOSTEMACEÆ 
is submerged and purely vegetative till almost the end of the 
wet season. The primary axis is commonly insignificant, and 
from its base there buds out, horizontally, an organ which 
may be, phylogenetically, either of “ shoot ” or “root” nature, 
the thallus^ from which arise, endogenously and usually 
acropetally, secondary shoots, which are at first only vegeta- 
tive, but ultimately bear the flowers in those cases where 
the thallus is of root nature. The morphology of the thallus 
is very interesting. The secondary shoots (or branches, in 
the case of “ shoot ” thalli) develop the flowers late in the 
season, often changing their structure and appearance in so 
doing, and when the dry weather commences the flowers 
open, in some cases emerging through the water, in others 
remaining closed till exposed to the air by the fall of the 
water. The thallus when exposed usually becomes much 
altered by withering and soon dies ; the seeds are shed on 
the rocks to await the next rise of water, and the life-history 
begins anew. It is thus easy to see that without study of 
these plants upon the spot where they grow, and at different 
times of year, the student is liable to fall into great errors 
and difficulties in dealing with them. 
The Primary Axis .— for the few species described 
below, this is unknown in the Asiatic species and most others 
of the order, the usual axes preserved in herbaria being the 
secondaries. The characters of the primary axis are often 
very interesting, and are probably of considerable value in 
settling the affinities of the different forms, but as yet we 
know too little of them to use them with much effect. It is 
very remarkable that in the Asiatic forms at any rate the 
primary axis hardly ever bears flowers. 
The Thallus . — Most of the Podostemaceæ possess some 
kind of thallus, using the term broadly to indicate the part of 
the plant that creeps on the rocks or lies near to them. There 
are two main types of this organ well illustrated in the Indian 
forms: the thallus of “shoot” nature, seen in Lawia,^'' and 
* Cf. Plates X.-XII. of subsequent paper. 
