192 
WILLIS : PODOSTEMACEÆ 
long leafy tips of thethalli are not fioriferons, do not become 
lignified, and drop as soon as exposed, leaving the short 
woody fioriferons basal portion;" in D. Wallichii,t on 
the other hand, the flowers are often more distal, but the 
marginal parts of the thallus are not lignified, and break 
away towards the flowering season, so that the thallus 
becomes linear, v/hich was before broadly ribbon-shaped, 
and lobed, which was before entire. Only when the thallus 
is very closely attached to the rock does it retain its original 
form, and even here great changes usually occur when the 
plant is exposed, owing to the fall of the leaves. There is 
no Indian species which is well represented in herbaria by 
specimens which clearly illustrate the morphology of the 
thallus in its mature living condition. 
Another complication is introduced by the great capacity 
for rejuvenescence exhibited by most thalli ; when broken 
or otherwise damaged, new growing points form behind the 
injury and grow out in various ways. 
The Secondary Shoots . — In the root thalli these are endoge- 
nously formed close to the apex, in the shoot thalli of Lawia 
further back. In the latter they are mere rosettes of leaves, 
unless they happen to be at the extreme edge of the thallus, 
when they soon form growing points just like the original 
growing points of the thallus, and ultimately give rise to 
flowers. In root thalli we get two types of secondary shoot, 
the one illustrated by Podostemon, Willisia, and Tristicha, 
the other by the remaining Indian genera. In the first, the 
secondary axis grows on continuously and branches (except 
in Willisia), forming new leaves, and reaching a considerable 
length, finally forming several flowers (one in Willisia) in 
the usual way. In the second, the axis remains obsolescent 
during the vegetative period, being represented merely by a 
fascicle of leaves emerging from an opening in the thallus. 
Towards the flowering season the axis elongates so as to 
* Cf. Plate XIX. of subsequent papei. 
t Cf. Plate XXI. of subsequent paper. 
