434 WILLIS : MORPHOLOGY OF THE PODOSTKMACEÆ 
time comes, the development of the flower, its expansion, and 
the ripening of the fruit take place with great rapidity. 
As a rule some starch is left in the thallus after the seeds 
are ripe, and this helps in regeneration of the growing points 
if it be again submerged. 
As regards sensitiveness to stimuli, there is as yet little 
to be said. The thalli do not seem at all rheotropic, but 
the creeping forms follow out every irregularity of the sub- 
stratum, whether owing to effects of contact or of light or 
gravity. At the flowering period the stems change their 
sensitiveness and stand erect, though while developing they 
were commonly horizontal. 
On the Dorsiventralify of the Podostemaceæ, 
and its bearing on current views of the 
Mechanism of Evolution. 
No other family above the liverworts shows so marked 
and far-reaching a dorsiventrality in organization, and the 
Podostemaceæ are often quoted in discussions of this pheno- 
menon. Their dorsiventrality has been described in 
general terms by Warming (43), Goebel (13, 14), and others, 
but several interesting points have as yet escaped notice, 
especially the remarkable series of stages in a progressive 
dorsiventrality exhibited by the members of this family, 
and the curious feature presented by so many of them, of 
erect anemophilous autogamous flowers, which at the same 
time possess the most extreme structural dorsiventrality 
which occurs among the flowering plants. 
We may best illustrate this by a series of actual instances, 
working downwards from the less modified types of the 
order. In Tristicha ramosissima and in Weddellina squa- 
mulosa there is but little structural dorsiventrality of the 
plant taken as a whole ; it is only expressed in the creeping 
position of the thallus. The secondary shoots stand up in 
the water, and are radially constructed, while the radially 
symmetrical flowers emerge through the water on erect 
