OF CEYLON AND INDIA. 
43f) 
In \¥eddellina they are entomophilous, in Tristiclia 
ramosissima aneinophilous. Perhaps entomophily may be 
regarded as the more primitive condition in the order, but 
there is little evidence to prove this, and the point does 
not matter to the present argument. In the other species of 
Tristicha there is rather more dorsiventrality in the vegeta- 
tive organs ; the thallus is similar to that in the preceding 
forms, but the shoots arising from it show a distinct bilater- 
ality, the leaves being in lateral and upper ranks, and the 
branches in one plane, much as in a Selaginella or in 
the dorsiventral branches of many flowering plants. The 
flowers still appear to stand erect and to emerge through 
the water, but they also show signs of zygomorphism, in 
that there is a suppression of the upper stamen and of one 
lateral stamen as compared with the flower of T. ramo- 
sissima. 
From these genera we branch off at once to Lawia, in 
which the vegetative organs already show the most extreme 
possible structural dorsiventrality, with a growing point 
symmetrical only about a vertical plane, and with upward 
displacement of the leaves. Nevertheless, perhaps because 
this dorsiventrality is of recent acquisition and heredity is 
strong, the flower is radial in symmetry, and stands erect 
as soon as it emerges from the cupule. This latter organ 
shows a strongly marked difference between its upper and 
lower sides, but though the flower is developed inside the 
cupule in a horizontal position, it does not itself show any 
sign of zygomorphism, unless it be in minor anatomical 
details. 
The next stage is seen in the Oenones and Marathrums. 
Here the root is very like that in the preceding forms, but 
perhaps with rather more structural dorsiventrality, while 
the secondary shoots are strongly dorsiventral and prostrate. 
The flowers, however, are still erect and radial, and are 
apparently developed in a more or less erect position in the 
inflorescences. They are apparently insect-fertilized, but 
have the perianth reduced and replaced by the spathe as 
