OF CEYLON AND INDIA. 
427 
Podostemon, and Griffithella, which are good examples of 
the isolobous and anisolobons ribbed, and the smooth fruits 
respectively. 
The natural history or ecology of the flower, like so many 
other features, also affords an interesting comparative study. 
The American Tristicheæ have a conspicuous entomophilous 
flower, projecting from the water on a long stalk, radial in 
structure, provided with a perianth, and freely visited by 
insects, to judge from the authors quoted by Goebel (13, p. 
330). So far as the herbarium specimens show, most of the 
flowers seem to set seed, and as the seeds are small and very 
numerous, the plant has probably every advantage needed 
in this respect. In the next groups we find the perianth 
replaced by the spathe, an organ apparently composed of one 
or more of the uppermost leaves of the secondary shoot. As 
to how the change occurred, and as to whether it was of any 
advantage to the plants, we are as yet ignorant ; further 
study is needed in South America. The perianth in these 
forms, e.g.^ in Oenone, is still represented by a ring of small 
scales alternating with the andrœceum ; the flowers are still 
insect- visited and emerge as before above the water, but as we 
progress along the series towards the Eupodostemeæ, we find 
a reduction of the size and conspicuousness of the inflo- 
rescence and of the individual flowers becoming more and 
more marked, and the flowers becoming dependent on the 
wind or on self-fertilization. At the same time the dorsi- 
ventrality of the flowers is increased by the loss of the upper 
stamens and perianth. 
In the Eupodostemeæ, finally, we find a most remarkable 
state of things. The flowers are small and very dorsi- 
ventral, the perianth being absent or represented only by 
the two thread-like organs at the sides of the andrœceum, 
which itself is reduced to a single or forked stamen on the 
lower side of the flower. Even the fruit is dorsiventral in 
structure in some forms. 
Dorsiventrality is usually supposed to be an attribute of 
entomophilous flowers, and tobe a feature of direct advantage 
(57j 
