440 WILLIS : MORPHOLOGY OF THE PODOSTEMACBÆ 
and union or branching of stamens or receptacular prolon- 
gation (according to the particular interpretation put upon 
the forked andrœceum of most of the Eupodostemeæ). 
We can scarcely suppose the floral organs to have led the 
way in the dorsiventrality, because throughout the family 
we find the vegetative organs to be the more dorsiventral, 
and we have also seen that the floral organs appear to 
derive no advantage from their zygomorphism ; further, 
zygomorphism is almost unknown in floral organs that 
are developed in erect positions on radially symmetrical 
organs. 
The general conclusion we have thus drawn is led up to by 
two separate lines of argument, one from the morphological 
construction of the organs in the whole series of Podostemaceæ, 
the other from a consideration of the ecological features of 
the flowers and fruits. If it be correct we ought to find 
confirmative arguments from the general phenomena of 
dorsiventrality in other plants. A general comparison of 
these phenomena shows at once that dorsiventrality is much 
more common in the vegetative organs than in the floral, 
but that it is also frequent in the accessory organs of the 
flower, less so in the essential organs, especially in the fruit or 
seed. Again, within the same family, compare, for example, 
the slight dorsiventrality of the vegetative and floral organs 
of Potamogeton with the great dorsiventrality of those of 
Zostera and others. There are many cases in which the 
vegetative shoots are not themselves dorsiventral in structure, 
but the zygomorphic flowers which they bear are developed 
in horizontal positions, a feature which we have seen 
to be one of importance in determining the degree of 
dorsiventrality in the Podostemaceæ ; even in these cases it 
is not to be forgotten that there is marked dorsiventrality 
in the leaves. 
One of the marked features in the flowers of the 
Podostemaceæ is that the absent stamens are always those 
of the upper side of the flower ; this again is an almost 
universal feature in other dorsiventral flowers, e.g,, in the 
