4T4 WILLIS : MORPHOLOGY OF THE PODOSTEMACEÆ 
mainly with the Indian forms, it is still fairly large, but not 
branched, and as a rule bears one terminal flower only. Its 
leaves are like those of the primary shoot, but in a different 
phyllotaxy, and at length they form scaly bases. The 
anatomy of the stem itself is also different from that of the 
primary axis, not merely in the presence of the stiffening 
layer of tissue needed to hold the stem erect, but also in the 
construction of the vascular tissue, which shows decided 
dorsiventrality and other points of difference. In Podoste- 
mon the reduction of the secondary axis has gone further, 
and it is short and more or less prostrate, though branched, 
and bears few flowers ; it still, however, does a major part 
of the work of assimilation in the plant, as its leaves are 
large. It is much larger and more complex than the primary 
axis in the one species in which it has been investigated. 
In the remaining four Indian genera we find an extreme 
reduction of the size of the secondary shoots, accompanied 
by a flattening and enlargement of the thallus, which does 
much of the work of assimilation. In Hydrobryum, 
Farmeria, Dicræa, and Griffithella, the secondary shoots are 
at first mere tufts of leaves, endogenously formed in acropetal 
succession on the root-thalli, but towards the flowering 
season they elongate and bear each a few bracts, a spathe, 
and a terminal flower. They are not very dorsiventral until 
the flowers develop, and then there is usually an extreme 
dorsiventrality, most marked in Hydrobryum and Farmeria. 
In these forms the primary axis is also reduced to a great 
degree. 
The most interesting general evolutionary features of the 
series of secondary shoots, then, are their gradual reduction 
in length, branching and complexity, and the diminution of 
the number of flowers to one, and finally the non-elongation 
of the axis until that one flower has to be developed upon it. 
The same kind of evolution seems to show in the American 
forms, from the large and complex secondary shoots of the 
Weddellinas and Moureras to the comparatively simple ones 
of the Podostemons and Mniopsides. 
