OF CEYLON AND INDIA. 
405 
extraordinary plasticity of the skeleton-less root, and the 
parallel dorsiventrality of the vegetative and floral organs, 
the latter point in particular leading to important deductions 
bearing on the importance of correlation in evolution. 
The Primary Axis."^ 
We now know the construction of this organ in Lawia, 
some Dicræas, Podosternon subulatus, Hydrobryum liche- 
noides, H. olivaceum, Farmeria metzgerioides, and probably 
Willisia selaginoides, while it is not improbable that the tall 
floriferous shoot of Sphærothylax, as described by Warming 
(42, IV.), is also a primary axis. It is very much to be 
desired that the primary axis should be investigated in the 
other Podostemaceæ, especially in some of the Tristicheæ, 
as we may hope from it to get some light on the difficult 
problem of the phylogeny of these plants. Looking at 
such cases as the probable primary axis in Willisia and 
Sphærothylax, and the well grown hypocotyledonary shoot 
of Hydrobryum olivaceum, it would seem probable that the 
order at first consisted of forms with well grown primary 
axes, which in all likelihood were floriferous. From this 
stage the evolution would appear to have pro.ceeded in the 
direction of a gradual reduction of the primary axis, as the 
secondaries, and afterwards the thalius also, took over its 
functions of assimilation and flower-bearing. The primary 
axis in Hydrobryum lichenoides, and still more in Dicræa 
and Farmeria, is reduced to a very insignificant object, and 
after germination is over, and the thalius established, it 
appears to be of no further value in the economy, and does 
not bear flowers. 
The phyllotaxy of the primary shoot is usually complex, 
but in Podosternon subulatus it is approximately distichous 
like that of the secondary shoots. The leaves are usually 
almost exactly like those of the secondaries, and are of very 
simple construction, as in most water plants. The anatomy 
And of, preceding paper, p. 190. 
