243 
The Rhizophore of Selaginella. 
and these expectations are clearly fulfilled in forms where the homo¬ 
geneous thallus of the lower forms has become thus specialised. The 
same is true of the Lichens. Again, the highest members of the 
Bryophytes, show the same intricate specialisation. In all Vascular 
plants precisely the same primary differentiation into shoot, leaf, 
and root occurs; we find these three organs universally distributed 
and each under such manifold forms, shewing evidence of having 
undergone such profound and varied modification, that we feel 
compelled to refer the origin of each organ to the very beginning 
of the evolution of vascular plants. 
Tints, from the primitive undifferentiated thallus out of which 
all plants have been evolved, there has proceeded, in all groups, as 
well in the gametophyte as in the sporophyte generation, a perfectly 
well-defined differentiation of the plant-body into three stereotyped, 
fixed categories of organs, viz., shoot, leaf, and root, and into none 
other. 
There can be no room in that region of the vegetable kingdom 
above the level of the homogeneous thalloid types for any organ 
which is neutral, intermediate or undifferentiated in character, for 
the preservation of any such organ is, as we have seen above, 
contrary to the whole trend of evolution, and there is no evidence 
for the existence of any such organ amongst the simpler types 
where we should first of all expect to find it. In these lower types, 
no matter in what group—Algae, Lichens, Bryophytes or Vascular 
Plants—we may choose to look, there exist the three organs : shoot, 
leaf and root, or their analogues, and (confining the discussion to 
the vegetative region, to which the rhizophore, which I shall here¬ 
after treat of, belongs) it seems impossible to discover any other 
organ but just these three. 
Supporters of the idea of the homologous alternation of 
generations, however, hold that certain organs, e.g., the sporangio- 
phore of the Pteridophytes, the rhizophore of Selaginella, the Stig- 
marian axis! may have originated as a specialised portion of the 
thallus of an Alga or Bryophyte before the categories of stem and 
leaf were differentiated. If the homologous alternation view is 
true, this may have been so, but is most unlikely. 1 cannot, how¬ 
ever, here enter in a discussion of this view, as it would involve 
that of the whole questioiTof alternation of generations, with regard 
to which I refer the reader to the literature on the subject. It is 
enough to say that in the present paper I write as a believer in the 
antithetic alternation of generations. 
