The Rhizophore of Selaginella. 247 
resembles the root of the same plant in the anatomical structure of 
its vascular parts, for it has a monarch structure of the stele with 
central protoxylem. Like a root, it is completely devoid of any 
lateral appendages in the form of foliar organs. Like a typical root 
it grows downwards to the soil and absorbs water through its 
tissues in a direction the reverse of that in which it has grown. It 
produces lateral endogenous roots. These characters are, in my 
opinion, not sufficient to warrant us placing the organ in the root- 
category. On the other hand it resembles a stem or shoot in the 
fact that it arises exogenously from the tissue of the leafy shoot at 
the region of forking ; but this in itself is not conclusive evidence 
in favour of its stem-nature, for exogenous roots are known in various 
plants. But as Bruchmann points out, the definite place of origin 
of the rhizophores at the place of forking, growing in a plane at 
right angles to that of the stem-fork, the whole forming a cross-like 
four-armed branching-system at that point, speaks strongly in 
favour of these organs being shoots. The absence of a root-cap is 
another character speaking in favour of its shoot-nature. The 
leafless character affords no argument against this idea; several 
cases are known of shoots which have for the purpose of some 
special function, lost the power of forming leaves, either over the 
whole or a part of their length, e.g., the rhizomes of Psilotacea:, 
the inflorescences of Cruciferae, the shoots of Tricliomanes 
ntembranaceuin. The anatomical structure is known to be a varying 
factor and is totally unreliable as an absolute determinant of the 
stem- or root-nature of an organ. For instance, the rhizome of 
Tricliomanes muscoides has a monarch stele-structure just like that 
of the rhizophore of Selaginella ; it has obviously been derived from 
the structure of other species by reduction of the metaxylem ; in the 
same way the rhizophore-structure could easily be obtained by 
reduction of one half of the xylem of the diarch stem-stele of a form 
like S. Martensii, along with elimination of the phloem on the outside 
of the protoxylem which is preserved. It is thus apparent that the 
stele-structure stands as a very small point in favour of the root- 
nature of the organ. It is only what we should expect, that the 
downward-growing direction, the leafless character, and fixation of 
the apical end in the soil should induce a root-like structure of the 
stele of an organ which was originally of shoot-nature. 
Pfeffer has placed on record some most interesting observations 
in which rhizophores of certain species (e.g., 5. inequalifolia) develop 
spontaneously into leafy shoots. 
