Theories of Transition from Root to Stem. 19 
The theories of seedling structure considered so far have little 
reference to the evolution of the vascular system. The study of 
stelar ontogeny in the Filicineae has yielded rich phylogenetic 
inspiration : and it was expected that the seedling anatomy of 
Phanerogamae would display primitive features which might cast 
light on the ancestry of the group. This expectation has not been 
realised in the same almost diagrammatic fashion as among Pterido- 
phyta; but two main phylogenetic theories have emerged. In both 
of these there is no discussion of the stelar theory: it is the 
structure of the bundles which is under consideration. 
Miss Thomas is responsible for the theory of the double bundle 
of which mention has already been made. Speaking of the simple 
diarch seedlings of the Ranales and Rhoeadales she remarks “The 
striking feature in the cotyledon is the presence of a peculiar V- 
shaped bundle, which is obviously composed of two very definite 
halves, and has therefore been called a * double bundle.’ The apex 
of the V is occupied by a common group of protoxylem, while at the 
extremity of each arm is seated a phloem group.” With this we 
may compare the conception of the “ divergeant ” of Bertrand and 
Cornaille: this anatomical unit is defined as “ une masse libero- 
ligneuse comprenant un groupe tracheen d’ou partent deux lignes 
de differentiation ligneuse qui vont en s’ecartant.” The divergeant 
is a unit of great flexibility, and by means of a series of modifi¬ 
cations—divisions, reductions, inflexions and so on—it is shewn that 
the vascular structures of the petioles of Ferns can be analysed into 
systems of divergeants : the whole being expressed in a curious 
shorthand notation. Miss Thomas’ notion of the double bundle is 
almost equally fluid. “The double bundle,” she says, “is sometimes 
represented by two quite separate bundles.” And again, “ Cycas 
revoluta may have its two central bundles replaced by a single 
one”—this also seeming to be the case in the “ Anetnarrheua 
type ” of Miss Sargant. “ The homology of the extreme forms is 
proved beyond question by the existence of a perfect series of 
intermediate cases. The function of these central bundles is 
always the same viz., to form between them one pole of the root.” 
It must be noted in passing that Miss Thomas’ figures shew, what 
indeed is often very conspicuous, that the median protoxylem group 
is quite separate from the two lateral metaxylems with their super¬ 
posed phloems : in fact the term “ double bundle ” is something of 
a misnomer, and “triple bundle” or‘more simply “triad” better 
expresses the facts in such cases. Though the presence of the 
