i7 
Theories of Transition from Root to Stem. 
expressed his view in a series of diagrams which may he sum¬ 
marised in column 3 of the figure. In the very young root rows 
of undifferentiated cells connect the first-formed xylem vessel with 
the first-formed phloem element. Along these files of cells dif¬ 
ferentiation proceeds simultaneously from phloem and xylem poles, 
as indicated by the arrows: the cells in between the last dif¬ 
ferentiated primary xylem and phloem elements remain meri- 
stematic and become cambium, represented .by dotted lines in the 
diagram. The essential feature of the transition from root to stem, 
according to Bonnier’s view, is the displacement inwards of the pole 
of xylem differentiation. This, acting concurrently with a relative 
approximation of the phloem poles and a separation of the xylem 
pole into two halves, is the chief cause of the different arrangements 
of vascular tissue in the seedling axis: the lines of differentiation 
always connect xylem and phloem poles like lines of force between 
opposite magnetic poles, and the points of first differentiation 
appear in moving to drag the lines of xylem and phloem initials 
with them. 1 
The views of Bonnier and Van Tieghem have much in common. 
Both ignore the median polar protoxylem of the hypocotyl : in this 
they are in contrast with the schemes of Sterckx and of Chauveaud. 
Both regard the conducting strands of the whole seedling as con¬ 
tinuous morphologically: in this they agree with Chauveaud, but 
stand in sharp opposition to the analytical ideas of Sterckx which 
we will now consider. 2 
The philosophical basis of the theory of Sterckx is the funda¬ 
mental distinctness of the morphological categories of leaf, stem 
and root. The plant is considered as capable of being cleanly 
dissected into the morphological units of which it is composed ; 
the anatomy necessarily follows suit, and we have special leaf- 
traces, and special root-bundles. In the hypocotyl there is no 
confusion of structure ; the respective traces of leaves, cotyledons 
and root are here put into contact with one another : there is no 
passage from root to stem structure, but only a juxtaposition of 
tissues for physiological convenience. The diagrams in column 2 
of the figure, will shew how the transition is supposed to occur in 
1 The uppermost figure is not giyen by Bonnier, who indeed says 
very little about the transition phenomena beyond what is 
implicit in his diagrams. 
2 Sterckx is selected as the representative of a school of thought 
since he has worked more particularly at seedling anatomy. 
Other names which must be mentioned are those of Gravis, 
Vuillemin and Dangeard. The diagrams in columns 1 and 2 
are taken with some slight alterations from Sterckx’s paper. 
