Theories of Transition from Root to Stem. 21 
the vessels are differentiated radially and centripetally, and groups 
of vessels alternate with groups of sieve-tubes round the circumference 
of the stele : this is the so-called “ root-structure ” and is represented 
in column 4 of the figure by vessels 1, 2 and 3, and by sieve-tubes 
labelled pa. The “ disposition intennediaire ” is that exhibited by 
the tangentially arranged vessels 4 and 5 and the sieve-tubes pi. 
and is intermediate between the alternate and the superposed 
arrangements. In the “ disposition snperposee ” the vessels are 
differentiated radially and centrifugally, and lie on the same radius 
as the sieve-tubes : this is what is commonly known as “ stem- 
structure,” and is represented by vessels 6 and 7 and sieve-tubes ps. 
These three types of arrangement are distinguished for the 
sake of convenience in description : actually they merge into one 
another. Chauveaud has shewn that in the ontogeny of certain 
parts of the Phanerogam axis these three phases of structure succeed 
one another in time—the alternate arrangement being the first to 
appear, this being followed by the intermediate and this again by 
the superposed : and he holds that this successive realisation in 
ontogeny is recapitulatory of vascular phylogeny. If we take sections 
of the base of the cotyledon of Beta vulgaris, for instance, in a very 
young state, we find two or three vessels and sieve-tubes in the 
alternate position. At a somewhat later stage more vascular 
elements are differentiated, these being situated in the intermediate 
position, the vessels forming a wing on each side of the alternate 
vessels, the latter being in the process of degeneration. At the 
fully developed stage a section through the same point shews both 
alternate and intermediate vessels re-absorbed, and the functional 
elements in the superposed position. This process is represented 
in the second diagram of column 4, those vessels and sieve tubes 
functional at the end of ontogeny being shewn black, the 
degenerate elements clear. 
It may be noticed that the “ disposition superposte ” is capable 
of indefinite extension by means of the cambium. It is par excellence 
the position for continued growth, and as such it is adopted in the 
secondary thickening of plant members generally, root, stem and 
leaf. 
This ontogenetic sequence of arrangements is found in a great 
number of Phanerogamic roots, hypocotyls and cotyledons, and 
also occurs in certain Cryptogamic stems. It is held by Chauveaud 
to be also a phylogenetic sequence, In the Angiosperm stem with 
the rarest exceptions the “ disposition snperposee " alone is realised : 
