R. H. Compton. 
*4 
this stage to give a comparative account of the different aspects 
which the anatomical transition has borne in the eyes of certain 
prominent writers on the subject. 
The early recognition of the stem and root as distinct mor¬ 
phological units, different in appearance, function, direction of 
growth, appendages, &c., directed attention to the region of the 
main axis where the change from root to stem is located. This 
region became known as the collet , a term to which the most diverse 
connotations were assigned. The early attempts at definition had 
regard only to external features, but in 1849 Clos introduced anato¬ 
mical criteria into the discussion. It was only in 1858 that Naegeli 
pointed out the characteristic difference in the arrangement of the 
bundles of xylem and phloem in the stem and root of Dicotyledons. 
The work of Van Tieghem in 1871 gave a great incentive to the 
study of hypocotyl anatomy, and important papers, dealing with the 
transition phenomena of vascular plants, were published by Dodel, 
Mile. Goldsmith and Gerard. The aim of the research was 
essentially the same in each case—to define the collet in terms of 
anatomy. Root and stem were regarded as by nature differently 
constructed, and the problem was to discover the device by which 
the two are put into connection. But though root and stem were 
regarded as objectively distinct, it was not doubted that the vascular 
strands are continuous throughout the whole axis. The transition 
[French ; passage ] was conceived as a process of rearrangement, 
and the vascular strands were considered to contrive the connexion 
by means of splittings, rotations and fusions. No special interest 
attached to the transition structures, except a curiosity as to how 
the plant might tackle the awkward problem. The Three Types of 
Transition of Van Tieghem are merely descriptions of the manner 
in which this problem is solved in different cases. There is no need 
to discuss these Types in detail: the essential features of the 
DESCRIPTION OF FIGURES. 
Diagrams of the transition from root-structure (below) to stem- 
structure (above) in Angiosperms according to various authors. 
Column 1. Gerard. Black=xylem ; clear=phloem. 
,, 2. Sterckx. wc=median cotyledonary trace ; k:=lateral 
cotyledonary trace ; r=xylem pole of root. 
,, 3. Bonnier. #£=xylem pole ; //“phloem pole. 
,, 4. Chauveaud. *=xylem; />=phloem ; <i=alternate ; i= 
intermediate ; s=superposed : 1,2,3" • * 
=order of differentiation of vessels ; circles 
=vessels ; squares=sieve-tubes ; black= 
functional; clear=degenerating. 
