of Dioscorea prehensi Us. 421 
Their course is sinuous, and as the apex is reached they 
anastomose with one another; hence just behind the extreme 
ap'ex of a young spine it is found that there are but few 
phloem-groups, and these are arranged in quite a normal 
manner (Fig. 9, a-e). 
This wide scattering of the phloem, coupled with the 
numerous connexions with the corresponding tissue of the 
main root, is probably to be correlated with the induration 
of the fully developed spine ; for it is obvious that during 
the process of lignification much material, relatively speaking, 
must be required and must be well distributed for the purpose. 
The arrangement of the tissue in question is excellently 
adapted to facilitate such a process of lignification on a large 
scale. 
Tracing the course of the xylem in a similar manner, it has 
already been stated that at the extreme base of the thorn the 
xylem is chiefly restricted to two large peripheral (just within 
the phloem-ring) masses, somewhat crescentic in shape, and 
that between these, smaller groups — which are the first to be 
connected with the xylem of the parent root — occur. The 
junction is effected chiefly by very short tracheides. 
Sections cut nearer the main root exhibit longitudinal 
strands of xylem between the two crescentic masses ; these 
represent the xylem rays of the parent root. 
Passing towards the apex of the spine it is found that the 
xylem is more evenly distributed, and is generally restricted 
to the more central regions. As the apex is reached it 
diminishes in amount and gradually takes up a position 
towards the centre of the stele (Fig. 10). At the extreme 
apex it is generally aggregated into three or four groups 
alternating with the phloem in a normal root-like manner. 
The abnormalities above described may at first sight appear 
remarkable, especially when a fully matured spine is examined; 
but it is to be borne in mind that no induration of the 
parenchymatous tissue sets in until the lateral root has 
attained its maximum size. The soft ground tissue of the 
basal region of the spine is the first to become lignified, 
