422 Hill and Freeman . — The Root-Structure 
and the peripheral parts of the stele thicken up before the 
more central. 
Fig. 1 1 illustrates the structure of a spine viewed in trans- 
verse section. A mature spine is seen to be made up chiefly 
of elements with very thick lignified walls, amongst which are 
scattered small isolated groups of phloem and conducting 
xylem-elements. The cortex of younger thorns possesses 
tannin-cells, and sometimes these elements may be found in 
the stele. 
In longitudinal section it is seen that fibres predominate ; 
the phloem pursues an irregular path, and cross-connexions 
occur between neighbouring groups. Sieve-plates may be 
observed at rare intervals, and the conducting xylem-elements 
are much longer in the central regions than those, already 
described, at the base. 
Summary. 
1. In the natural state the tuberous stem is aerial. 
2. The tuber possesses large spine-bearing roots, and smaller 
roots chiefly absorbent in function. The lateral rootlets of the 
latter, after dying off, leave behind a small hard spine similar 
to those of the larger roots. 
3. The large spines of the material examined, when in the 
young state, tapered off into a normal root-ending. This 
absorbing end of the lateral spinous root eventually separates 
off, leaving behind a perfect spine. 
4. Induration of the vascular cylinder of the thorn does not 
set in until the maximum size has been attained. 
5. The apex of the large spine-bearing roots has no definite 
calyptrogen layer. 
6. The central vessels of these organs are multinucleate 
and appear to be produced not merely by the obliteration of 
the end-walls of elements situated one above the other, but 
also partly by the breaking down of the lateral walls of con- 
tiguous*' cells in a transverse plane. 
7. Certain of the inner vessels of the xylem-rays, the 
