THE SEEDLING AND YOUNG PLANT 31 
the axial cylinder, and the comparison of numerous 
transverse sections at different levels of the root would 
prove that the smallest vessels are the first to develop; 
whence we learn two facts—namely, that the xylem 
vessels of the young root are developed in centripetal 
order, and that the later ones have a larger calibre 
than those formed earlier. 
If longitudinal sections are compared with these 
transverse ones—and I may here observe that it is only 
by means of numerous such comparisons that these 
matters have been gradually discovered—it is found that 
each vessel is a long tube, usually containing air and 
water when complete, the lateral walls of which are 
curiously and beautifully marked with characteristic 
thick and thin ornamentation. It must suffice here to 
say that the small, outer, first-formed vessels are marked 
with a spiral thickening, reminding one of caoutchouc 
gas-tubing kept open by means of a spiral wire inside; 
while the larger ones, developed later, usually have 
numerous small pits on their walls, reminding one of 
mouths, and the structure of which is very curious. Con- 
sequently these groups of xylem vessels are said to consist 
of spiral and pitted vessels, and their chief function is 
to convey water up the root to the stem (cf. fig. 16). 
Packed in between these vessels are certain cells known 
as the wood-cells. 
Returning to the transverse section, we saw that 
between each xylem group described above there is a 
group of structures differing from the latter in their less 
