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 

 tli an 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 



