THE SEEDLING AND YOUNG PLANT. 31 



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 

 caliber 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. 

 Consequently these groups of xylem vessels are said to 

 consist of spiral and pitted vessels, and their chief func- 

 tion 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. 



Keturning 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 

 distinct outlines ; these alternate groups are known as 



