78 OAK. [CH. V 



paper between them, and then fixed into a hole cut in a 

 thick piece of card. The outline of the screw-head is the 

 outline of the united watch-glasses where they are let 

 into the card : the groove in the screw-head is the 

 oblique cleft which leads into the space between the 

 glasses. The structure will be understood from the 

 bordered pits shown in section in the walls of the 

 tracheid (3) in figure 34. A bordered pit is in fact a 

 thin place in the wall which allows water to pass laterally 

 from surrounding tissues into the cavity of the vessel; 

 the function of the protective "border" (the watch-glasses) 

 need not be discussed. 



The remaining elements of the xylem are wood-cells, 

 wood-fibres and tracheids. The cells of the wood paren- 

 chyma, as seen in longitudinal section, or when isolated 

 by maceration, are not unlike the medullary ray, seen in 

 tangential section. That is to say, they consist of what 

 was originally a single cambium cell divided into chambers 

 by horizontal walls. The wood parenchyma retains vitality 

 in its constituent cells, which like the medullary rays are 

 loaded with starch grains, especially in the winter. 



The tracheids (tr, fig. 37) and wood-fibres (f, fig. 37) 

 resemble the vessels and differ from wood parenchyma 

 and medullary rays in having no living protoplasmic 

 contents. The tracheids are in fact closely allied in 

 character and in function to vessels ; if in a line of 

 tracheids the transverse walls were to disappear such a 

 line would be a small vessel. Lake the vessels too, they 

 serve for water transport. In accordance with this 

 relationship we find that the pits (which are organs of 



