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A TEXTBOOK OF THEORETICAL BOTANY 





no part whatever in conduction. Water storage in the duramen is confined 

 to the Hving cells of the medullary rays and xylem parenchyma, but as the 

 conducting elements pass out of action they are often filled with resinous or 

 gummy substances and tannins, and the walls may be impregnated with 

 colouring matters, an example of which is Haematoxylin, which oxidizes to the 

 well-known dye, Logwood. Mineral substances, such as Calcium carbonate 

 in Fagus and Uhmis, Silica in Tectona graudis (Teak) or Calcium oxalate in 



Fig. 881. — Laburmim anagyroides. Transverse section of a 

 woody stem showing alburnum and duramen traversed 

 by light-coloured medullary rays. 



Chloroxyloti (Satinwood) may also impregnate the heart wood, adding greatly 

 to its weight and durability and enhancing its value as a strong core in the 

 tree trunk. 



Vessels which are passing out of action often become partially or wholly 

 filled by sac-like ingrowths from the living parenchyma cells which are in 

 contact with them. The pits between parenchyma and vessels are not 

 bordered on the side of the former, so that they present a large surface of pit 

 membrane, which is expanded by turgor pressure into the cavity of the 

 vessel. Such ingrowths are called tyloses (Fig. 882). They are balloon- 

 like, cylindrical or even branched, and have thin walls, but occasionally they 

 may be lignified, either like stone cells or like the walls of the vessel in which 

 they lie. What determines these differences is unknown. 



The young vessel elements usually expand laterally very rapidly, reaching 

 their full diameter even while still in contact with the cambium, and creating 

 considerable tissue pressures, while tracheids do not thus expand ; yet there 



