I02 TRANSPORT -OF WATER AND SOIL SOLUTES 



The Tracheal Tubes. — The tracheal tubes, or water tubes, 

 are fitted for carrying water by being essentially ' continuous 

 tubes from the finest branches of the roots up through the stem 

 and into and throughout the leaves. In some instances cross 

 walls have been found in them every 45 to 91 centimeters apart, 

 but the amount of resistance which these walls afford to the 

 ascent of water must be extremely slight compared to that which 

 would come from the 15,000 to 30,000 cross walls in a chain of 

 ordinary cells reaching to the same height. 



The average diameter of the tracheal tubes is approximately 

 .05 mm. They average the smallest in submerged water plants 

 where the need for them is not great, and the largest in tall-grow- 

 ing climbing or clambering plants where stems of small diameters 

 have to carry relatively large amounts of water through long 

 distances. In shrubs and trees the water tubes are larger in 

 the early growth of spring and summer, when the new crop of 

 leaves is to be supplied with water, than in the later growth 

 when the demand for a larger water-carrying capacity has been 

 almost or quite satisfied. 



It has been told in Chapter II that the walls of the tracheal 

 tubes are thickened after various patterns, spirally and annu- 

 larly (Fig. 19) in those that are first formed by the procam- 

 bium, and more extensively, leaving thin places in the form 

 of circular or transversely elongated pits in those that are last 

 formed by the procambium and later by the cambium (Fig. 

 19). The thickenings of the walls are needed to hold the tubes 

 open against growth and turgor pressures from the surrounding 

 tissues, pressures great enough to burst the bark even when this 

 is reenforced by a continuous sclerenchyma ring, as in the case 

 of Aristolochia (compare Figs. 23 and 24), and to crowd the 

 wood in to the complete obliteration of the pith. 



The thin places between the spiral and annular thickenings 

 and in the pits are of course needful to allow water and solutes 

 rea3ily to pass out to supply the surrounding tissues as they 

 need them, and to permit reserve food solutes to pass in when 

 after a period of storage, in the wood parenchyma and medul- 



