TRANSLOCATION OF WATER 



193 



as these bubbles would interfere with the translocation of water 

 and would, moreover, rupture water threads at such high 

 tensions as are observed in the trunks of trees. It is known now 

 that air is present only in the older wood, which has ceased to 

 function for water conduction. The younger parts, through 

 which water moves, contain no air. Air that enters the vessels as 

 a result of mechanical injury, for instance in wounding, is not able 

 to move farther than through the first septum. Such injuries will 

 cause the exclusion of only small portions 

 of the general system of water supply 

 (Fig. 89). The presence of cross-septa in 

 the vessels, which show of course a con- 

 siderable resistance to the translocation 

 of water in the wood, is an indispensable 

 condition for the regular functioning of 

 the whole water-conducting system. The 

 air which clogs the vessels is frequently 

 dissolved again and the elements having 

 temporarily suspended their functions 

 begin once more to take part in the con- 

 duction of water. 



64. The Rate of Water Movement in 

 Plants. Resistance of Wood. Participa- 

 tion of Intermediate Factors in Water 

 Transportation. — As wood consists of 

 exceedingly narrow tubes, which more- 

 over are provided with septa, it must show a strong resistance 

 to the ascending water. Naturally, this resistance becomes an 

 additional encumbrance to the cells of the leaf parenchyma. 



This additional resistance was formerly rated very high, con- 

 sidering the pressure necessary to force water through septa in the 

 wood. The comparatively low rate at which the water moved in 

 the plant, however, was not taken into consideration. Exact 

 determinations by Ursprung have shown that with increase in 

 height in the trunk, the suction tension of the parenchyma cells 

 adjoining the vessels increases. But this increase constitutes only 

 0.3 to 0.4 atmosphere per meter in height. This is a small value 

 indeed when compared with the increase in tension of water moving 

 through the parenchyma. In this movement from cell to cell, the 

 suction force increases by 0.1 atmosphere. If the average dimen- 



Fig. 89. — Vascular bundle of 

 a wilting leaf, into which air 

 has entered in cutting. Vessels 

 filled with air cross-hatched; 

 vessels filled with water in 

 outline (after Bode). 



