184 TEXTBOOK OF PLANT PHYSIOLOGY 



order to traverse 207 cells, a total difference in suction tension of 

 20.5 atmospheres was required, or about 0.1 atmosphere per cell. 

 Although in other cases somewhat different figures were obtained, 

 still they were close to this value. 



The strong resistance met by water in moving from cell to cell 

 shows that long distances cannot be covered in this way ; hence, the 

 other means of translocation of water through dead, empty, 

 elongated cells with numerous pores, the tracheids, or still better 

 through hollow tubes with very thin septa, the tracheae or vessels. 

 Lower plants, such as the lichens and mosses, have no true vas- 

 cular systems. They, therefore, are small in size. The develop- 

 ment of tracheids in ferns was an important step forward in the 

 evolution of plants. 



61. Translocation of Water in the Vessels. Dynamics of the 

 Water Current. The Influence of Transpiration. — The vessels 

 and tracheids, being mere tubes filled with water, possess neither 

 suction nor any other power that may cause the movement of 

 water. Because of the very small dimensions of those elements, 

 capillarity might be considered as the only element developing 

 considerable force. Confronted by the problem of the upward 

 movement of water through the vessels, early investigators, there- 

 fore, tried to explain this phenomenon by capillarity. The source 

 of force causing the capillary rise of fluids in narrow tubes is the 

 force of adhesion shown by the concavity of the upper meniscus. 

 But the plant vessels are completely filled with water. No men- 

 isci are found in them. Moreover, if even such menisci existed, 

 with an average diameter of the vessels = 0.1 mm., water would 

 be able to rise on the strength of capillarity only about 30 cm. In 

 trees, the water has to be lifted many meters. Hence the theory 

 explaining the upward movement of water in the plant by capil- 

 larity was soon discarded. 



The water-conducting tissues being passive elements, the active 

 movers of the water current are the living cells which adjoin the 

 vascular system in its upper and lower end. At the base of the 

 plant, this work is performed by the cells of the root parenchyma 

 which are in close contact with the vessels, while above, it is accom- 

 plished by the cells of the leaf parenchyma, which clasp the tips 

 of vascular bundles (Fig. 82, also details in Fig. 80). As a result of 

 the activity of these cells, the water current moves through the 

 dead tissues of the wood. 



