THE CONDUCTING CHANNELS 215 



with which the water is transferred in such channels causes almost the 

 whole supply to pass through them, for the upward suction is pro- 

 pagated extremely slowly in badly conducting tissues. Thus when, for 

 example, the youngest annual ring conducts water best, the transpiration- 

 current will pass mainly through this layer, but if the continuity of the 

 younger wood is destroyed, the older alburnum layers may still be able 

 to supply all the water required, the acceleration of the current through 

 these channels being apparently attained by an increase in the motive 

 power. Such increase would be produced by a diminution in the amount 

 of water present in the transpiring cells and the tissues connected with 

 them, for in this way an increased potential energy of absorption and 

 a more pronounced suction-force would be generated. 



An increased propulsive force becomes necessary when the sectional 

 area of active conducting tissue undergoes any diminution, for the current 

 must be so accelerated that the same amount of water shall pass through 

 in a given time. A local narrowing, as, for example, in a water-tap, only 

 exercises a slight influence upon the amount of water that issues forth, 

 whereas the interpolation of a long narrow tube produces a pronounced 

 effect. Similarly, it is of great importance in the conduction of water 

 whether a long or a short distance needs to be traversed in channels 

 which convey it with less freedom. 



These relationships and their attendant consequences must all be 

 considered in discussing the problems connected with the movements of 

 water in plants. As a matter of fact, the resistance to its passage varies at 

 different points, while at each point of connexion of different systems of 

 vascular bundles, such as those of the stem and leaf, the structural relation- 

 ships are such as to indicate that the transference of water must take place 

 to a certain extent in a different manner from that which is characteristic 

 of either system. When the continuity of the younger wood is interrupted 

 by a saw-cut, sufficient water may still pass through the older alburnum 

 layers, but it does not necessarily follow that the same would be the case 

 if, by removing a long cylinder of the young wood, the water were forced 

 to travel for a considerable distance through the older layers. All our 

 knowledge of the poorly conducting tissues shows that under normal 

 conditions the amount of water which they can transfer in virtue of their 

 relative sectional area does not suffice for the needs of the plant, although 

 in transpiring leaves the conductivity of the parenchyma cells is sufficient 

 for all normal requirements. Similarly in the root, owing to its enormous 

 absorptive surface, the slow transference through the parenchyma conveys 

 an abundance of water from the soil to the conducting channels. 



As their function demands, the fibrovascular strands, including the 

 tracheal elements, form a connected system, composed of a series of longi- 

 tudinal conducting channels. Owing to the lack of vessels in the rudi- 



