320 CONDUCTING SYSTEM 



regarded even as approximately accurate. The attraction of the tori 

 cannot, of course, lead to an actually air- or water-tight sealing of the 

 pits ; Pappenheim and Strasburger have, in fact, shown experimentally, 

 that it merely results in a great reduction of the permeability of these 

 structures towards water and air. 



Precisely what part the valve action of the bordered pits plays in 

 connection with the ascent of sap, is not clear. Strasburger supposes, 

 that these pits represent an automatic mechanism for preventing the 

 low tension that arises in a partially depleted water-channel from being 

 relieved by the entrance of air, before the deficiency of water can be 

 made good ; for this reason, depleted vessels do not necessarily lose their 

 water-conducting capacity, whereas they undoubtedly would do so, if 

 large quantities of air were to gain admittance. 



In conclusion, a few sentences may be devoted to the hitherto like- 

 wise unsolved problem of the sources of the energy employed in raising 

 water to the tops of tall trees. Since this question really belongs to 

 the domain of pure physiology, it will, in the first instance, suffice to 

 remark that neither capillarity, nor atmospheric pressure, nor root- 

 pressure can supply the requisite energy, while it is quite inadmissible 

 to suppose that the water moves either by osmosis alone, or in the form 

 of vapour. A very considerable amount of energy may, however, 

 become available in consequence of transpiration, since this process 

 can, in certain circumstances, give rise to a very powerful osmotic 

 suction, particularly when the removal of water is rapid enough to 

 cause a large diminution of turgor in the mesophyll, which is the actual 

 site both of the evaporation and of the osmotic action. Under ordi- 

 nary conditions, however, no very great diminution of turgor actually 

 takes place. Hence the suggestion recently put forward by Dixon and 

 Joly, and independently by Askenasy, to the effect that the cohesive 

 power of water probably enables the suction produced by transpiration 

 to be transmitted to the root system, cannot seriously affect our 

 estimate of the value of osmotic suction as a source of energy. 



Elaborate calculations, and a careful consideration of the general be- 

 haviour of Jamin's chains, led Schwendener to conclude that the upward 

 suction produced in these chains as a result of transpiration is usually 

 restricted to the smaller twigs, and in all probability seldom extends 

 below the base of the leafy crown. As, on the other hand, the action 

 of root-pressure cannot as a rule raise water more than 1 or 2 metres 

 above the ground, in the case of our native deciduous tre^s, " the move- 

 ment of the Jamin's chains in that part of the trunk which lies 

 between the aforesaid limits can only be due to forces resident in the 

 trunk itself." Schwendener accordingly assumes, that the requisite 

 energy is furnished in some as yet unexplained fashion by the living 



