98 LECTURE VI. 



immediately ; on reversing the position of the two ends the 

 phenomenon will be repeated. This cannot be explained by 

 assuming that the water placed on the upper surface simply 

 runs through open vessels to the lower surface, for the escape 

 of water at the lower surface takes place first from the young 

 wood (alburnum) which, in the Fir and other Conifers, contains 

 no vessels, but consists of completely closed wood-cells 

 (trachei'des). The true explanation is that the slight pressure 

 exercised by the layer of water on the upper surface is at once 

 transmitted to the water which saturates the walls .of the 

 wood-cells and which forms a continuous column, and con- 

 sequently an immediate escape of water takes place at the 

 lower surface. 



Although the experimental evidence given above applies 

 only to Dicotyledons and Conifers, yet it is equally true that 

 in Monocotyledons and in Vascular Cryptogams, the stems 

 of which differ considerably in their anatomy from those of 

 Dicotyledons and Conifers, the transpiration-current travels 

 in the lignified cell-walls. In these plants the amount of 

 wood in the fibro-vascular bundles is very small in relation to 

 the bulk of the stem ; it is probably insufficient to serve 

 as a channel for the supply of water to the leaves. It is 

 probable, as Sachs suggests, that the deficiency is made up 

 by the conduction of water through the walls of the. lignified 

 sclerenchymatous cells which form so large a portion of the 

 ground-tissue in the stems of these plants. 



Another account of the conduction of the transpiration current has 

 been given by Bohm. He states, and in this he is confirmed by Elfving 

 and by Hartig, that the conducting cells of the wood always contain some 

 water, even when transpiration is most active. He considers the mechan- 

 ism of conduction to be this ; that when water is withdrawn from a 

 conducting cell, the air which it contains becomes rarefied and the 

 tension in the cell is then less than in the neighbouring cells ; as a conse- 

 quence water is forced by filtration under pressure through the pits in the 

 wall of the cell until equilibrium is restored. Inasmuch as the air in the 

 conducting cells of the leaves is constantly undergoing rarefaction in 

 consequence of transpiration, a current is set up towards the leaves from 

 the stem. 



According to Hartig the duramen usually contains water, and it seems 



