776 SUMMARY OF CURRENT RESEARCHES RELATING TO 



51 • 6 cell-walls 45 • 8-56 • 5 per cent. ; Buxus sempervirens, cell-cavities 

 of wood-cells 7 '9, cavities of vessels 9*8, walls of vessels and cells 

 82 '3 per cent.; Morus alba, cell-cavities (without vessels) 10-6-25; 

 cell-walls 75-89-4 per cent. 



The author retains his opinion that the cell-cavities and vessels of 

 wood are in no way necessary for the transport of the sap. This 

 movement takes place entirely in the cell-walls, in consequence of a 

 little-known property belonging to their internal nature. It is, no 

 doubt, to a certain extent influenced also by transpiration. 



Movement of Water in the "Wood.* — Both the prevalent theories 

 with regard to the causes of the ascent of the sap in woody plants — 

 that of imbibition, that it ascends through the porous walls of the 

 vessels, while the cell-cavities are filled with air, and that of gas- 

 pressure, that at the time of greatest transpiration the vessels are 

 filled partly with sap, partly with bubbles of rarefied air — depend on 

 the hypothesis that the cell-cavities or vessels of the wood contain air 

 under normal conditions. M. Scheit throws grave doubts on the 

 elementary fact on which both these theories are founded. The air- 

 bubbles constantly found in the vessels in microscopical sections have 

 probably entered in the process of dissection, and those said to have 

 been observed in sections under oil are certainly in some cases bubbles 

 of aqueous vapour. There are only two possible ways in which air 

 can reach the tracheids, through the stomata or through the root. 

 The first hypothesis is excluded by the fact that there is no direct 

 connection between the stomata or the intercellular spaces and the 

 vessels ; the second is very improbable ; it is difficult to understand 

 how air could pass through the fluid which permeates the parenchyma 

 and collect in bubbles. By a number of actual experiments on Ahies 

 halsaminea and excelsa, Taxus haccata, Acer platanoides, and Pteris 

 aquilina, Scheit also determined the impermeability to air of moist 

 wood and of the closing membrane of pits; the water-conducting 

 organs contain nothing but water either in the liquid or gaseous state. 



The author believes that the passage of water from the parenchyma 

 into the tracheids is greatly facilitated by the bordered pits. The 

 water is absorbed from the soil by the youngest parts of the roots and 

 the root-hairs by means of osmose ; the osmotic pressure is greatest 

 at the thinnest spots of the cell-wall, the pits; and, as far as the 

 elasticity of the closing membrane of the pits permits, this membrane 

 is pressed in towards the cavity of the adjoining vessel, and brought 

 into a position for filtration, so that water can now readily pass into 

 the vessel. The manometer indicates that this root pressure may 

 amount to as much as one atmosphere. The water thus pressed 

 into the empty vessels rises through capillarity, and the root pressure 

 has thenceforward nothing more to do than to place the closing 

 membrane of the pits in a position for filtration ; a continuous column 

 of water being thus formed in the plant. The whole plant is per- 

 meated by a system of capillary tubes having its lower end in a tissue 

 which absorbs water, the parenchyma of the root ; its upper end in a 



* Bot. Ztg., xlii. (1884) pp. 177-87, 193-202. 



