OUTLINES OF THE PHYSIOLOGY OF NUTRITION 227 



trees more than a hundred years old. In others, as 

 in the Oak, it is only the outer layers, forming the 

 sap-wood or alburnum, by which water is conducted. 

 In the latter case, if we repeat our ringing experiment, 

 but make a deeper incision than before, so as to remove 

 the outer, more lightly-coloured wood, as well as the 

 phloem, we get quite a different result. The leaves 

 wither and dry up, and after a time all transpiration 

 ceases ; the water supply has evidently been cut off 

 from the part of the tree above the incision 



Experiments in which plants are made to absorb 

 coloured fluids have proved that it is through the 

 vessels and tracheides of the wood that the water 

 ascends ; the fibres take no part in the process. 

 Whether the living cells of the woody parenchyma 

 and of the medullary rays do or do not help in the 

 conduction of water is still an open question. 



As to the causes of the ascent of water in the 

 wood, we are still very much in the dark. The 

 pressure in the living cells of the root, due to osmosis, 

 no doubt, has much to do with the forcing of water 

 into the vessels in the first instance. The constant 

 giving-off of watery vapour by the transpiring leaves 

 is also of great importance, for it causes a pull on the 

 water in the vessels below. The extremely low press- 

 ure in the vessels and tracheides must help the 

 movement of the short columns of water which they 

 contain, and this movement will tend to be in the 

 right direction, for it has been proved that the 

 pressure becomes still less as we approach the leaves 

 than it is in the lower parts of the stem. 



