K.— BOTANY 205 



beautifully demonstrated in the trunk of Fraxitius excelsior, in spring, 

 when the newly formed vessels lie just below the cambium. These 

 vessels are very long ; they have been traced, without a cross wall, for 

 more than 25 ft., and they often have a diameter appro.ximating to 

 o • I mm., so that they are readily seen with the naked eye. If the cambium 

 is exposed and the vessels are then cut open under Indian ink or coloured oil, 

 the liquid can be seen to enter, moving both upwards and downwards 

 in the tracheae with an astonishing speed, often more than i ft. in 

 three seconds. In this way vessels will rapidly fill often to a length of 

 more than 10 ft. from the point of injection. As the liquid enters, there 

 is no marked change in diameter of the vessel such as would suggest 

 a great release of tension, and, if the vessel had been originally full of 

 water, it is very difficult to explain where this water has been accommo- 

 dated when it is so rapidly displaced, particularly as vessel after vessel 

 can be injected, the liquid entering at practically the same speed. But 

 that the vessel originally contains water under tension seems to be com- 

 pletely negatived by a simple modification of this experiment. In most 

 trees the vessels are much shorter, and many closed vessels in isolated 

 branches, stripped of all leafy shoots, can be injected when they are cut 

 open under suitable liquids. These injection experiments, carried out 

 extensively in a different form by von Hohnel, seem to admit of no other 

 interpretation than that, in many of the tracheal elements, as the leaf 

 surface expands, water vapour displaces water. 



Sap Wood and Heart Wood. — Water vapour displaces water in the Ayood 

 beneath the opening buds both in the old wood and in the new ring of 

 wood in direct communication with the leaves ; and during the summer 

 water vapour continues to displace water in the tracheae of the older wood 

 throughout the axis, but is found first in the outer rings and then appears 

 progressively further inwards. In the autumn water absorption exceeds 

 evaporation even during the day, and the tracheae will begin once more 

 to fill with water, save that if any air has accumulated from the water 

 as the result of release from solution with rising temperatures, this air 

 cannot be driven out but is only slowly redissolved. In the older rings 

 of wood, where the tracheal content has fluctuated between water and 

 vapour over many seasons, air has gradually accumulated, in some trees 

 to a very considerable extent. This older wood is then often structurally 

 modified, so that it is distinguished as heart wood from the relatively air- 

 free sap wood, and the accumulation of air in the tracheae may prove to 

 be causally connected with these structural changes. Such heart wood 

 gives buoyancy to logs floating down streams to the lumber mill, because 

 the air within the tracheae is not readily displaced, but it probably plays 

 little part in the movement of water in the living tree, which takes place 

 mainly through the sap wood. 



The Ascent of Sap. — Through the complex tracheal system of the 

 sap wood the water supply is maintained to the foliage of even the 

 tallest trees, and the facts reviewed in the previous section are relevant 

 in this connection, though they do not in themselves supply a complete 

 explanation of the mechanism by which this movement is brought 

 about. 



