iv.] VARIOUS THEORIES, &c. 83 



Sachs and others assumed to be the conducting 

 agents here the result was as before, and I may pass 

 over the details. The same with Dicotyledons, and 

 here again it is interesting to note that the injected 

 cacao-butter penetrated through the vessels, not only 

 into surrounding elements, but into the starch cells 

 of the medullary rays. Mere sucking brought the fat 

 up the vessels and into the tracheides, &c. surrounding 

 them. 



Elfving expressly points out that this remarkable 

 injection of the xylem-parenchyma and medullary 

 rays always occurred by merely sucking with the 

 mouth, though afterwards a pressure of 25 cm. of 

 mercury failed to drive water through. Elfving's 

 conclusions are that wood loses its "conductibility " 

 as soon as the lumina are blocked, and that the rapid 

 ascent of water under consideration does not take 

 place in the walls of the elements. Since tracheae in 

 the widest sense (i.e. vessels and tracheides) are the 

 only elements which never fail, and are sometimes the 

 only elements present in the wood, and since they 

 always contain some water, and are provided with the 

 easily permeable pits, he concludes that they are the 

 conducting elements. The bordered pits are filters, 

 the ring being a support : the same is true for the rings 

 of the spiral, the thin parts acting as pit- membranes. 



G 2 



