38 TRANSPIRATION AND ASCENT OF SAP ch. 



wherein the lumen is choked, could account for so rapid 

 a flagging of the soft parts. However, to set this doubt 

 at rest, we inserted branches in water which had been 

 boiled and cooled in vacuo, and coated with oil after the 

 insertion of the branch. These, however, remained per- 

 fectly fresh ; indeed, they seemed in no way affected. 



Lumen blocked with ice. In order to investigate 

 this subject more fully, an additional series of experiments 

 was devised and carried out upon the passage of water 

 through the wood of Taxus at low and at high temperatures. 

 For it was very certain that in the one case the formation of 

 ice, and, in the other, the formation of steam, would occur 

 in the lumen before occurring in the wall, rendering the 

 former non-conducting without the introduction of any 

 foreign substance. 



It was necessary to determine first of all the freezing- 

 point of water in the lumen by direct microscopic observa- 

 tion. To effect this a special form of cold stage was used, 

 the construction of which will be easily understood from 

 Fig. 8. In this stage the object under examination is 



Fig. 8. 



completely surrounded by the cooling liquid, which also 

 flows round the bulb of the thermometer, t. The tempera- 

 ture is, therefore, accurately known. The bottom of 

 the cell is of glass ; a ring screwing out upon the top 

 serves to permit the lifting of a cover-glass acting as a 

 water-tight window, this being luted on the edge with a 

 little white lead. The object is luted between two cover- 

 glasses, and carried upon an open support within. It is 

 necessary to protect the upper window from moisture pre- 



