TRANSPIRATION. 277 



scenes of its activity, and consequently in fully formed wood-cells and vessels livino- 

 protoplasmic contents are wanting. They must be regarded in a certain sense as 

 dead structures, for they have no further power of growth, and the reciprocal 

 pressure of wall and contents observable in absorptive cells and other cell-cavities 

 occupied by living protoplasm, which has been termed " turgescence ", is never seen 

 in them. 



In the walls of the wood-cells as well as of the vessels, woody material (Lignin) 

 is deposited. It appears to be in consequence of this that they are much less 

 capable of swelling than are cell-walls which consist chiefly of cellulose. The 

 amount of sap which presses its way in between the groups of molecules of the 

 lignified walls, and with which these walls are saturated, is also comparatively very 

 small. On the other hand, of course, this imbibed sap is conducted much more 

 quickly through the lignified walls of the cell chambers and tubes than through non- 

 lignified walls. More fluid is carried up by the intermolecular stream through the 

 woody walls of the cells and vessels than by the ascension of the raw nutritive sap 

 in the interior of the wood-cells and tubes. If no evaporation is going on from the 

 leaves, or if this is only very slight, the vessels and cells become filled with sap. As 

 soon as transpiration becomes active, part of the sap is taken up, and if fresh 

 supplies do not arrive quickly enough a limited amount of air can get in temporarily, 

 which of course must be in a very rarefied condition on account of the obstacles 

 which oppose its entrance. The passage of the sap is quicker through the non- 

 septate vessels than through the much shorter woody cells. The sap on its way 

 through the latter, to the transpiring leaves, must filter through innumerable trans- 

 verse walls. This filtration will of course be materially helped by the bordered pits 

 with which the wood -cells are so regularly provided; for the extremely delicate 

 membrane which is stretched between the two cavities of such an apparatus at any 

 rate allows the sap to pass through very easily. The bordered pits are exactly like 

 clack-valves, and they also appear to regulate the sap-stream, though the way in 

 which they do this is not yet completely understood. The nearer the path of the 

 raw sap approaches to the spots in which evaporation is being carried on, the greater 

 is the number of cells in the sap-conducting strands, while the vessels in the same 

 become fewer and fewer. The termination of the whole sap-conducting apparatus 

 consists entirely of cells whose walls are stifiened by spiral bands on the inside. 

 Between this termination and the transpiring cells some parenchymatous cells with 

 living protoplasmic cell-contents are interposed, whereas, it must again be insisted, 

 the tubes and chambers composing the sap-conducting apparatus have no living 

 protoplasm in their interior. 



The whole mechanism for the transmission of the raw nutritive sap may be con- 

 sidered as a system of tubes and chambers provided with clack-valves, into which 

 the fluid taken up by the absorbent root-cells is forced, and through which it is con- 

 ducted to the transpiring cells of the green leaves or of the green cortex, which takes 

 the place of the green leaves in leafless branches. This does not exclude the activity 

 of cells at certain levels, as it were at intermediate stages of the road traversed by 



