212 MOVEMENTS AND EXCHANGES OF FLUIDS 



ried out with no great physical difficulty, and no special differen- 

 tiation of tissue for this purpose is necessary. When the leafy 

 crown to which the current of water must be led is separated 

 from the roots by a distance of many meters, on a huge trunk, 

 mostly composed of dead cells, special provisions may be looked 

 for to ensure the proper supply of water to the transpiring sur- 

 faces. The upward movement of sap takes place through the 

 xylem, and for the most part through the vessels and tracheids. 

 In woody stems of perennial plants, such as trunks of trees, the 

 movement is largely through the most recently formed layers, 

 and is most rapid in the elements formed in the spring or begin- 

 ning of the active season of growth, the mature wood partly losing 

 its capacity to transport water with age. Trees with a large 

 amount of sap wood, or alburnum, cany the water supply up 

 through a comparatively thick cylinder of wood, and hence must 

 be girdled deeply to be killed. The beeches and birches are 

 examples of this kind. On the other hand the wood of the oaks, 

 pines and cherries ages rapidly with respect to this capacity, and 

 a shallow girdling of such trees will cause death because of the 

 incapacity of the older wood to carry an adequate supply of water 

 to the crown. The path of sap through stems may be demon- 

 strated by allowing the plant to absorb and carry up reagents 

 which will stain the elements through which they pass or be 

 easily detected by chemical reactions. 



An earlier theory as to the method of the ascent of sap sup- 

 posed that fluids were conducted from the roots to the shoots 

 through the walls of the elements concerned only, and that the 

 lumina of the cells were empty or served as reservoirs for sur- 

 plus supply of the liquid. A certain amount of water undoubt- 

 edly does traverse the entire body of the plant along the walls, 

 but the quantity which might be conveyed from the roots to 

 the shoot by this path would be wholly inadequate to meet the 

 needs of actively transpiring leaves. That the current of water 

 from the base to the crown traverses the cavities of the vessels 

 and tracheids seems to be conclusively demonstrated by the fact 



