Grafts et al. — 156— Water in Plants 



based on evidence that the tracheal elements of the xylem contain gas. 

 CoPELAND (1902), in his review of work on the rise of sap, concluded that 

 the existence of chains of bubbles in the xylem (Jamin's chains) precludes 

 the possibility of tensile columns. He postulated atmospheric pressure as 

 the cause of the rise of sap. Much of the early work on this problem, which 

 CoPELAND reviewed, is invalidated by the fact that early workers did not 

 realize the ease with which bubbles could be formed in a tensile water column 

 by manipulation. 



Four principal methods have been used in modern studies on the state 

 of water in xylem ducts: (1) dye penetration; (2) freezing of the tissue; 

 (5) transpiration measurements; and (4) direct observation of the xylem 

 contents. 



Dye Penetration: — On the basis of injection of dyes Priestley (1935) 

 concluded that the mature wood of trees acts as a reservoir ; only differentiat- 

 ing elements conduct water. He contended that water is drawn from the 

 non-living vessels by the differentiating elements, and by some unexplained 

 process is driven into the expanding tissues of the leaves. By isolating in 

 early summer a branch from a tree having diffuse porous wood, Priestley 

 found that upon removing the bark he could inject the tracheae all around 

 the branch, the injections proceeding in rapid succession around the stem. 

 At a loss to explain where the water originally present before injection 

 went, he concluded that the tracheae had contained some gas at low pres- 

 sure. 



If the sap originally present in the branch was under tension, it would 

 be surprising if during the process of isolating the branch the tensile columns 

 had not broken. Probably much of the gas in the injected elements resulted 

 from the isolation treatment. Where, due to excessive stress, bubbles are 

 actually formed in stems, they may collapse during later periods when water 

 is more abundant. If water remains wanting, then such elements are iso- 

 lated by gum or tylose formation and new elements of smaller diameter may 

 be formed as a result of cambial activity. In ring porous woods there is 

 usually a direct correlation between the diameter of xylem elements and the 

 availability of water. Whether this is merely a reflection of the effect of 

 limiting water on turgor expansion during growth, or an adaptation on the 

 part of the plant to greater tensions in the xylem, the result is the same, 

 namely the production, as stress increases, of narrower and narrower ele- 

 ments that are less liable to bubble formation and more suitable for collapse 

 of bubbles once they are formed. 



MacDougal, Overton, and Smith (1929) studied the direction and 

 path of dye movement in various woods and concluded that they were able 

 to introduce the dye into the closed system of the conducting tubes. This 

 was accomplished by forcing the dye under one or two atmospheres pres- 

 sure into a hole in the stem, tightly fitted with a metal tube. The dye moved 

 tangentially from the hole into elements of the xylem which had not been 

 injured. From these experiments they concluded that since different species 

 of trees showed different dye patterns, the patterns are determined by the re- 

 lation of gas-filled to water-filled elements. Radial movement of the dye 

 from one annual ring to another was not observed. 



Baker and James (1933) criticized the results obtained by Mac- 

 Dougal, Overton, and Smith, and emphasized the fact that the introduc- 

 tion of a dye into the conducting system of a plant necessarily requires open- 

 mg it at some point with an alteration of the original conditions of tension. 



