Chapter X — 185 — Loss and Retention 



may so alter the water supply (either by increasing the resistance due to 

 plugging of the tracheae with air, latex, or mucilage, or by decreasing the 

 resistance through the removal of the roots and much of the conduction 

 resistance) that completely abnormal results may be obtained (Table 45). 

 Cutting under water at a time of day when little or no tension exists should 

 avoid the plugging of xylem conductors with air and hence provide experi- 

 mental plant material most like that of the normal plant. A dramatic illus- 

 tration of the plugging action of latex can easily be observed on Poinsettia. 

 The flowers of this plant, when cut and placed in water, often begin to wilt 

 within a few hours. If the shoots are removed from the water and im- 

 mediately held with the lower 1 to 2 inches of their bases in a flame, the 

 heat and generated steam cause the latex plugs in the xylem to be blown 

 out. If then the ends, while still hot, are placed in water, a very rapid 

 recovery from wilting is often observed. The recovery may be so rapid, if 

 wilting has not progressed too far, that the leaves will move visibly, and 

 the drooping tip becomes erect within a few minutes. A similar effect 

 upon a wilted shoot plugged with air may be shown by sealing its base in 

 a bottle of water and aspirating it (Stocking, 1948). 



Table 45. — Transpiration of Solidago virga aurea in milligrams per gram fresh weight 

 per minute. Cut surface renewed hourly {data of Leick, 1939) : — 



Rooted plant Adjacent cut plant Difference in 

 Time of day richly watered standing in water per cent 



A further modification of the loss in weight method is one in which the 

 cut part is immediately weighed without placing in water and rapid sub- 

 sequent weighings made. Under favorable conditions the initial loss in 

 weight of the cut piece approaches that from the piece before cutting. 

 Studies have been made by this method which has proven fairly successful 

 for plants that transpire slowly (Huber, 1923; Ivanov, 1924, 1928) but 

 not for rapidly transpiring plants ( Maximov, 1929a) , 



Recently Weinmann and le Roux (1945) have made a critical analy- 

 sis of the rates of transpiration of barley, maize, oats, wheat, and tall 

 fescue plants before and after cutting. The transpiration of the intact 

 plant was measured in each case by weighing on a torsion balance and im- 

 mediately afterwards under identical atmospheric conditions the transpira- 

 tion of the cut shoot was determined. These experiments were carried out 

 on relatively young plants and agreement in the rate of water loss for three 

 minutes before and after cutting was poor. The average discrepancies 

 ranged from 49 per cent in maize to 106 per cent in barley. In only ap- 

 proximately 35 per cent of the experiments with wheat and only 10 per cent 

 of those with the other plants did the transpiration rates agree within 20 

 per cent. This further emphasizes the fact that extreme caution must be 

 used in interpreting the results of transpiration experiments on detached 

 plant parts. 



