496 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 38 



experiments seem to show conclusively that both objections are 

 invalid. "Root-pressure" is certainly a very real phenomenon, going 

 on in uninjured normally metabolizing roots and showing a striking 

 diurnal rhythm that is reminiscent of many vital processes. The 

 fact that a bacterial contamination will stop secretion so suddenly 

 indicates the metabolic character of the process. And the force 

 developed is of a magnitude not to be despised. Six atmospheres 

 pressure is sufficient to sustain a 200-foot water column. This is 

 far higher than any tomato plant ever grows. Yet such a column 

 appears to be small in comparison with what the lifting power of 

 tomato roots is capable of sustaining. That is a force which is to be 

 reckoned with. It has been unappreciated in the past because, 

 before the development of the root-culture technique, only moribund 

 and abnormally metabolizing tissues could be studied. 



It is not suggested that mechanical factors, such as transpiration- 

 pull, cohesion, capillarity, etc., do not play a considerable role in sap 

 movement. Under conditions of high transpiration they probably 

 do account for the movement of large volumes of water through the 

 plant. It is merely pointed out that under certain conditions — such 

 as those prevailing in the spring when the maple sap is flowing, 

 though leaves have not been developed — some or all of these me- 

 chanical processes cease to function. At such times root-pressure or 

 its equivalent, stem-pressure, is quite adequate to provide for the 

 proper functioning of even the tallest trees. 



No attempt will be made at present to explain how this force is 

 developed. It may be said, however, that we are much interested 

 in determining if there is a diurnal rhythm in respiration rate to 

 correspond with the observed rhythm in rate of secretion. It will 

 take some rather delicate methods to determine that. But it is 

 believed that we have here a phenomenon which can be studied in 

 detail and which may throw some light on the mechanism of secretion 



SUMMARY 



Experiments have shown that excised tomato roots growing in 

 vitro secrete sap continuously and rhythmically from their proximal 

 ends. Methods of measuring the force of this secretion have been 

 developed. It is not retarded by opposed pressures of 90 pounds 

 per square inch. The secretion force, therefore, probably greatly 

 exceeds this value. Since a pressure of 90 pounds per square inch is 

 sufficient to raise water to a height of 200 feet, and since the existence 

 of such secretion pressures has been demonstrated in normally metab- 

 olizing, actively growing roots, it is concluded that "root-pressure" 

 may be a far more important factor in sap movement than has been 

 generally conceded. 



