1917] CURRENT LITERATURE 163 



will have value no doubt in preventing useless attempts along the same lines. 

 However, they provide Jost the opportunity to discuss the cohesion theory 

 and to present his own views of it. 



The actual results add little that is new. He found that suction on the 

 root causes an increase in water delivery, especially in plants which normally 

 exude sap on being cut, and that greater suction causes a greater root excretion 

 of water than low suction. But there seems to be no proportion between 

 amount of suction and rate of water delivery, as should be the case if the root 

 acts merely as a filter in water intake. The maximum suction possible with 

 an air pump could never cause a delivery sufficient to cover even a moderately 

 estimated transpiration need. Completely surrounding the root with water, 

 or replacing the atmosphere about the roots with hydrogen, or excessive cooling 

 of the roots, leads to noticeable decrease of water delivery, even with strong 

 suction. 



Tensions existing in the water columns of the intact plant were demon- 

 strated by the rapid intake of water by freshly cut tops, although the plants 

 previously had been kept in condition of low transpiration. Suction on the 

 cut end of the top, or pressure exerted upon it, produces only a temporary 

 decrease or increase respectively in water intake by the top. It was found 

 that plants could continue transpiration essentially undiminished at real 

 negative pressures of 15-25 cm. 



Jost comes to the conclusion that, even in low plants like Cobaea, Chamae- 

 cyparis, etc., just as in tall trees, very considerable negative pressures must 

 exist, if we assume that the osmotic pull of transpiring leaves must pull in 

 sufficient water through a passive filter-like root. 



In the concluding section he questions whether it is possible for high 

 negative pressures to exist continuously in the stems of plants, and proceeds 

 to answer the question by subjecting stem tissues to high gas pressures. In 

 Ficus Carica he found all the vessels easily penetrated by air, this experience 

 running counter to the recent work of Renner and Holle, which shows that 

 there may be two kinds of vessels present, storage and conducting, the latter 

 being more or less impermeable to air, and providing the cohering water 

 columns. In such a case as Ficus, Jost thinks that those who hold to the 

 cohesion theory must suppose either that there are no cohesion phenomena in 

 plants with nothing but tracheae, or that certain vessels remain with cohering 

 water columns by pure accident, or that in closed vessels entirely different 

 conditions obtain from those which have been cut. The question of unbroken 

 columns is vital to the cohesion theory, and those who adhere to it must expect 

 to be called upon to prove the existence of cohering columns of water which 

 can remain unbroken under the reduced or negative pressures involved in nor- 

 mal transpiration. The conservative view of so prominent an authority on 

 plant physiology will help us to maintain a balanced perspective with reference 

 to this important problem. — Charles A. Shull. 



