Transpiration and the Ascent of Sap. 23 



According- to this theory ') the evaporating cells of the leaf lose 

 water and tlieir cell-walls are pressed inwards by atmospheric pressure. 

 Then the elasticity of the walls asserts itself, expands the cells and 

 draws water into them again. The water extracted from the tracheae 

 below reduces the air pressure in them and so the bubbles in still 

 lower tracheae expand and drive the water upwards. Each trachea 

 is supposed to act as a suction pump on the water in the tracheae 

 below, and the bubbles at each level are assumed to have a greater 

 pressure than those above. So that ultimately the external atmospheric 

 pressure is responsible for driving the water into the roots and for 

 lifting it to the leaves. Böhm believed that the weight of the water 

 was removed by the resistance opposed by the cross partitions, and- 

 by the immobility of the Jamin's chains. He concluded that the 

 movement of the water is a phenomenon of filtration depending on 

 differences of pressure in neighbouring tracheae. 



R. Hartig-) a few j^ears later adopted a view which, so far 

 as one can understand either, closely resembles B ohm's air-pressure 

 hj'pothesis. In the final form of his theory Hartig argued that the 

 function of the air-pressure was to make the pit-membranes permeable 

 and to press the water from one cell to its neighbour ; but it did not 

 act to raise the water from one organ to another. To emphasise this 

 point of view he prefers to call his theory the gas-pressure theory in 

 contradistinction to B ohm's air-pressure theory. 



Both investigators^) later recognised the inadequacy of their 

 explanations, and, as we shall see shortly, Böhm evolved a completely 

 different view as to the state of the water in the stem, retaining, 

 however, his peculiar ideas as to the function and action of the 

 evaporating cells. 



Open as these gas-pressure theories are to almost obvious physical 

 objections, the upholders of them did good service in maintaining 



') J. Böhm, Ueber die Ursache des Saftsteigens in den Pflanzen. Sitzuugsber. 

 d. Akad. d. Wiss. in Wien, 47, 1. 1863. Idem, Wird das Saftsteigen in den Pflanzen 

 durch Diffusion, Capillarität oder durch Luftdruck bewirkt? Sitzungsber. d. Akad. 

 d. Wiss. in Wien, 50, 1. 1864. Idem, Les causes de l'ascension de la sève. Ann. 

 des Sciences Nat. Bot., VI, 6. 1878. Idem, Ueber die Ursache der Wasserbewegung 

 und der geringen Lufttension in transpirirenden Pflanzen. Bot. Ztg., 1881, p. 801. 

 Idem, De la cause du mouvement de l'eau et de la faible pression de l'air dans les 

 plantes. Ann. des Sciences Nat. Bot., VI, 12. 1881. 



*) E. Hartig, Ueber die Vertheih;ng der organischen Substanz, des Wassers 

 und des Luftraumes in den Bäumen, und über die Ursache der Wasserbewegung in 

 transpirirenden Pflanzen. Unters, d. Forst. Bot. Inst. München, IL Berlin 1882. 

 Idem, Die Gasdrucktheorie. Berlin 1883. Idem, Die Wasserbeweguug in den Pflanzen. 

 Bot. Ztg., 41, 1883, pp. 250—255. 



^) ß. Hartig, Holzuntersuchungen. Berlin 1901, p. 9. J. Böhm, Papers 

 siuce 1889 passim. 



