1902] rise' OF THE TRANSPIRATION STREAM . 187 



Hohners work (1879). He found (p. 113) that a suction of 

 40-50^"" Hg was not rare as late in the year as the end of Octo- 

 ber, Bohm (1877) found a suction by Crataegus of 61'^"'; by 

 Syringa and some Pomaceae of 25"^°^; and by Platanus of only 

 16''"'; and (Bot. Zeit. 1881.: 824) even in March a suction by 

 Crataegus of 40^"". In one experiment (Bot. Zeit. 1S81 : 819) 

 mercury was jerked into a vessel of Robinia 117^"^, but this 

 was not a rise of that distance. 



In 1890 Bohm said that Salix with boiled roots would exert 

 a suction sufficient to lift mercury, as already quoted, **Stets 

 bis zur Barometerhohe." In 1893 he said that it was in 

 '-'successful" experiments that the mercury could be drawn as 

 high as the barometer by shoots whose lower end had been 

 cooked ; but only three experiments in 400 succeeded. In 

 one of these three the indicated suction was 2.2'^'" over an 

 atmosphere. We have already noted his results with conifers. 

 Strasburger ( 1 89 1 : 787) reports as the greatest observed 

 suction in any herb 28^" Hg [Amarantlms caudatus)\ in live 

 branches of dicotyledonous trees, 54''"'; in branches whose 

 lower end was boiled, 6y'''^\ and in conifers similarly treated, > 

 70^"^. Vines (1896) reported suctions of 116'^'^ and of II 

 inches by Helia?itluis mimins, 3 2 o™™ by Prumis Latirocerastts, 



^1% inches by Fagiis sylvatica, and 22]/^ inches by Taxus. 



These pressures are to be subtracted not from zero, but from one 

 atmosphere (Vines, Annals of Botany 10 : 644). Data obtained 

 by Th. Hartig, Schwendener (i8S6),and others, by inserting 

 manometers in the trunks of transpiring trees, agree that tensions 

 of zero are not closely approached. A part of these figures may 

 be lower than they should be, because they only indicate the 

 point at which air came out of the cut end of the stem and pre- 

 vented the absorption of any more water. 



On the other hand, figures obtained after removing air by 

 boiling or sucking it out of the cut end are likely to be too high. 

 There is no doubt that the transpiring cells could draw water 

 from the tracheae in spite of an absolute pull; that tensions of 

 zero or less are not transmitted and used to absorb water i^ 



