262 BOTANICAL GAZETTE [October 



7 



tion, but not on the height^ is not an objection to the theory, 

 because in the crown there must be a poor opportunity to test 

 the tension in the same path twice ; and the transverse equili- 

 bration of tensions is limited even in Tilia, where it is easiest. 



The time and place of the occurrence of low tensions in trees 

 permit no doubt as to their being the result of transpiration. 

 They occur in the youngest wood, whence the water is drawn to 

 the leaves; and they are greatest in the summer season of most 

 active transpiration, in spite of (and also as a cause of) the fact 

 that absorption by the roots is most active then. The usual daily 

 variation is very conspicuous. In mosses with specialized con- 

 ducting strands energetic transpiration results in the same way 

 in the appearance of greatly rarefied gas (Haberlandt : 1883, and 

 1886: 416). Not only is it a general rule that the tension is 

 lower during the day, but in many cases the tension exceeds an 

 atmosphere by night, giving way to a relative suction when 

 transpiration becomes active, as was mentioned in discussing 

 root pressure. In our climate this occurs in summer only in 

 herbs, but Molisch (Ann. Jard. Bot. Buit. 1898) reports it in 

 lofty Javanese w^oody plants. 



That the lower tension following transpiration will have as 

 its result a forcing up of water from below until the tension is at 

 least in part eliminated is almost self-evident. I do not recall 

 seeing this questioned, except in one or two passages by 

 Strasburger. After finding that branches of Taxus and Tsuga 

 can absorb water under almost no outside pressure, he denies 

 (1891: 796) that the tension of bubbles in the wood has any 

 importance. And in speaking of the possibility of low tension 

 in the upper part of a tree (1893 : 60), where he says **So miisste 

 fiir diese Erscheinung eine andere Ursache als die der Wasser- 

 hebung gesucht werden," I am uncertain what ''Ursache" may 

 mean. But in many places Strasburger takes the other view. 

 For instance ( 1 89 1 : 871), " Die auf tracheler Saugung beruhende 

 Wasseraufnahme aus dem Boden kann dann (after transpiration 

 ceases to be active) fortdauern, bis dass die Bahn mit Wasser 

 gesattigt ist," and (1893: 81) **Vorallem muss aber die negative 



