Band 138. Nr. 22. XXXIX. Jahrgang. II. Bd. Nr. 22. 



Botanisches Centraiblatt 



Referierendes Organ 



der 



Association Internationale des Botanistes 

 für das Gesamtgebiet der Botanik. 



Herausgegeben unter der Leitung 

 des Präsidenten: des Vice- Präsidenten: des Secretärs: 



Dr. D. H. Seott. Prof. Dr. Wm. Trelease. Dr. J. P. LotSf. 



und der Redacttons-Commissions- Mitglieder : 



Prof. Dr. Wm. Trelease, Dr. G. Bonaventura, Ä. D. Cotton, 



Prof. Dr. 0. Wehmer und Mag. G. Christensen. 



von zahlreichen Specialredacteuren in den verschiedenen Ländern, 



Dr. J. P. Lotsy, Chefredacteur. 



No. 48. 



Abonnement für das halbe Jahr 15 Mark 

 durch alle Buchhandlungen und Postanstalten. 



1918. 



Alle für die Redaction bestimmten Sendungen sind zu richten an: 

 Redaction des Botanischen Centralblattes, Haarlem (Holland), Spaarnei?. 



Bailey, J. W., The structure of the bordered pits o.f co- 

 nifers and its bearing upon the tension hypothesis of 

 the ascent of sap in plant s. (Botan. Gazette. LXII. p. 133 — 

 142. 2 Fig. PI. 1. 1916.) 



Exception is taken to Statements that the merabranes in the 

 bordered pits of conifers form complete septa that are entirely * 

 impervious to finely divided solids and undissolved gases. 



Perforated pit membranes are clearly visible in properly stained 

 sections ol Larix and Sequoia. Owing to the minute size and tenuity 

 of pit membranes, their detailed structure is more or less obscured 

 by the thick, overhanging secondary walls. 



Aqueous Solutions, containing finely divided particles of car- 

 bon, can be made to pass through the membranes in the bordered 

 pits of sap wood taken from the stems of large specimens of Larix, 

 Sequoia, and other conifers. 



The presence of perforations in the membranes is also indicated 

 by the fact that large quantities of gases can be forced rapidly 

 through the bordered pits of tracheids that are thoroughly saturated 

 with sap. 



The surface tension of sap in the pit of membranes of various 

 conifers can be overcome by pressures of less than 3 atmospheres. 



The tension hypothesis of the ascent of sap in plants, as inter- 

 preted by Dixon, requires continuous columns of water, which at 

 the tops of trees, 75—300 feet in height, are subjected to tensions 

 of 5—20 atmospheres. In other words, the bordered pits must be 

 impervious to undissolved gases under pressures of 5—20 atmo- 

 spheres. 



Botan. Centraiblatt. Band 138. 1918. 22 



