Transpiration and the Ascent of Sap. w 



Vital Theories. 



G od lew ski M in 1884 formulated a view which won many sup- 

 porters, and which, with modifications, still seems to appeal to many. 

 He assumed a periodic change in the permeability of the osmotic 

 membranes of the parenchymatous cells contained within the wood 

 in order to bring about a pumping action which would account for 

 the raising of w^ater in the tracheae of the stem. Thus, supposing 

 a cell of a medullary ray in contact with 8 tracheae — 4 on each 

 side — to draw water into itself and to increase its turgor so that 

 its protoplasmic membrane is considerably stretched, and assuming the 

 osmotic pressure of the cell and the resistance to filtration of the 

 membrane opposite to one trachea to be periodically and suddenly 

 diminished owing to a chemical change, then it is evident that the 

 contractility of the protoplasm wall cause water to escape through 

 the most permeable spot of the membrane, viz. into the trachea 

 opposite to which filtration is most easy. Once in the trachea God- 

 1 e w s k i assumed it to move upwards until it was drawn into a 

 medullary ray cell lying at a higher level in the stem. The reason 

 given for the motion upwards in the trachea rather than dow^nwards 

 in obedience to the gravitational force, is because the air pressure in 

 the tracheae above is less than in those at lower levels. For this 

 difference of pressure in the air bubbles in the stem G o d 1 e w s k i 

 relies on Hartig's results.^) Hart ig had indeed previously shown 

 that the percentage of air in the higher parts of stems is less than 

 that in the lower. Godlewski claimed for his hypothesis that it 

 explained the relations of the tracheae to the parenchymatous tissues, 

 the radial position of the bordered pits, which facilitates a staircase 

 motion of the water upwards in the stem, and the radial intercellular 

 spaces along the medullary rays, which afford the aeration necessary 

 for the respiratory liberation of energy in these cells. 



Janse^) supported Godlewski in a general way and was one 

 of the first to point out that, if the low^er part of a branch be killed, 

 the leaves above fade as a rule wdthin a few days. This result is 

 ■obtained even when the branch remains attached to its supporting 



^) E. Godlewski, Zur Theorie der Wasserbewegung in den Pflanzen. Jahrb. 

 f. wiss. Bot., 15, 1884, p. 569 et seq. 



'■*) R. H artig, Ueber die Vertheilung der organischen Substanz, des Wassers 

 und Luftraumes in den Bäumen, und über die Ur.sache der Wasserbewegung in 

 transpirirendeu Pflanzen. Untersuchungen aus d. Forst. Bot. Inst, zu München, II 

 und III. 1882—1883. 



•■') J. M. Janse, Die Mitwirkung der Markstrahlen bei der Wasserbewegung 

 im Holz. Jahrb. f. Wiss. Bot.. 18. 1887. 



