64 Eenry H. Dixon. 



AVith these facts befoi-e us it is evident that the theoiy of the 

 tensile sap not onlj" accounts for the rise of water in trees, but also 

 for its rise in sufficient quantities. ^) 



2. The greatest misunderstanding's have arisen with regard to 

 the necessity of continuous water-columns. It has been already pointed 

 out tliat, owing to the permeability of the woody walls, the water in 

 the wood of plants does form a continuous S)\stem which the fortuitously 

 placed bubbles are powerless to sever. This fact forbids us, as J o s t ^) 

 points out, to rigidly compare the w^ater in the wood to a column in 

 a glass-tube, but the permeability of the walls secures the continuity 

 on which, strangely enough, Jost casts doubt. It is by no means 

 necessary for the transmission of tension that the water should be 

 continuous in vertical series of tracheae. If one or more in such a 

 series contains air, the tension is transmitted in the tracheae filled 

 with water around those which contain air. The continuity of the 

 tensile water is not destroyed but its form is rendered more tortuous. 

 How devious maj' be the upward course of the water without 

 destroying the transpiration supply is shown by incision experiments 

 such as those carried out by S t r a s b u r g e r. ^) 



3. The observation of motion in water beside bubbles occupying 

 tracheae or other tubes has been responsible for the perfectly un- 

 w'arrantable suggestion that this motion takes place past all the 

 bubbles in the transpiration stream. The streaming of water past 

 bubbles has, so far as is recorded^), been observed only at small 

 heights above the water supply. In these cases the surface tension 

 forces of the bubbles have been sufficient to resist the forces tending 

 to distend the bubbles, and when evaporation takes place above, atmo- 

 spheric pressure shoves a stream past the bubbles which lie quite 

 passive in the stream. The water in these cases ascends past the 

 bubbles in just the same way as oil rises in a wick past the innumerable 

 menisci formed on its sides. 



The motion past the bubbles in the plant in this manner can 

 only take place in the lower parts and when the action of root 

 pressure or of atmospheric pressure is lifting the transpiration stream. 

 For the tension set up at the evaporating surfaces could not be trans- 

 mitted in this film. 



^) L. Jost, Lectures on Plant Physiology. Eng-, trans, by R. J. Harvey 

 Gibson. Oxford 1907, p. 72. 



^) L. Jost, loc. cit., p. 72. 



•■') Ed. Strasburg er, Ueber den Bau und Verrichtungen der Leitungsbahnen 

 in den Pflanzen. Jena 1901, pp. 599 et seq. 



^) J. Vesque, Observation direct du mouvement de l'eau dans les vaisseaux 

 des Plantes. Ann. des Sei. Nat. Bot., VI. Ser., Tome 15, 1882, pp. 5 et seq. Idem, Sur 

 l'interprétation d'une experience de Hales concernant le rôle des vaisseaux. Comptes 

 rendus, 97, 1883, p. 1085. Ed. Strasburger, loc. cit., pp. 697 et seq. 



