MOTION OF THE SAP THROUGH PLANTS. 517 



almost entirely forgets the result of his own observations, and speaks 

 of it as taking place in the vessels, as if it were unnecessary to bring 

 forward proof of the fact. Link* evidently intends to bring forward 

 some testimony which will be found nearly approaching to the truth, 

 but he twice changes his views with regard to the contents of the 

 vessels ; I think, however, that he puts forth his views without having 

 studied the subject sufficiently. A very clever observer, who bestowed 

 eight days in the summer upon two hundred plants, in order to inquire 

 into this subject completely, convinced himself of the fact, that plants 

 in their perfectly formed spiral and porous vessels contain air only; 

 therefore, when quickly brought under water and examined, they 

 always appear dark. This holds good of our annual and perennial 

 plants, and of the tropical ones, at least in our hothouses. The repe- 

 tition of these experiments will convince every one, that no change 

 of seasons, or time of day, brings about any alteration in this fact, 

 except perhaps in some perennial dicotyledonous trees of our own 

 climate during some weeks of the spring, and under especial and 

 unnatural circumstances. Should this fact be once fully established, 

 there is no further place for what the botanists in general say about 

 the motion of sap, and a new course must be sought out. In what 

 follows I divide the subject into two parts: first, the question con- 

 cerning the absorption of the sap ; and, secondly, the course which the 

 sap takes through the plant. 



On the subject of the absorption of the sap, people have used the 

 unmeaning phrases, vital activity of the plant, vital attraction of the 

 sap through the vessels, &c. Dutrochet first noticed the phenomenon of 

 endosmosis, which gives a satisfactory explanation of the matter: no other 

 explanation has at present been given. The conditions of the existence 

 of endosmosis, namely, a fluid containing in solution gum, sugar, or 

 albuminous (mucous) substances, separated from the water of the soil, 

 impregnated with small particles of foreign substances, by membranes 

 easily penetrated, are found to exist in all plants ; so that, in order to 

 ascertain the force with which the sap rises in the plant, it is necessary 

 to observe carefully the process of the endosmosis. A solution of sugar 

 of IHOsp. gr., according to Dutrochet, caused the quicksilver in an 

 endosmotic apparatus, during two days, to rise 45" 9'", exhibiting a 

 pressure two and a half times greater than that of the atmosphere. In 

 all the experiments of Hales, Meyen, Mirbel, &c. on the vine, the quick- 

 silver never rose in so short a time above 15". If it even be allowed 

 that the sap ascends in the vessels as continuous tubes, there yet remains 

 a superabundance of power for the endosmose. This is, however, not 

 the case, and the endosmotic action is only exerted from cell to cell. 

 In this manner the pressure of the fluid above upon the universally dif- 

 fused endosmotic membranes is reduced to a minimum; and in the second 

 place, it is not probable that its collective effect is thereby increased : 

 but on this subject we have no experiments. There is here, however, 

 a great range of problems to solve, as, besides the various endosmotic 

 experiments in relation to the action of different kinds of endosmosis 

 one upon another, there is the consideration of its effects observed in 

 living plants, especially with regard to the contents of the cells, their 

 specific gravity, their elementary constitution at different heights in the 

 plant, &c. All this occurs in the rising of the spring sap in the trees of 



' Wiegmann's Archiv, 1841, vol. ii. p. 278. 



L L 3 



