How Substances are Transported and Removed 215 



2. Atmospheric pressure. This will suffice, when the suitable 

 conditions are provided, as they are in a pump, to raise water 

 some 32 feet, but no more; in the plant, however, the requisite 

 conditions are wanting, while this height is obviously quite in- 

 adequate. Therefore this cannot be the explanation. 



3. Capillarity. This is the power, as the reader will recall, 

 by which water, driven by its own internal molecular energy, 

 rises in small tubes, the higher the smaller the tube. But even 

 the slenderest ducts known to occur in plants are not small 

 enough to raise the water more than a few feet even if all the other 

 conditions were most favorable, which indeed they are not. 

 Therefore this cannot be the explanation. 



4. Imbibition. This was the favorite theory of the great 

 botanist Sachs, who defended it to the end of his life. He con- 

 ceived of the wall-system of the plant as a kind of gigantic con- 

 tinuous membrane, extending all the way from the root hairs to 

 the cells of the leaf; into this membrane, by forces and method 

 already considered, water was absorbed by imbibition, and raised 

 by the same energy, to be finally removed by evaporation at 

 the leaf-cells. The theory is simple and plausible, but is shattered 

 by one fatal fact, viz. it requires that the transpiration stream 

 shall move in the walls of the ducts, not their cavities (which 

 Sachs took simply for reservoirs), whereas experiment proves 

 beyond question that the water does move in the cavities. There- 

 fore this cannot be the explanation. 



5. Propulsion. This theory maintains that the water is 

 forced or propelled upwards by some action of the living cells 

 distributed along the course of the ducts, each living cell being 

 supposed to draw water from a lower duct and force it out into a 

 higher. It really is an extension of root pressure to the whole 

 stem, the living cells passing water from one duct to another 

 precisely as the root hairs and cortex pass it from the soil into the 

 ducts, and by the very same physical power and method, which 

 is still unknown in detail. It differs from the preceding explana- 



