Vegetable Physiology. 121 



little cup at its extremity : under a microscope, the structure 



f these is very beautifully developed. 

 The force with which the roots absorb fluid, and the quan- 

 tity of sap thereby formed, are remarkable. Experiments have 

 been made, by which this force may be measured, and it is 

 found to be very great. If the stem of a vine be cut off, in the 

 soring when the sap is ascending, and a piece of bladder be 

 tied firmly over the section of the part which remains, it will 

 soon burst from the force with which it is distended. The 

 attention of several distinguished observers has been directed 

 to the explanation of the means by which the absorbing action 

 of the roots and the force which accompanies it, are produced, 

 and such investigations have resulted in the adoption of the 

 theory of Endosmose and Exosmose. M. Dutrochet was the 

 first to promulgate this theory, and his experiments seem to 

 have established its truth. It has been before stated, that 

 although vegetable membrane has no visible pores, yet liquid 

 will pass through it. The same is found to be the case with 

 animal membrane. If, then, a thick syrup, or a mucilage, be 

 enclosed in a funnel-shaped glass vessel, over the mouth of 

 which a piece of bladder is tied, and then immersed in water, 

 a portion of the sugar or gum will pass through the bladder 

 into the water, and a portion of the water will pass upwards 

 into the vessel. It is the same with milk or any other liquid. 

 If, on the other hand, the vessel were filled with water, and 

 immersed in syrup, the result would be reversed. The gene- 

 ral law, deduced from such experiments, according to M. Du- 

 trochet, is, that when two fluids of unequal density are sepa- 

 rated by a membrane, the denser fluid will attract the less 

 dense. The principal current, or that from without inwards, 

 he calls Endosmose, (flowing in,) and the one from within out- 

 wards, Exosmose (flowing out). Now all the conditions of this 

 action are found in roots. The fluid already absorbed by 

 them is rendered denser than the water around, by the mixture 

 of the descending sap ; and the partitioning membrane is sup- 

 plied by the spongiole. Thus, as long as the fluid within is 

 more dense than that without, absorption will continue. The 

 Exosmose is shown to take place, by growing a plant with its 

 roots in water, when the fluid is soon found to be impregnated 



