FUNCTION.] GENERAL MOTION OF SAP. 325 



the earth at the foot of the tree freezes ; and, whether at that 

 time the roots are mechanically compressed by it, or whether 

 the duration of the cold causes contraction by a vital action, 

 the roots commence causing a considerable discharge of fluid 

 from the lower part of the apparatus. This goes on night and 

 day, except when the pipes to carry off the sap are frozen. 

 As soon as a thaw comes on and the earth is relaxed, the 

 roots, emptied of their juices, find themselves below their 

 point of saturation; they then emit nothing, but on the 

 contrary absorb the descending juices. I satisfied myself of 

 this not only by my apparatus, but in sawing through the 

 trunk of a large poplar tree, a yard from the ground. The 

 surface of the section of the stump was dry, but that of the 

 trunk itself dripped with water." *" 



The motion of the sap appears to be of two kinds ; 1. general, 

 and 2. special: these must be carefully distinguished. The 

 former is what has been alluded to in the preceding observa- 

 tions ; the latter is altogether of a different nature. 



Of General Motion. 



The course which is taken by the sap, after entering a plant, is 

 the first subject of consideration. The opinion of the old bota- 

 nists was, that it ascended from the roots, between the bark 

 and the wood : but this has been long disproved by modern 

 investigators, and especially by the experiments of Knight. 

 If a trunk is cut through in the spring, at the time the sap is 

 rising, this fluid will be found to exude more or less from all 

 parts of the surface of the section, except the hardest heart- 

 wood, but most copiously from the alburnum. If a branch 

 is cut half through at the same season, it will be found that, 

 while the lower face of the wound bleeds copiously, scarcely 

 any fluid exudes from the upper face ; from which, and other 

 facts, it has been fully ascertained that the sap rises through 

 the wood, and chiefly through the alburnum. It is related 

 by Berthellot, that the people of the Canaries tear off the 

 bark of the poisonous Euphorbia canariensis, and suck the 

 limpid sap of the alburnum, which, during its ascent in that 

 part, undergoes but little alteration from its condition when 



