328 CAUSE OF GENERAL MOTION. [BOOK n. 



circumstance prevented bearing its crop one year, the sap 

 that would have been expended accumulates, and powerfully 

 contributes to the abundance and perfection of the fruit of 

 the succeeding year. 



The cause of the motion of the sap is a subject which has 

 greatly excited curiosity, and given rise to numberless con- 

 jectures. It was for a long time believed that there was a 

 sort of circulation of the sap of plants, to and from a common 

 point, analogous to that of the blood of animals; but this 

 was rendered improbable by the well-known fact that a plant 

 is more analogous to a polype than to a simple animal ; that 

 it is a congeries of vital systems, acting indeed in concert, 

 but to a certain degree independent of each other, and that, 

 consequently, it has myriads of seats of life. It was, more- 

 over, experimentally disproved by Hales. This excellent 

 observer, whose Statics are an eternal monument of his 

 industry and skill, thought that the motion of the sap, the 

 rapidity of which he had found to be greatly influenced by 

 weather, depended upon the contraction and expansion of the 

 air, which exists in great quantities in the interior of plants. 

 Others have ascribed the motion to capillary attraction. 

 Knight was once of opinion that it depended upon a hygro- 

 metrical property of the plates of silver grain (medullary 

 rays), which traverse the stem in all directions. The same 

 physiologist considered that the mechanical agitation of stems 

 and branches by wind was favourable to the motion : he con- 

 fined the stem of a tree, so that it could vibrate only in one 

 plane; and, at the end of some years, he observed that its 

 section was an ellipse, whose greater axis lay in this plane. 

 Other theorists have called to their aid a supposed irritability 

 of the vessels ; but no contraction of the vessels has ever yet 

 been noticed, except under the influence of frost, as shown 

 by Biot. Du Petit Thouars suggests that it arises thus : In 

 the spring, as soon as vegetation commences, the extremities 

 of the branches and the buds begin to swell : the instant this 

 happens a certain quantity of sap is attracted out of the cir- 

 cumjacent tissue for the supply of those buds : the tissue, 

 which is thus emptied of its sap, is filled instantly by that 

 beneath or about it : this is in its turn replenished by the 



