268 ANATOMY OP STEMS. 



turn, or movement downwards, is the greater owing to 

 the pressure of the ascending sap, it must necessarily 

 advance ; and this contraction being repeated in eve- 

 ry successive coil, the fluid is moved forward with a 

 sufficient impetus ; while the new quantity of sap 

 which supplies the place of that carried forward, and 

 which rushes into the coil at the instant of its relaxa- 

 tion, forming the basis of resistance to the return of 

 the portion before it, and at the same time exciting a 

 renewal of the contraction, its progression must be 

 uninterrupted. These appear, indeed, to be the on- 

 ly vegetable vessels endowed with contractility, or 

 which act in any manner analogous to the arteries of 

 animals. If this hypothesis be tenable, the spiral ves- 

 sels are the sap vessels of the succulent stem and the 

 annual shoot of dicotyledonous ligneous plants ; and 

 their spiral structure is essential for the performance 

 of their conducting function, in the spongy Medullary 

 Sheath, or cellular parenchyma in which they are im- 

 bedded. 



Malpighi regarded the spiral vessels as bronchia, or 

 air-vessels, and the same opinion is supported by 

 Grew, Hales, and Du Hamel. This supposition pro- 

 bably originated from their always appearing empty 

 when examined ; and on the same account the ani- 

 mal arteries were regarded as air-vessels by the an- 

 cients and their followers, until Harvey demonstrated 

 them to be blood-vessels. Grew also suggested the 

 idea of the spiral vessels acting as sap-vessels, and Du 

 Hamel supposed he had detected them in the per- 

 formance of this function, as did also the celebrated 

 Hedwig. Dr. Darwin may perhaps, however, be re- 

 garded as actually the first who taught that the spiral 

 vessels convey fluids. 



The Medulla or Pith. Returning to our shoot 

 of Horse Chesnut, we find the tube which is formed 



