70 SPIRALS NATURE OF THE FIBRE. [BOOK i. 



is internal, because it projects beyond the inside of the vessel, 

 at every turn. Mr. E. J. Quekett proved this by the evidence 

 derived from fossil Palm- wood : he observed that a portion of 

 this substance readily broke down into minute fragments, 

 which, on examination under the microscope, were seen to be 

 composed of cylinders more or less elongated, and minute 

 rounded granules. Round the cylinders was wound a perfect 

 screw (with either a single or compound helix) fashioned from 

 the interior of the spiral vessel, and thus affording the most 

 satisfactory evidence that the spiral fibre is really formed in 

 the interior of the vessel, as most recent observers have main- 

 tained. (Annals of Natural History, vol. xv., p. 495.) 



It is more difficult to determine whether the fibre is solid, 

 or tubular, or flat like a strap ; and Amici has even declared 

 his belief that the question is not capable of solution with 

 such optical instruments as are now in use. When magnified 

 500 times in diameter, a fibre appears to be transparent in 

 the middle, and more or less opaque at the edges ; a circum- 

 stance which has no doubt given rise to the idea that it is a 

 strap or riband, with the edges either thickened, according to 

 De Candolle, or rolled inwards, according to Mirbel. But 

 it is also the property of a transparent cylinder to exhibit 

 this appearance when viewed by transmitted light, as any one 

 may satisfy himself by examining a bit of a thermometer tube. 

 A better mode of judging is, perhaps, to be found in the way 

 in which the fibre bends when the vessel is flattened. If it 

 were a flat thread, there would be no convexity at the angle 

 of flexure, but the external edge of the bend would be straight. 

 The fibre, however, always maintains its roundness, whatever 

 the degree of pressure that may be applied to it. (Plate II. 

 fig. 10.) This I think conclusive as to the roundness of the 

 fibre; but it does not determine the question of its being 

 tubular or solid. Bischoff, who has investigated the nature 

 of spiral vessels, asserts (De verd vasorum plantarum spiralium 

 Structurd et Functione Commentatio, 1829), that it is solid, 

 and this almost everybody is now agreed upon. But M. Girou 

 de Buzareingues asserts that it is hollow and contains fluid, 

 and he gives numerous excessively magnified figures to illus- 

 trate his statement. Hedwig also long since believed that, 



