24 ORGANOGRAPHY. BOOK I. 



It is much more difficult to determine whether the fibre is 

 solid, or tubvilar, 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 circumstance 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 ti'ans- 

 parent cylinder to exhibit this appearance when viewed by 

 transmitted light, as any one may satisfy himself by examin- 

 ing a bit of a thermometer tube. A better mode of judo-ino- 

 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 pres- 

 sure that I have been able to apply 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. I should have been induced to think, with BischofF, 

 who has investigated the nature of spiral vessels with singular 

 skill {De vera Vasorum Plantarum spiralium Structurd et Func- 

 tione Commentation 1829), that it is sohd, if it did not appear 

 to have been ascertained by Hedwig that, when coloured 

 fluids rise in spiral vessels, they follow the direction of the 

 spires. This fact may, however, be explained upon the sup- 

 position that they rise in the channels formed by the approxi- 

 mation of cylindrical fibres, and not in the fibres themselves ; 

 in which case there could be little doubt that the fibres are 

 really solid. 



The nature of the termination of spiral vessels is now 

 placed beyond all doubt, by the preparations of Mr. Valentine, 

 above alluded to, and by some observations of my own. 

 It was stated by Nees von Esenbeck, in his Handhuch 

 der Botanik, published in 1820, that they terminate in a 

 conical manner; and hi 1824 Dutrochet asserted, that they 



