24 
ORGANOGRAPHY. 
BOOK I. 
It is much 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 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 trans- 
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 judging 
is, perhaps, to be found in the way in which the fibre bends 
when the vessel is flattene*^. 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 11. 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 solid, 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 Botamk^ published in 1820, that they terminate in a 
conical manner; and in 1824 Dutrochct asserted, that they 
