CHAP. I. 
VASCULAR TISSUE. 
29 
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 ther- 
mometer 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 con- 
vexity 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. Bischolf, wTo has in- 
vestigated the nature of spiral vessels, asserts [De vera vasorum 
plantarum spiralium Structura et Functione Commentatio, 1829 ), 
that it is solid, and this agrees with my own observations. 
But M. Girou de Buzareingues states that it is hollow and 
contains fluid, and he gives numerous excessively magnified 
figures to illustrate his statement. Hedwig also long since 
believed that, when coloured fluids rise in spiral vessels, he 
saw them follow the direction of the spires. This last fact 
may, however, be explained upon the supposition that they 
rise in the channels formed by the approximation of cylin - 
drical fibres, and not in the fibres themselves; in which case 
there could be little doubt that the fibres are really solid ; and 
I must declare that I can find no such appearances as those 
described by M. de Buzareingues. 
The last-mentioned physiologist states, that the fibre often 
runs between two cylindrical tubes, so that there is not only 
an outer membrane, but an inner one also. He adds that the 
inner tube contains air, but that fluid is lodged in the space 
between the two tubes. These observations cannot be re- 
peated, for the learned author on no occasion names the plants 
in which he has remarked these peculiarities of structure, which 
have hitherto escaped the most skilful vegetable anatomfists. 
