CHAP. I. WOODY TISSUE. 19 



smaller. It must, however, be observed, that the fibres of 

 this plant, as used in linen-making, are by no means in a 

 state of final separation, each of the finest that meets the 

 naked eye being in reality a bundle of tubes. While some do 

 not exceed 3-5^0^ of an inch in diameter, others have a diameter 

 as considerable as that of ordinary cellular tissue itself; in 

 Coniferee the tubes are often ^ho o^' 3005 and in the Lime they 

 average about 1^^. Link states {Elementa, p. 85.) that they 

 are very large in trees of hot countries, as, for instance, the 

 Brazilian coffee. 



There are three distinct kinds of woody tissue : — 

 L That in which the walls are not occupied with either 

 granules or glands sticking to them, or in which the former 

 are of very rare occurrence. (Plate IL fig. L) This is the 

 finest and the commonest of all ; and is also the most genuine 

 state of woody tissue. 



2. That in which the walls have uniformly considerable 

 numbers of granules of regular size sticking to them in a 

 scattered manner. (Plate IL fig. 3, 4, 5.) These granules 

 have been and are still considered by many anatomists as 

 pores in the sides of the tissue. They have been, in parti- 

 cular, so described and represented lately by Adolphe Bron- 

 gniart in Cycadeae, in which the tubes are large, and the 

 appearance very conspicuous. ' 4nnales des Sciences, vol. xvi. 

 tab. 2L) But I thuik it possible to demonstrate that this is 

 an optical deception, and that the supposed perforations are 

 semitransparent granules. In the first place, no colourless 

 light passes through the supposed pores in any case ; on the 

 contrary, they are dark, and have a solid appearance at all 

 times, except when, at a certain distance out of the focus of 

 the microscope, they become luminous. Secondly, if they 

 were holes, they would, at least, be seen open when the tissue 

 is dry and contracted, although they might close up when it 

 becomes swollen with moisture. That, however, they never are : 

 on the contrar}', they are more opaque when dry than when wet. 

 Thirdly, they become more and more opaque as the magnify- 

 ing power with which they are viewed is increased; a circum- 

 stance which seems incompatible with perforations. Finally, 

 and it is this which will possibly be regarded most conclusive, 



c 2 



