26 
ORGANOGRAPHY. 
BOOK I. 
siderably above the wall of the tube. In a specimen of Pinus 
Strobiis I have found them surrounded with an irregular 
elevated rim, as at Plate II. fig. 7. and 8., as if the lining of the 
tube was growing over them. It is not at all uncommon to 
find them in what may be supposed a nascent state, merely 
looking like tumours, with a pore in the middle, as is shown 
at Plate II. fig. 5. 
If the disks of coniferous wood are examined with a good 
eighth of an inch object glass, and a low ocular, they will be 
distinctly seen marked with concentric circles as represented 
at Plate II. fig. 6. The highest oculars with a lower objective 
will not separate the circular lines. M. Valentin, who first 
noticed them (Repertorium^ vol. i. t. i.), considers them to be 
the projecting edges of numerous layers of woody matter 
concentrically deposited round a space which they gradually 
close up, except a narrow opening into an air chamber, the 
layer next the centre being the youngest. I have not suc- 
ceeded in obtaining any section which will show this structure. 
It has been imagined that this glandular Pleurenchyma is 
confined to Gyinnosperms, but Dr. Brown long since remarked 
it in Tasmannia, and Mr. Griffith finds it common in aromatic 
trees. At Plate II. fig. 20. is a view of it as I see it in 
Spliaerostema. 
The nature of the disks has been examined by M. Guil- 
lemin, who, in a paper laid before the Academy of Sciences, 
Dec. 19, 1836, considers them to be tumours, and calls them 
(Edemata. He supposes them to be flattened vesicles, the 
central circle being either a pore or minute cell; and he 
imagines them to be filled with a colourless volatile oil, which 
changes to turpentine when it has been exuded from the cen- 
tral luminous point. He also adverts to the existence of 
similar appearances in aromatic woods, especially Drimys 
chilensis, but says they are not to be confounded with 
(Edemata. ( Comptes Rendus, iii. 761.) 
Pleurenchyma constitutes a considerable proportion of the 
ligneous part of all plants ; it is abundant in liber, and forms 
the principal part of the veins of leaves, to which it gives 
stiffness and tenacity. 
