CHAP. I. 
WOODY TISSUE. 
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countries. The sides of woody tissue become thickened, as 
they advance in age, by the successive deposits of layer after 
layer in their interior (see Jig. 1.) ; this is particularly ob- 
servable in the liber, and hence, perhaps, the reason why the 
toughest kinds of fibre are obtained from that part. 
There are two distinct kinds of Pleurenchyma : — 
1. 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. {Fig. 7.) This is the finest 
and the commonest of all ; and is also the most genuine state 
of woody tissue. 
2. The second kind of woody tissue is the glandular. This 
has hitherto been examined chiefly in Coniferae, in which it 
uniformly occurs. Its dimensions are more considerable than 
that of the last-mentioned form, and it has been described 
as perforated with pores. The markings of the tubes are 
vesicular, and usually transparent, with a darkened centre 
(Plate II. fig. 3.), which last is what has been described 
as a pore, the vesicle itself being considered a thickened rim. 
Kieser figures the glands as pores in Pine- 
wood {Jig. 8.), in Ephedra, and other 
cases. They may be most conveniently 
found by examining with a microscope 
a thin shaving of common Pinewood 
(Pinus Strobus), when they will be 
seen in the form of transparent globules, having a dark 
centre, and placed upon the walls of the pleurenchyma. 
The structure of coniferous glands has of late attracted the 
attention of many anatomists, and, at last. Professor Mohl 
seems to have discovered their real nature. He states them 
to be circular spaces, thinner than the rest of the tube, 
and placed opposite each other, so that when the walls are 
examined by transmitted light they appear more transparent 
than the rest of the tube. A plan of this structure is given 
in a cross section of two tubes in Plate II. fig. 4., where a a 
represent depressions in the centre of the elevation, which 
depression is supposed to cause the appearance of a central 
pore. With patience sections may be obtained so as to show 
the glands in profile, and then they are seen to project con- 
