KINDS OF PLEURENCHYM. 



[BOOK i. 



jig. 8. 



as they advance in age, by the successive deposits of layer 

 after layer in their interior (see fig. 1 .) ; this is particularly 

 observable 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 Pleurenchym : 

 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. 



. The second kind of woody tissue is the glandular. This 

 has hitherto been examined chiefly in Conifers, in which it 

 uniformly occurs. Its dimensions are more considerable 

 than those of the last-mentioned form, and it has been de- 

 scribed as perforated with pores. The marking 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 

 Pinewood (fig. 8.), in Ephedra, and 

 other cases. They may be most con- 

 veniently 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 wall of 

 the pleurenchym. 



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 ex- 

 amined 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- 



