210 CONIFERS CYCADS. [BOOK i. 



In Coniferous wood (fig. 35.) there is scarcely any mixture of 

 bothrenchym among woody tissue, as in most other exogenous 



fig- 35. 



plants ; in consequence of which a cross section exhibits none 

 of those open mouths which give what is vulgarly called 

 porosity to wood. Instead of this, the wood generally con- 

 sists exclusively of that kind of tissue which has been described 

 at p. 65, under the name of glandular, with the exception of 

 the medullary sheath, in which spiral vessels are present in 

 small numbers. The Yew and Abies Douglasii are the prin- 

 cipal exceptions : in the former the woody tissue is the same 

 as that of other Conifers ; but many tubes have a great quan- 

 tity of little fibres lying obliquely across them at nearly equal 

 distances, sometimes arranged with considerable regularity, 

 sometimes disturbed as it were, so that the transverse 

 fibres, although they retain their obliquity, are not parallel, 

 and sometimes, but more rarely, so regular as to give to 

 the tubes of woody tissue the appearance of spiral vessels, the 

 coils of which are separated by considerable intervals. The 

 latter only is represented by Kieser, at his tab. xxi. fig. 103, 

 104. ; but the former is by far the most common appearance. 



In Cycads the vascular system is destitute of vessels, as in 

 Conifers; their place being supplied by such bothrenchym 

 as has been already described at p. 58. But the zones of 

 wood are separated by a layer of cellular substance resembling 

 that of the pith, and often as thick as the zones themselves, 



