76 ORGANOGRAPHY. BOOK I. 



attempts to explain this curious mode of growth upon the sup- 

 position that each leaf forms three fascicles of woody matter, 

 whereof the central is the most powerful, and produces the 

 mass of the stem; and the lateral ones, which are much 

 weaker, give origin to the accessory axes; — and he states, 

 that in climbing Sapindaceous plants the same phenomenon 

 occurs, only to a far greater extent. He represents that in 

 those cases the fibres of each leafstalk separate into three or 

 four principal branches, each of which applies itself to one of 

 the internal woody axes of the stem, which, in time, consists of 

 from four to eight distinct axes, the central being larger than 

 the others, and each having its own cortical integument. The 

 fact is exceedingly curious, but I doubt very much whether 

 the explanation is just. {Arch, de Bot, ii. 492.) 



In Coniferous wood {Ji(/. 33.) there is ^^ 



scarcely any mixture of vessels among 

 woody fibre, as in other exogenous plants ; 

 in consequence of which a cross section 

 exhibits none of those open mouths which 

 are caused by the division of vessels, 

 and which give what is vulgarly called 

 porosity to wood. Instead of this, the 

 vascular system generally consists exclu- 

 sively of that kind of woody tissue which 

 has been described at p. 20., under the 

 name of glandular, with the exception of 

 the medullary sheath, in which spiral vessels are present in 

 small numbers. The Yew is the principal exception : in this 

 plant the woody tissue is the same as that of other Conifera? ; 

 but many tubes have a great quantity of little fibres lying ob- 

 liquely 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 fibre 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 Cycadea? the vascidar system is destitute of vessels, as in 



