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 Coniferae ; 
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 Cycadeye the vascular system is destitute of vessels, as in 
