44 
ON THE PHYSICAL STRUCTURE OF PLANTS', 
the cells form layers of considerable thickness, and are united like a 
honeycomb, either in a vertical or horizontal position. The inner bark 
or liber is a tissue of fibres and intermediate cellular matter united, and 
discharged from the current year’s alburnum in the autumn. This is so 
regularly beautiful in some trees, that it resembles the finest Brussels 
lace; the fibres in crossing each other are inosculated, and remain con¬ 
nected after the accompanying cells have disappeared. 
Tubes are formed of conjoined cells placed in circles, strengthened 
lengthways by fibres passing upwards through them ; they (the tubes) 
are sap-conductors, or receptacles of gaseous fluids or common air. 
The woody axis of a tree is formed by these different organs com¬ 
bined, the fibres and the tubes having a vertical, or at least a direction 
from the root outwards, and the cellular fabric lying in horizontal strata 
between. 
The elementary matter of which the membrane or coats of the cells' 
are formed, we cannot name, it being a compound substance formed 
by the union of several chemical bodies, its normal structure being 
endowed with the wonderful power of subdivision and extension, and 
changeable from a small mass of succulent membrane up to a consider¬ 
able volume of ligneous matter, and by annual accretion arrives at a vast 
bulk. It is a creature of Almighty power and Omnipotent design, no 
less astonishing in its organic forms than in its organic action. 
The manner of accretion by extension of vegetable membrane is 
most wonderful. The elongation of a fibre takes place by the consecu¬ 
tive enlargement of the cells from the base upwards, there always 
remaining at the point an interminable number developable in future, 
or as long as the growth continues, as represented in fig. a, page 43. 
A tissue or web is expanded by the enlargement of the incipient cells 
lying in the inner edge of the margin, or hem, as it may be called, and 
in which hem there also appears to be an indefinite number of expan¬ 
sible cells, which may or may not be inflated, fig. d, page 43. The 
tubes or openings seen in a cross section of a stem, (not the spiral ves¬ 
sels already mentioned,) appear to be incidental passages, formed by the 
currents of sap or air passing through them, rather than identical organs 
of themselves, seeing that they have no regular boundaries. 
Besides the longitudinal organs of a stem already alluded to, there are 
other very conspicuous members which enter into its composition; these 
are what are commonly described as medullary rays, and have been sup¬ 
posed to be the diverging tracks of buds from the pith to the exterior of 
the bark; but later observers have discovered that they are, in fact, 
convergent partitions of dense cellular tissue, which divide the circum- 
