286 
PHYSIOLOGY. 
BOOK II. 
or, which is the same thing, parenchyma. Now suppose a 
number of these petioles to be separated from their blades, 
and to be tied in a bunch parallel with each other, and, by 
lateral pressure, to be squeezed so closely together that their 
surfaces touch each other accurately, except at the circum- 
ference of the bunch; if a transverse section of these be 
made, it will exhibit the same mixture of bundles of woody 
tissue and parenchyma, and the same absence of distinction 
between pith, wood, and bark, which has been noticed in the 
cormus, or first plate, of monocotyledons. 
As soon as the plate has arrived at the necessary diameter, 
it begins to lengthen upwards, leaving at its base those leaves 
which were before at its circumference, and carrying upwards 
with it such as occupied its centre ; at the same time, new 
leaves continue to be generated at the centre, or, as it must 
now be called, at the apex of the shoot. 
As fresh leaves are developed, they thrust aside to the cir- 
cumference those which preceded them, and a stem is by 
degrees produced. Since it has not been formed by additions 
made to its circumference by each successive leaf, it is not 
conical, as in dicotyledons; but, on the contrary, as its in- 
crease has been at the centre, which has no power to extend 
its limits, being confined by the circumference which, when 
once formed, does not afterwards materially alter in dimen- 
sions, it is, of necessity, cylindrical: and this is one of the 
marks by which a monocotyledon is often to be known, in the 
absence of other evidence. The centre, being but little acted 
upon by lateral pressure, remains loose in texture, and, until 
it becomes very old, does not vary much from the density 
acquired by it shortly after its formation ; but the tissue of 
the circumference being continually jammed together by the 
pressure outwards of the new matter formed in the centre, in 
course of time becomes a solid mass of woody matter, the 
cellular tissue once intermingled with it being almost ob- 
literated, and appearing among the bundles it formerly 
surrounded, like the interstices around the minute pebbles of 
a mosaic gem. 
Such is the mode of growth of Palms, and of a great pro- 
portion of arborescent monocotyledons. But there are others 
