GROWTH OF MONOCOTYLEDONS. 
117 
height. A seed germinates ; the plume rises ; the cambium, in de- 
veloping, gradually becomes less capable of extension ; at length, 
when it is converted into wood, its circulation ceases. The layer of 
wood then exhibits the form of an elongated cone ; at the summit of 
the cone a bud is formed, from which a new shoot issues; a new 
layer of alburnum organizes upon the surface of the cone ; this, in 
turn, becomes perfect wood, covering the layer first formed ; and 
thus the tree goes on increasing in height and in diameter. The ter- 
minal bud is formed each successive year. After a hundred years 
of vegetation, a hundred cones might be found boxed within each 
other in the manner first described ; the spaces comprised between 
the summits of the cones would show the succession and elongation 
of the annual shoots. 
As the wood is formed by the conversion of cambium into albur- 
num, so from the same hquid the inner layers of bark are formed to 
renew the waste occasioned by the destruction of the epidermis. 
While the wood is growing externally, that is, at an increasing dis- 
tance from the centre, the bark is forming internally, and the new 
layers are pressing outward. 
Growth of Monocotyledonous Plants. 
The growth of trunks, as hitherto considered, has relation only to 
woody plants ; but between plants which grow from seeds with one 
cotyledon, and such as grow from seeds with two cotyledons, there 
is a great difference as to the mode of organization and growth. 
The first kind of plants are called monocotyledonous ; the second 
dicotyledonous. Their stems, on account of their different modes of 
growth, have been distinguished into endogenous, signifying to grow 
inwardly ; and exogenous, signifying to grow outwardly. The dis- 
covery of the different modes of growth in these two great divisions 
of plants, is of recent origin, and constitutes an important era in ve- 
getable physiology. 
The stems of monocotyledonous or endogenous plants have seldom 
a bark distinct from the other texture; they have no liber, or albur- 
num disposed in concentric layers; they have no medullary rays; 
and their pith, instead of being confined to the centre of the stem, 
extends almost to the circumference. 
The wood is divided into fibres running 
longitudinally through the stem, (see Pig. 
119, where the dots represent the fibres;) 
each of these-fibres seems to vegetate sepa- 
rately ; they are ranged around a central 
support, and are so disposed that the oldest 
are crowded outwardly by the develop- 
ment of new fibres in the centre of the 
stem; this pressure causes the external 
layers to be very close and compact. 
This mode of increase, httle favourable to 
growth in diameter, produces long and ' 
straight stems, nearly uniform in size 
throughout their whole extent; as the 
palms and sugar-canes of the tropics, and 
the Indian corn of our chmate. Most of these plants present us with 
roots of the fibrous kind. 
Describe the manner in which the tree increases in height 7 -Difference in the 
growtir ot wood and bark— Remarks on the different organization of plants— Mono- 
cotyledonous plants— VS^hy called endogenous 7—Exogenous plants— Describe the 
Btcm oi monocotyledonous or endogenous plants— Describe the stem of a monocoty- 
ledonous plant. 
