112 
PHYSIOLOGICAL VIEWS. 
creases in higlit. A seed germinates ; the plumule rist/6 , un 
axis is developed, with its nodes which throw oif branches ; tno 
cambium performs its office, but gradually becomes less capable 
of extension, and when it is converted into wood its circulation 
ceases. The layer of wood then exhibits the form of an elon- 
gated 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 hight and in diameter. The terminal bud is 
formed each successive year. After a hundred years of vegeta- 
tion, a hundred cones might be found boxed within each other ; 
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 alburnum, 
so from the same fluid 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 
distance from the center, the bark is forming internally, and the 
new layers are pressing outward. 
Growth of Monocotyledonous Plants. 
135. The growth of trunks as hitherto considered has reia 
tion 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 difibrence as to the mode 
of organization and growth. Their stems, on account of their 
difierent modes of growth, have been distinguished into endo- 
gpMOits^ signifying to grow inwardly, and exogenous^ signifying 
to grow outwardly. The discovery of the difi'erent modes of 
growth in these two great divisions of plants constitutes an im- 
portant era in vegetable physiology. 
The stems of monocotyledonous or en- 
dogenous plants have seldom a bark 
distinct from the other texture ; they 
have neither liber nor alburnum dis- 
posed in concentric layers ; they have 
no medullary rays ; and their pith, in- 
stead of being confined to the center 
of the stem, extends almost to the cir- 
cumference. The wood is divided into 
fibers running longitudinally through 
the stem (see Fig. 126, where the dots 
134 Describe the manner in wiiich the tree increases in hight— Difference in the growth of wood 
and hark.— 135. Difference in the growth of plants of th« two great classes -Describe the growth o» 
the endogenous stem. 
