GROWTH OF PLANTS. 
139 
the base exhibits all the layers which have been formed, the ex- 
tremity of the branches contains under the bark only the continua- 
tion of an annual layer. 
We can now show you how the tree grows in height as well 
as circumference. A seed germinates; the ascending part rises; 
the liber, by the vegetative power, is urged upward ; but in de- 
veloping, it gradually becomes less capable of extension ; at 
length, when it is converted into wood, its growth 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 liber organizes upon the surface of the cone; 
this new liber in turn, becomes a woody cone covering the one 
first formed ; and thus the tree goes on increasing in height and 
in diameter. The terminal bud is formed each successive 
year. After a hundred years of vegetation a hundred cones 
might be found boxed into each other in the manner first descri- 
bed ; the spaces comprised between the summits of the cones 
would shew the succession and elongation of the annual shoots. 
As the wood is formed by the conversion of the cambium into 
the alburnum, so from the same liquid, the inner layers of bark 
are formed which renew the waste occasioned by the destruc- 
tion of the epidermis. While the tree is growing inward, 
that is from the alburnum towards the pith, the bark is grow- 
ing outward, or the inner layers are pushing to the surface. 
The growth of trunks, which we have hitherto considered, has 
relation only to woody plarfts. But there is a marked difference 
in the growth of plants, which seems to originate in the pecu- 
liar formation of their seeds. You will recollect that in the des- 
cription given of the seed, it was observed that some seeds have 
but one cotyledon ; others two, and a few more than two. Be- 
tween 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. 
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 outward- 
ly. The discovery of the different modes of growth in these 
two great divisions of plants, is of recent origin, and constitutes 
an important era in vegetable physiology. 
The stems of monocotyledons, or endogenous plants have 
seldom a bark distinct from the other texture ; they have no 
liber, or alburnum disposed in concentric layers ; they have no 
Advance of the tree in height as well a3 circumference — Remarks on the 
different organization of plants — Of Monocotyledons — Dicotyledons — Stems 
distinguished into endogenous and exogenous-Growth of endogenous plants. 
