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GROWTH OF PLANTS. 
bark. The new buds seem to originate from the points at which 
they terminate. 
The pith has been compared to the spinal marrow in animals; 
it appears to be an important part of the vegetable substance, 
though its offices are perhaps less understood than those of the 
other parts. 
You are not to expect that every stem, or branch of a plant 
will present you with all the various parts which we have de- 
scribed as constituting the vegetable body ; neither when they 
exist are they always distinct, for as there is a chain of connex- 
ion between them, so they often pass into each other in such a 
manner as to leave their boundaries difficult to define. Many 
species of plants have no distinct layers of bark, and many have 
such a similarity between the alburnum and the perfect 
wood as to render a distinction very difficult. 
GROWTH OP A PLANT. 
Let us now take a rapid view of the growth of a woody plant. 
Before germination, the substance of the plume or ascending 
part of the embryo, exhibits a delicate and regular cellular tex- 
ture ; where the liber and medullary rays are to be formed, traces 
of cambium appear. When the germination commences, the 
vascular system begins to organize around the pith, and to form 
the medullary rays ; the extremities of these rays exhibit cel- 
lular texture which is soon converted 'into liber. This liber at 
first expands, then hardens, and is at length converted into a lay- 
er of alburnum ; this alburnum gradually acquires tenacity ; 
the cells appear merged into vessels of a firmer kind and it is 
no longer a layer of alburnum but of perfect wood. While 
this change is taking place, the cambium, which may almost be 
termed a fluid cellular texture, flowing between the bark and 
the wood, reproduces a new layer of liber, which in its turn 
becomes alburnum and then perfect wood ; to this succeeds a 
third and fourth layer, and thus the growth of the vegetable 
goes on until death completes its term of existence. Each lay- 
er of wood is generally the product of one year’s growth ; but 
it is only near the base of the trunk, that the number of layers 
of wood is a criterion of the age of the tree ; for in trees where 
one hundred layers may be counted near the base, no more than 
one can be found at the extremity of the branches. These lay- 
ers then do not extend through the length of the tree ; but while 
Pith compared to the spinal marrow in animals — Various parts not ah 
ways distinct in different plants. 
Growth of a plant — Its commencement and progress io the formation of 
wood — 
