GROWTH OF DICOTYLEDONS. 
Ill 
thej 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 under- 
stood than those of the other parts. The letter ^, Fig. 125, 
represents the medullary rays as proceeding from the pith, and 
terminating in the cellular integument. 
132. We are not to expect that every stem or branch of a 
dicotyledonous plant will present all the various parts described 
as constituting the vegetable body ; neither when they exist are 
they always distinct, for they often pass into each other in such 
a manner as to render it difficult to define their boundaries. 
Many species of plants have no distinct layers of bark, and in 
many others there is such a similarity between the alburnum 
and perfect wood as to render it difficult to distinguish them. 
Growth of a Dicotyledonous Plant, 
133. Let us now review the most important circumstance in 
the gi'owth of a woody plant. Before germination the substance 
of the plumula exhibits a delicate and regular cellular tissue, 
and 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 the 
medullary rays to form ; the extremities of these rays exhibit 
cellular tissue, which is soon converted into liber. (See/*, Fig. 
125, which shows the extremities of the medullary rays, and the 
points where the liber is formed.) While this change is taking 
place, the cambium, which may be considered a fluid cellular 
mass, flowing between the bark and the wood, hardens into a 
new layer of liber ^ and a new layer of alhurnum — the latter har- 
dens still more, and becomes perfect wood / these new layers are 
formed each year, and thus the growth of the vegetable goes on 
until death completes its term of existence. Each layer of wood 
is, generally, the product of one year'' 8 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 layers, then, do 
not extend through the length of the tree ; but while the base 
exhibits all the layers which have been formed, the extremity of 
the branches contains under the bark only the continuation of 
an annual layer. The age of branches may be determined by 
the number of layers of wood at the base of each branch. 
134. We will now consider the manner in which the tree in- 
132. Various parts not always distinct in different plants. — 133. Aopearance of a dicotyledonous plant 
before ptrmination — Change — Process in the formation of perfec' wood— Number of layers of wood 
near the base of the trunk, a criterion of the age o " a trse — HoV may the age of brarches be detei^ 
miaed 1 
