116 
PHYSIOLOGICAL VIEWS. 
plants, are filled with fluids when young, but in old branches, the 
fluids disappear, and the cells are filled with air. In general, herbs 
and shrubs have a greater proportion of pith than trees. It is also 
more abundant in young than old vegetables; it extends fi-om the 
root to the summit of the trunk or stem of the plant. 
The medullary'^ rays are lines which diverge from the pith towards 
the circumference ; they are fi]?rous textures interwoven in the wood, 
the alburnum, and the different layers of the 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 offi- 
ces are perhaps less understood than those of the other parts. The 
letter e, Fig. 118, represents the medullary rays as proceeding from 
the pith and terminating in the cellular integument. 
You are not to expect that every stem or branch of a dicotyledon- 
ous plant will present all the various parts which we have described 
as constituting the vegetable body ; neither when they exist are they 
always distinct, for they often pass into each other in such a man- 
ner as 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. 
Let us now review the most important circumstance in the growth 
of a woody plant. Before germination, the substance of the plum.e 
or ascending part of the embryo, exhibits a dehcate and regular 
cellular texture ; where the liber and medullary rays are to be 
formed, traces of cambium appear. 
When the germ-ination commences, the vascular system begins to 
organize around the pith, and the medullary rays to form ; the extrem- 
ities of these raj^s exhibit cellular texture, wiiich is soon converted 
into libers. (See /, Fig. 118, which shows the extremities of the me- 
dullary 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 alburnum— the latter is 
at length changed to this; each year a new layer succeeds, 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'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 maybe 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. 
We will now consider the manner in which the tree increases in 
* So called from medulla^ marrow, a name often given to the pith. 
Medullary rays— Pith, to what compared 7 — Various parts not always distinct in 
different plants— Appearance of a dicotyledonous plant before germination, or while 
in embryo — Change at the commencement of germination — Process in the formation 
of perfect wood— Number of layers of wood near the base of the tmnk, a criterion of 
the age of a tree— How may the age of branches be determined ? 
