152 PHYSIOLOGICAL VIEWS. 



which we have described as constituting the vegetable body ; 

 neither when they exist are they always distinct, for as there 

 is a chain of connexion between them, so they often pass into 

 each other in such a manner as to leave their boundaries diffi- 

 cult to define. Many species of plants have no distinct layers 

 of bark, and in many there is such a similarity between the 

 alburnum and the perfect wood, as to render a distinction very 

 difficult. 



Growth of a Plant. 



Let us now take a rapid view of the growth of a woody plant. 

 Before germination, the substance of the plume or UM-< ndini: 

 part of the embryo, exhibits a delicate and regular cellular 

 texture ; where the liber and medullary rays are to be formed, 

 traces of cambium appear. 



When the germination commences, the vascular system be- 

 gins to organize around the pith, and to form the medullary 

 rays ; the extremities of these rays exhibit cellular texture, 

 which is soon converted into liber. See /, Fig. 93, which 

 shows the extremities of the medullary rays, and the points 

 where the liber is formed. This liber at first expands, then 

 hardens, and is at length converted into a layer of alburnum ; 

 the 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, cel- 

 lular texture, flowing between the bark and the wood, repro- 

 duces a new layer of liber, which in its turn becomes alburn- 

 um 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 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-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 con- 

 tains 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. 



Appearance of a woody 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 a criterion of the age of a tree. 



