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 frjm 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 fibrous 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 Avill 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 })ass 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 plume 

 or ascending 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 begins to 

 organize around the pith, and the medullary rays to form ; the extrem- 

 ities of these rays exhibit cellular texture, which is soon converted 

 into libers. (See/, Fig. 118, which shows the extremities of the me- 

 duhary 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 gi-^owth 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 

 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 ?— 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 trunk, a criterion of 

 the age of a tree— How may the age of branches be determined 1 



