GROWTH OF DICOTYLEDONS. * 111 



they terminate. The pith has been compared to the spmal 

 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 6, Fig. 125, 

 represents the medullary rays as proceeding from the pith, and 

 terminating in the cellular integument. 



132. Wc are not to exj)ect that every stem or branch of a 

 dicotyledonous plant Avill 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 growth of a icoody 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, flowhig between the bark and the wood, hardens into a 

 new layer of liber ^ and a new layer of alburnum — the latter har- 

 dens still more, and hecomes 2:)ei feet tvood ^' these new layers are 

 formed each year, and thus the growth of the vegetable goes on 

 until death completes its term of existence. jE'ccch layer of wood 

 is^ generally^ the product of one year''s groiotli; but it is only near 

 the base of the trunk that the number of layei'S 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 

 tlie number of layers of wood at the base of each branch. 



131. We will now consider the manner in which the tree in- 



132. Various parU not always distinct in different plants.— 133. Appearance of a dicotyledonous plant 

 before germination— Change— Process in the formation of perfect wood— Nnmber of layers of woo.l 

 near the base of the trunk, a criterion of the age of a tree— How may the age of braM.lu's b.- iVu-t- 

 mined 1 



