BOTANY. 



153 



a layer of wood and a layer of bark, organized under the in- 

 fluence of a liquid called the sap. Hence it happens that the 

 oldest layers of wood are at the centre, and the more recent at 

 the circumference, while, with the bark, the reverse is the 

 case. 



The development in height is effected by buds, which, like 

 so many young plants, contribute, by their growth, to aug- 

 ment the diameter of the base, and the extent of the plant 

 considered in its ensemble. This particular mode of growth 

 has caused the trunk to be called the ligneous exogenous 

 stem. 



The Pith. 



A white substance, composed of hexagonal cells, abundant 

 and moist in the shoots, and dry in the old branches; it is 

 contained in a canal formed principally of tracheals, and 

 called the Medullary Sheath, or Canal. 



The Ligneous Layers. 



Formed of ligneous fibres, compacted together, and disposed 

 in concentric beds ; the oldest, generally of a deeper colour, 

 are called the Wood, or heart, the new, of a brighter hue, the 

 alburnum. 



The Bark 



Is formed of three membranes, placed one over the other. 

 For the description of the epidermis, see what is elsewhere 

 said concerning Elementary Tissues. The herbaceous enve- 

 lope is a plate or layer of cellular tissue placed beneath the 

 epidermis, the parietes of whose cells contain little green 

 grains, called globuline; it likewise contains the proper juices. 

 The liber is formed of a series of superimposed laminse, com- 

 posing a vascular net, the meshes of which are filled with 

 cellular tissue; the old liber forms the proper cortical cover- 

 ing. The medullary rays are laminae of cellular tissue pass- 

 ing in the direction of the thickness of the trunk, from the 

 centre to the circumference, and, in a horizontal cut of the 

 tree, resembling the horary lines on a dial : they serve to esta- 

 blish a communication between the pith and herbaceous en- 

 velope. 



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