54 Vegetable Organography and Physiology. 



of vegetable life, and analogous to the nervous system in animals, 

 is still an interesting question for the vegetable physiologist. 



Immediately surrounding the pith is the medullary sheath. 

 Whilst the former is composed of cellular tissue, the latter consists 

 of spiral vessels and ducts. The office of the medullary sheath 

 is not well understood. Lindley, who believed it to be in direct 

 communication with the leaf-buds, and the veins of the leaves, 

 supposed that it was the medium through which the ascending 

 sap is transmitted to the leaves. Dutrochet and some other phys- 

 iologists are of the opinion that the office of circulating the as- 

 cending sap is confined to the lymphatic tubes of the woody fibre, 

 and that the spiral vessels of the medullary sheath belong to the 

 function of respiration. As these vessels are found almost invaria- 

 bly to contain air ; and, moreover, have direct communication 

 with the leaves, which are in fact the lungs of the plant, it is 

 more than probable that the opinion of Dutrochet is correct. 



Deposited in concentric layers around the medullary sheath, 

 and lying immediately upon it, is the wood. It is composed of 

 cellular tissue, woody fibre, and ducts. Each year a distinct layer 

 is deposited. The concentric layer of the first year, or the one 

 lying in immediate contact with the medullary sheath, consists of 

 woody fibre and ducts ; but each succeeding layer has an interior 

 membrane of cellular tissue, (the same tissue of which the pith is 

 composed,) and an external stratum of woody fibre and ducts. 

 The wood is usually subdivided into the denser portion, which 

 is called lignum ; and external to this, a softer portion called al- 

 hurnum. 



The former consists of the internal layers surrounding the me- 

 dullary sheath, which being fully formed, have ceased to afford 

 a passage for the circulating fluid. It is also called heart-wood. 

 External to this is the softer wood, called the alburnum ; which 

 is also deposited in concentric rings, between the true wood and 

 the bark. It is through this part of the plant, the alburnum, that 

 the fluid drawn from the earth, the ascending sap, is principally 

 transmitted. The hark is composed of the same elementary tis- 

 sues that enter into the composition of the wood, viz. cellular tis- 

 sue, woody fibre and ducts ; but whilst the layers of wood con- 

 sist of an interior stratum of cellular tissue, and an outer stratum 

 of woody fibre and ducts, those of the bark are reversed : they are 

 composed of a layer of woody fibre and ducts inside, and of cellu- 



