76 THE PITH THE MEDULLARY SHEATH. 



of large size, and may be well examined in the pith of the elder. Their colour is 

 green whilst they freely perform their function ; but subsequently the tissue is nearly 

 colourless. In the old age of the plant the pith often assumes a colour which it has 



obtained from the juices which have been 



deposited within it. In a majority of 

 instances the pith forms a solid cylindrical 

 mass ; but in certain fast-growing plants, 

 as in the hollow stems of the Umbelli- 

 ferce, it is torn, and vacuities are left. 

 Fig. 130. Chambered pith in the WALNUT TREE. I n a f ew plants the ruptured pith assumes 

 a very regular form, and is thence termed chambered pith, since it is divided into a 

 series of compartments which pass across the column in small stems. Such is the case 

 in the walnut (Fig. 1 30), as may be readily seen by selecting a very young shoot and 

 slicing away a portion of one or both sides. According to the researches of Professor 

 Morrison this change depends upon the lateral elongation of the cells, and the conse- 

 quent disappearance of the contents of the cells, and is induced immediately by the 

 absorbing action of the leaf 'bud. 



The connexions of the pith are highly important, and demand special enumeration. 

 It has already been intimated that it does not exist in the root, at least, of tolerably 

 grown plants, and therefore its functions are confined to the stem. First. It is in 

 direct and unbroken connexion with every branch, leaf, bud, and flower, and is the 

 structure which first conveys fluids to, and receives fluids from, the newly-developed 

 leaf. It thence becomes the main organ of nutriment ; and, at the same time, the chief 

 depository of the secretions. Secondly. It is in equally direct and unbroken connex- 

 ion with the bark, through the medium of the medullary rays ; and thus becomes the 

 centre of all the movements of sap which proceed in the horizontal system ; it is that 

 system which more especially presides over the life of the plant. 



The mode in which its ultimate disappearance occurs has been a matter of doubt 

 and speculation. It seems quite clear that it is not converted into wood, as was asserted 

 by Mirbel, and there are certain facts which militate against the opinion that it is 

 gradually compressed by the wood ; but since it is known that in the growth of the 

 plant much compression of the previously formed wood must occur, and since this 

 compression is a reasonable theory by which to account for the disappearance of the 

 less resisting pith, it is now pretty generally admitted to be at least one of the causes 

 of this occurrence. 



As a general rule, the pith, so long as it exists, is not mingled with other than 

 cellular structures ; but, in a few instances, woody fibre has been found with it ; and 

 in others, as Nepenthes, spiral vessels have been detected. 



The economic uses of pith have not been numerous, but amongst them must be 

 mentioned the rice-paper used in China, and prepared by cutting the pith of the 

 -ZEschynomene (Fig. 48), and the Aralia papyrifera, in a circular manner, so as to obtain 

 large, thin, and evenly cut sheets. It is used for drawing and for writing. The 

 cellular pith-like stems of the JEschynomene aspera, called "shola," have been forwarded 

 to this country from India, and have been made into various ornaments, models of 

 buildings, hats, boxes, and life-buoys. Its lightness, and non-conducting property of 

 heat, render it very fitted for the manufacture of hats. 



Medullary Sheath. Immediately surrounding the pith of all exogenous plants 

 there is a layer of vascular tissue, which has received the name of medullaiy sheath 



