40 EXOGENOUS STEM. WOOD. 



pith is ruptured irregularly, and forms large cavities, as in the fistular 

 stem of Umbelliferous plants. Circumscribed cavities in the internal 

 cellular portions of stems are by no means unfrequent, arising either 

 from rupture or absorption of the cells. In some rare instances, ves- 

 sels occur in pith, as in Elder, Pitcher-plant, and Ferula; and occa- 

 sionally its cells are marked by pores indicating the formation of 

 secondary deposits. The extent of pith varies in different plants, and 

 in different parts of the same plant. In Ebony it is small, while in 

 the Elder it is large. In the Eice-paper plant, a species of _<Eschyno- 

 mene, the interior of the stem is occupied almost entirely by cellular 

 tissue, which may be called pith; from this the paper is made by 

 cutting thin sections in a circular manner. The same kind of tissue 

 occurs in the Papyrus of the Nile. When the woody circle of the 

 first year is completed, the pith remains stationary as regards its size, 

 retaining its dimensions even in old trunks, and never becoming oblit- 

 erated. 



76. The Medullary Sheath, is the fibro-vascular layer immediately 

 surrounding the pith. It forms the inner layer of the vascular bundle 

 of the first year (fig. 97 i), and consists chiefly of true spiral vessels, 

 which continue to exercise their functions during the life of the plant, 

 and which extend into the leaves. With the spiral vessels there are a 

 few woody fibres intermingled. The processes from the pith are pro- 

 longed into the medullary rays between the vessels of the sheath. 



77. Woody Layers. During the first year, the vascular circle con- 

 sists of an internal layer of spiral vessels forming the medullary sheath, 

 and external bundles of porous and ligneous vessels. In subsequent 

 years the layer of spiral vessels is not repeated, but concentric zones 

 of porous vessels (fig. 98 ter.), and pleurenchyma are formed, consti- 

 tuting what are commonly called the woody circles of trees. The 

 vascular bundles, from their mode of development in an indefinite 

 manner externally, have been called Exogenous; and, for the same 

 reason, Schleiden has denominated them Indefinite. Exogenous plants 

 have sometimes received the name of Cydogens (xvx^os, a circle), 

 in consequence of exhibiting concentric circles in their stems. On 

 a transverse section, each zone or circle is usually seen to be separ- 

 ated from that next to it by a well-marked line of demarcation. This 

 line, as in the Oak (figs. 99, 100), and in the Ash, is indicated by 

 holes which are the openings of large porous vessels; the remainder 

 of the tissue in the circle being formed by pleurenchyma, with 

 thickened walls and of smaller calibre. In some trees, as the 

 Lime, Hornbeam, and Maple, the line is by no means so well marked, 

 as the openings are smaller and more generally diffused; but there 

 is usually a deficiency of porous vessels towards the outer part of the 

 circle. In cone-bearing plants, as the Fir, in which the woody layers 

 consist entirely of punctated woody tissue (fig 47), without any 



