350 PEOFESSOR W. C. WILLIAMSON ON THE PLANTS ' 



cast of this hollow internal cavity, which has been filled with 

 various inorganic materials. In some cases the formation of 

 the internal mould has been the result of chemical infiltra- 

 tion ; but it more commonly consists of sandstone, which 

 has entered mechanically at the patent extremities of the frag- 

 ment. The transverse lines and ridges with which its surface 

 is always sculptured, prove that the interior of the medullary 

 cavity has not been smooth, but has exhibited a modification 

 of the curious medullary type termed discoid or disciform. 



In a few recent trees, belonging to the widely separated 

 orders of Jasmineaceae and Juglandaceae, the pith, which is 

 at first solid and homogeneous, soon undergoes a change. 

 Portions of it become absorbed, whilst the remainder exists 

 as a thin layer, lining the medullary sheath, from which 

 layer these discoid laminae are extended across the medul- 

 lary cavity (fig. 12). In the first instance these laminae 

 are thick and in close contact; but by the continued absorp- 

 tion of the medullary matter, their thickness is diminished, 

 and the intervening spaces proportionately enlarged. It is 

 possible, also, that the growth and consequent elongation 

 of the woody zone may cause some further divergence of 

 these laminse; but if so, the change which they undergo 

 from this source is very small, since their average distances 

 in small twigs and in thick stems is very similar. This type 

 of pith does not appear to characterise any particular groups 

 of recent plants. It is very obvious in the common white 

 Jasmine (Jasminuui officinale), but does not occur in the 

 yellow species (Jasrainum humile and revolutum). It 

 exists in the Juglans regia, or common walnut, as well as in 

 some species of Carya or hickory ; but is absent in others. 

 In all cases where it occurs, the pith is, in the first instance, 

 solid ; its subsequent laminated aspect being the result of 

 a secondary process of absorption. The laminas are 

 thickened at the base (12 c); so that in a vertical section, 

 the intervening spaces present a concave peripheral 



