A character to which authors tend to attach excessive importance as a diagnostic feature is the almost invariable tendency of the parenchymatous pith of _Cordaites_ to break up on contraction into transverse diaphragms, thus producing what is known as a discoid pith. In the stem shown in fig. 473 the pith is represented by a more or less cylindrical cast characterised by fairly regular transverse ribs and narrow grooves; in the upper part of the fossil the peripheral tissue of the pith is preserved in the form of narrow plates projecting from the inner face of the wood. As Renault[637] pointed out, this type of pith is the expression of certain conditions of growth and is not a satisfactory distinguishing feature of any particular genus or family. The same tendency to form a discoid pith is characteristic of _Mesoxylon_, and it occurs also in some other Palaeozoic genera. Corda long ago figured a stem attributed by him to _Lomatofloyos_ with a typical discoid pith, and a similar pith is recorded in a stem of _Dicranophyllum_[638]. Among recent plants _Juglans regia_ affords perhaps the most familiar instance of an identical form of pith: the same type occurs in the white Jasmine, in _Ceropegia peltata_, and some other flowering plants. An interesting case is that of the tree Groundsel, _Senecio praecox_ D.C.[639], of Mexico: in this plant, which grows in arid districts, the pith serves as a water-store and as the water is drawn off the thick turgescent discs contract and form thin transverse diaphragms separated by wide spaces, as is also the case on drying in some succulent _Euphorbia_ stems. It may be that in _Cordaites_ the medullary region also served as a water-reservoir and the depth of the medullary discs would vary according to the state of their contents.