The fossil trunks or stems called Stigmariæ, or Variolæ, (from the pits or areolæ with which they are studded,) occur as abundantly in the coal formation as the Sigillariæ, of which tribe of plants unequivocal proof has at length been obtained that they are the roots. These bodies are more or less regularly cylindrical, and vary in length from a few inches to fifteen or twenty feet, the largest being several inches in diameter. Their surface is covered with numerous oval or circular depressions, in the middle of each of which there is a rounded papilla, or tubercle. These variolæ are disposed round the stem in quincunx order. When these roots are broken across, a small cylindrical core or pith is exposed, which extends in a longitudinal direction throughout the stem, like a medullary column. This central axis, which is often separable from the surrounding mass, is composed of bundles of vascular tissue disposed in a radiated manner, and separated from each other by medullary rays. This internal organization presents the same correspondence with that of the stems of Sigillariæ, as does the structure of the roots of a dicotyledonous tree with that of its branches and stems.
The Stigmariæ are almost invariably present in the bed called the "Under Clay," which underlies the coal, and when observed in this situation, long tapering sub-cylindrical fibres are found attached to the tubercles; and these processes or rootlets are often several feet in length. Their form and mode of attachment are shown at C, D; the rootlets terminate in bifurcations, as seen at A, B.
The specimen here figured is part of a root nearly six feet long, and three inches in diameter; some of the rootlets were two feet long. It is imbedded in shale; from Elsecar colliery.[19]
[19] A Stigmaria with rootlets, many feet in length, is placed over the doorway in the room devoted to fossil vegetables in the Gallery of Organic Remains in the British Museum.
Plate XXII.
PLATE XXII.