. The anatomy of woody plants. Botany -- Anatomy. THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 265 wood, which is triangular in configuration, with the small elements of the protoxylem at the angles. The primary wood presents no features of special interest as viewed in transverse section beyond the fact that it is protostelic in its organization and consequently lacks a parenchymatous medulla or pith. The secondary wood which surrounds the primary structure is characterized by the radial seriation of its elements and by the presence of medullary rays. There are no other parenchyma- tous struct
. The anatomy of woody plants. Botany -- Anatomy. THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 265 wood, which is triangular in configuration, with the small elements of the protoxylem at the angles. The primary wood presents no features of special interest as viewed in transverse section beyond the fact that it is protostelic in its organization and consequently lacks a parenchymatous medulla or pith. The secondary wood which surrounds the primary structure is characterized by the radial seriation of its elements and by the presence of medullary rays. There are no other parenchyma- tous structures in the secondary wood ex- cept the rays. The radial parenchyma of the Sphenophyllaceae is peculiar in the fact that its cells, instead of being strictly elon- gated in the radial direction and at right angles to the longer axis of the tracheids, •frequently have their greatest dimensions in the vertical plane. This situation leads to the extension of the cells of the rays along the edges upward and downward among the tracheids in a manner simulating true wood parenchyma. This is, however, merely an appearance, for longitu- dinal storage cells of the type ordinarily known as wood parenchyma have not yet been found in any Paleozoic wood of secondary origin. Wood parenchyma, indeed, as has been indicated in an earlier chap- ter, was primitively intimately associated with the phenomenon of annual rings which appeared for the first time in the Mesozoic age. It is clear from the description of the wood of the Sphenophyllaceae supplied in the present connection that it shows, as indeed might be expected, a general resemblance to that of the more ancient representatives of the Lycopodiales. The tracheids were somewhat. FIG. 188.—Transverse section of the stem of Sphenophyllum Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly re
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