. The anatomy of woody plants. Botany -- Anatomy. 256 THE ANATOMY OF WOODY PLANTS among the tracheids, particularly toward the central region of the stele. This peculiar organization of the median area of the stele in protostelic lepidodendrids is responsible for a hypothesis as to the origin of the medulla or pith. Quite generally it is con- sidered that by continuing the process of transformation of tracheids, first into short tracheary elements and then into paren- chymatous cells, there is formed in the center of the stele a pith of stelar origin. In accordance with this view the central r
. The anatomy of woody plants. Botany -- Anatomy. 256 THE ANATOMY OF WOODY PLANTS among the tracheids, particularly toward the central region of the stele. This peculiar organization of the median area of the stele in protostelic lepidodendrids is responsible for a hypothesis as to the origin of the medulla or pith. Quite generally it is con- sidered that by continuing the process of transformation of tracheids, first into short tracheary elements and then into paren- chymatous cells, there is formed in the center of the stele a pith of stelar origin. In accordance with this view the central re- gion of the stele in many protostelic lepidodendrids is called a "partial ; There is no conclusive evidence, however, that the so- called " partial pith " in reality gives rise to the true medulla in those lepidoden- droid types which possess it. More- over, the evidence in the case of the Pteropsida, which are very much better displayed in the period of time which we are able to investigate, is distinctly against the validity of the stelar origin of the pith, since the medulla in the large-leaved vascular cryptogams shows very marked indications of derivation from the fundamental system. It seems on the whole more likely that the medulla in the lepidodendrids, where such a structure is found, is an inclusion of fundamental tissues on the part of the stele. This conclusion is particularly favored by conditions found in lepidodendroid stems in which there is no indication of secondary growth, as, for example, in Lepidodendron Spenceri, shown in Figs. 183 and 184. Here the medulla is largely occupied by dark-brown sclerenchymatous tissues similar. FIG. 183.—Transverse section of the stem of Lepido- dendron 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 resemble the original Jeffrey, Edward
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