. A manual of the North American gymnosperms [microform] : exclusive of the cycadales but together with certain exotic species. Bois; Trees; Gymnosperms; Gymnospermes; Arbres; Wood. "^""tion IfTflT "'^j-CHUM. Medullary ray showing the structure and posi- t on of the p ts on the lateral walls; the straight ray cells and the thin straight, terminal walls, x 280 ^ j- ana ine tnin, change in such a way that the one type passes gradually into the other. When these variations are viewed collectively and taken together with the general fact that the thin-walled cells are a
. A manual of the North American gymnosperms [microform] : exclusive of the cycadales but together with certain exotic species. Bois; Trees; Gymnosperms; Gymnospermes; Arbres; Wood. "^""tion IfTflT "'^j-CHUM. Medullary ray showing the structure and posi- t on of the p ts on the lateral walls; the straight ray cells and the thin straight, terminal walls, x 280 ^ j- ana ine tnin, change in such a way that the one type passes gradually into the other. When these variations are viewed collectively and taken together with the general fact that the thin-walled cells are a feature of the higher types of organization, we may reason- ably conclude that the thin-walled cells have been derived from the thick-walled through a process of arrested development. The cause of such alterations is to be sought for, and it will doubt- less be found in connection with another component of the ray. The terminal walls of the ray cells present three typical vari- , —(,) thin-walled and entire, (2) thin-walled and locally thickened, and (3) thick-walled and coarsely pitted.^ The first ' See Appendix 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 Penhallow, D. P. (David Pearce), 1854-1910. Boston : Ginn & Co.
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