. Contributions from the Hull Botanical Laboratory. Plants. 1919] MILLER—CYC AS MEDIA 15 they meander back and forth tangentially, so that only short patches can be caught here and there in a single radial section. The meandering habit is not so pronounced in the traces, and consequently longer stretches of primitive xylem elements can be seen and identified as such. Secondary xylem of the normal cylinder is composed of tra- cheids which are characteristically pitted, the pits being confined largely to the radial walls, as described by both Chamberlain and Jeffrey. The phloem situation of the
. Contributions from the Hull Botanical Laboratory. Plants. 1919] MILLER—CYC AS MEDIA 15 they meander back and forth tangentially, so that only short patches can be caught here and there in a single radial section. The meandering habit is not so pronounced in the traces, and consequently longer stretches of primitive xylem elements can be seen and identified as such. Secondary xylem of the normal cylinder is composed of tra- cheids which are characteristically pitted, the pits being confined largely to the radial walls, as described by both Chamberlain and Jeffrey. The phloem situation of the normal cylinder adds emphasis to the fact of the latter's procam- bium origin, for proto- phloem is as distinct here as it is in any of the typical monoxylic cycad stems. Fig. 7 illustrates the upper phloem region of this cylinder, showing the crushed cellular sub- stance which once was organized protophloem. This dark crushed mass has the appearance of a thick irregular ring in transverse section, entirely surrounding the normal cylinder and immediately inside the centripetal limits of the first cortical cylinder (fig. 8). The ring of course is inter- rupted here and there by medullary rays, but in. many cases it extends unbroken across them, being squeezed in between the cells of the pith or cortical medulla. From this protophloem center primary and secondary phloem extend, fanlike, outward and downward in typical fashion. The rather startling character of the secondary phloem is its large number of suberized bast fibers compared to the number of sieve tubes. The former far. Fig. 6.—Cycas media: radial section of stem, showing centripetal end of bundle of innermost cylinder; m, pith; px, protoxylem distinctly spi- ral; s, scalariform tracheids of primary xylem, left one also having spiral thickenings; Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illust
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectplants, bookyear1895
