The thin-walled spiral or annular tracheae of the protoxylem allow of longitudinal stretching brought about by the active growth in length of the neighboring living parenchymatous cells of a growing organ.
These fibre-tracheids are easily confused on superficial view with the true wood-fibres belonging to the parenchymatous system; but their pits are always bordered, though in the extreme type they are reduced to mere slits in the wall.
The typical structure of the vascular cylinder of the adult primary stem in the Gyrnnosperms and Dicotyledons is, like that of the higher ferns, a hollow cylinder of vas- Structure of cular tissue enclosing a central parenchymatous pith.
The characteristic companion-cells of Angiosperms are represented by phloem-parenchyma cells with albuminous contents; other parenchymatous elements of the bast contain starch or crystals of calcium oxalate.
The pith is encircled by a cylinder of secondary wood, consisting of single or multiple radial rows of tracheids separated by broad medullary rays composed of large parenchymatous cells; the tracheids bear numerous bordered FIG.
Within this muscular tube lies a parenchymatous tissue which may be uniform (Cestodes) or differentiated into a central or digestive, and a peripheral portion (some Turbellaria), or finally the central portion becomes tubular and forms the digestive sac (Trematodes), while the peripheral portion is separated from it by a space lined in some forms by a flattened epithelium (most Planarians).
The cylinder is surrounded by a mantle of one or more layers of parenchymatous cells, the pericycle, and the xylem is generally separated from the phloem in the stem by a similar layer, the mesocycle (corresponding with the amylom sheath in mosses).
The interfascicular cambium may form nothing but parenchymatous tissue, producing merely continuations of the primary rays.
Which is essentially a parenchymatous tissue containing chloroplasts, and is penetrated by a system of intercellular spaces so that the surfaces of the assimilating cells are brought into contact with air to as large an extent as possible, in order to facilitate gaseous interchange between the assimilating cells and the atmosphere.
The whole sac, with its epithelial wall and its contained genital cells, arises ultimately from some of the parenchymatous cells of the body.