Periderm; c, cortex; ph, phlocni with alternating strands of fibres, sieve-tubes and parenchyma; ~r.r, principal ray; Sr., subordinate rays; ca, cambium.
A peculiar modification of periderm is formed by the phellogen in the submerged organs (roots or stems) of many aquatic or marsh-loving plants.
In other cases, a similar formation of spongy but dead periderm tissue may occur for the same purpose in special patches, called pneumatodes, on the roots of certain trees living in marshy places, which rise above the soil in order to obtain air.
The tissues of an animal or plant are all under a certain pressure, caused, in the one case, by the expulsive action of the heart and the restraint of the skin and other elastic tissues, and, in the other case, by the force of the rising sap and the restraint of the periderm or bark.
One chief means employed by nature in accomplishing this object is the investment of those parts of the organism liable to be attacked with an armour-like covering of epidermis, periderm, bark, &c. The grape is proof against the inroads of the yeastplant so long as the husk is intact, but on the husk being injured the yeast-plant finds its way into the interior and sets up vinous fermentation of its sugar.
Occasionally happens that groups of (After Worsdell.) xylem and phloem are developed pd, Periderm in leaf-bases.
U1 - U', leaf-traces, numbered according to the phyllotaxis, It 5 belonging to the lowest leaf of the five; ph, a group of primary phloem; pd, periderm, formed from pericycle.
In the old stems the primary cortex was replaced by periderm, giving rise to a thick mass of bark.
At the same time a deep-seated periderm arose, by which the primary cortex was soon entirely cut off.
The primary cortex between stele and periderm has perished.
The development of periderm was a constant feature, and this tissue attained a great thickness, consisting chiefly of a phelloderm, produced on the inner side of the formative layer, and no doubt subserving a mechanical function.
The secondary cortex or periderm attained a great development, and in some cases shows considerable differentiation.