plants have thick bark

by danielle luise castro
(makati city,philippines)

The term bark refers to all tissues of a woody stem or root occurring just outside of the vascular cambium, i.e., all tissues that could be stripped away from the woody core. The petiole or rachis of a long-lived leaf can theoretically also form patches of bark, but this normally is not observed. Stems of monocotyledons, especially trunks of arborescent types, often have an outer cover, sometimes appearing like a form of bark, but in these cases the origin of the cover is very different from that of dicotyledons and gymnosperms. Ferns and other seedless vascular plants never form bark of any type.

Inner and Outer Bark

Where present, the bark most typically serves two very important functions. The outer, mostly dead tissues (outer bark) form a protective barrier between the plant axis and the abiotic and biotic environment. The inner tissue (secondary phloem), including living cells (inner bark), is where sugar transport for the plant occurs, and the inner bark also can have defenses against herbivores, such as cells with tough cell walls (secondary phloem fibers or sclereids) or cells and tubes filled with bitter or toxic chemicals.

Bark is a very complex structure, consisting of cells that formed from lateral meristems (cambia producing secondary tissues) but often containing some cells (living or dead) that remain from the first-formed plant (primary tissues, such as epidermis, cortex, and primary phloem). In general, the inner bark is defined very narrowly as the innermost sector consisting only of secondary phloem, which is produced to the outside of and by the vascular cambium. The outer bark always includes all tissues formed by the lateral meristem known as the cork cambium (phellogen) or a number of individual and often discontinuous cork cambia.

Periderm and its Cell Types

Each cork cambium produces a sector of cells called a periderm. The dividing cells (initials) of the cork cambium (phellogen) divide to produce cells, most of which are "pushed" toward the outside, and some may be pushed toward the inside. Cells pushed to the outside generally have layering of suberin in their walls, and thereby become cork cells (also called phellem). Suberin, often composed of suberic acid and phellonic acid, is a fatty and waxy substance, which makes the cells mostly impervious to water and unable to exchange gases and nutrients, hence these cells soon die and entrap air. In some species, cells pushed to the outside of the cork cambium can develop with thickened cell walls but lacking suberin (phelloids). Cells pushed from the cork cambium to the inside are called phelloderm, and these can remain alive because they have unthickened and unspecialized cell walls and, hence, can exchange gases and obtain nutrients.