Plants that inhabit wetlands range from very tiny plants to large trees and display various structural adaptations to wet areas. Most plants retain an epidermis that in roots may produce hairs and are one to several cell layers thick. Large air spaces (aerenchyma) are often found in the cortex and/or pith of stems and roots and the bark of some plants. Aerenchyma begins as small spaces between cells and then progresses through one of the three developmental paths: schizogeny, lysigeny, and expansigeny. In roots, the aerenchyma is delimited by an inner barrier (endodermis) and an outer barrier (hypodermis). Modifications to these layers may include Casparian bands, suberin lamellae, or secondary cell wall development. In roots, vascular tissues may be comprised of a single xylem element or multiple strands of alternating xylem and phloem. In stems, vascular tissues may be found in ring(s), as scattered bundles or with root-like configurations.
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