The spongy mesophyll layer in leaves is composed of parenchyma cells. These cells are loosely arranged with large intercellular spaces, which facilitate gas exchange, such as the diffusion of carbon dioxide and oxygen, essential for the process of photosynthesis within the leaf structure.

The central region of a plant stem, often referred to as the stele, primarily consists of vascular tissues, specifically xylem and phloem. These tissues are responsible for the transport of water, minerals, and nutrients throughout the plant body. In many vascular plants, the arrangement of these tissues is central to the structural integrity and physiological function of the stem.

A collateral vascular bundle is one in which the xylem and phloem are arranged on the same radius, with the phloem positioned on the outer side (abaxial) and the xylem on the inner side (adaxial). This is the most common arrangement in the stems of many seed plants.

In many dicotyledonous plants, the mesophyll tissue within the leaf is organized into two distinct layers: the palisade parenchyma, which is located towards the upper epidermis and is rich in chloroplasts for photosynthesis, and the spongy parenchyma, which is located towards the lower epidermis and contains air spaces for gas exchange. Together, these two tissues constitute the mesophyll.

The pith is the central part of the root or stem, composed primarily of parenchyma cells. In many monocot roots, the pith is well-developed and occupies the center, whereas in dicot roots, the central region is often occupied by the vascular cylinder or xylem, though the term pith is generally used to describe the central ground tissue.

In plant anatomy, the cortex is the region between the epidermis and the vascular cylinder. The inner layers of the cortex can differentiate into various specialized tissues. In certain contexts, the spongy parenchyma or spongy tissue is derived from the inner cortical regions, particularly in leaf mesophyll or specific stem structures, providing space for gas exchange.

In the primary structure of many plant stems, the outermost layer is the epidermis, which serves as a protective covering for the underlying cortical tissues. While the endodermis is a specialized layer often found deeper within the root or stem, the epidermis is the standard single-layered boundary for the cortex in most young stems.

The mesophyll of a dorsiventral leaf is differentiated into palisade and spongy parenchyma. The palisade cells are elongated and arranged vertically just beneath the upper epidermis. They contain a high concentration of chloroplasts to maximize light absorption for photosynthesis. In contrast, the spongy parenchyma cells are more loosely arranged with larger intercellular spaces, facilitating gas exchange.

In plant anatomy, the pith (or medulla) is the central tissue of a stem or root, typically composed of soft, spongy parenchyma cells. It is surrounded by the vascular tissue. In many pteridophytes, the presence or absence of pith is a key diagnostic feature in the evolution and classification of different types of steles, such as protosteles versus siphonosteles.

In dorsiventral leaves, the palisade parenchyma is a layer of elongated cells containing chloroplasts, situated directly beneath the upper epidermis. This positioning maximizes light absorption for photosynthesis. The epidermis serves as the protective outer layer, and the palisade cells are organized to capture sunlight efficiently as it enters the leaf structure.