MEDICAL BOTANY - A.G. Serbin - 2003

CHAPTER 1. ANATOMY

PLANT TISSUES

Plant Tissues evolved through the gradual transition to terrestrial life. A true tissue is defined as a group of Cells sharing a common origin, similar Structure, and unified Functions. True tissues are characteristic of higher plants, although their formation is also observed in highly organized lower plants, such as red and brown Algae. More primitive lower plants and Fungi possess false tissues, which differ in that their constituent cells have independent origins.

Based on morphological characteristics, tissues are classified into living (containing a protoplast) and dead (devoid of a protoplast), parenchymatous and prosenchymatous (according to Cell shape), thin-walled and thick-walled, as well as dense (lacking intercellular spaces) and loose (with intercellular spaces). A distinction is made between meristematic tissues and their derivatives—permanent tissues (Table 1.2). In turn, permanent tissues are subdivided into primary tissues, which form during the differentiation of primary Meristems, and secondary tissues, originating from secondary meristems or permanent ground tissues. According to their STRUCTURE AND FUNCTIONS, permanent tissues are grouped into protective tissues (epidermal and mechanical) and METABOLISM-related tissues (absorptive, conductive, secretory, and fundamental).

MERISTEMATIC TISSUES

Meristematic tissues, or meristems, give rise to all permanent tissues and drive organ growth. They are categorized based on their origin into Primary and secondary, as well as by their localization within the plant body (Fig. 1.11) and physiological functions. Apical meristems originate in the plant embryo, persist at the apexes—the ROOT tip and SHOOT apex—and ensure the longitudinal growth of Organs. Lateral meristems extend along the axis of organs, causing their secondary thickening. These include primary lateral meristems (procambium, pericycle) and secondary ones (cambium, phellogen or cork cambium). Intercalary meristems are primary meristems located at the base of internodes in shoots and leaves, facilitating their elongation. Traumatic, or wound, meristems are secondary tissues that develop at sites of injury. Cell Division in meristems is stimulated by phytohormones.

Class="center">Fig. 1.11. Meristems of the shoot (A) and root tip (B): 1 — apical; 2 — intercalary; 3 — 5 — lateral (of the central cylinder): 3 — procambium; 4 — cambium; 5 — pericycle; 6 — apical initial cells; 7 — ground meristem of the primary cortex; 8 — protoderm

Meristematic cells (Fig. 1.11, 3, 6) are parenchymatous, living, thin-walled, and tightly packed, featuring dense Cytoplasm, a large nucleus, and an Abundance of Ribosomes. Plastids are represented by proplastids and leucoplasts, while vacuoles are either absent or extremely small. Meristematic cells that remain in a state of continuous division are termed initials, whereas those derived from them that undergo differentiation are known as derivatives of initials, or ground meristem.