Explain the different types of tissue systems found in the various organs of plants.
Answers
Plant body organization
Like animals, plants are multicellular eukaryotes whose bodies are composed of organs, tissues, and cells with highly specialized functions. The relationships between plant organs, tissues, and cell types are illustrated below.
Plant Organ Systems
Vascular plants have two distinct organ systems: a shoot system, and a root system. The shoot system consists of two portions: the vegetative (non-reproductive) parts of the plant, such as the leaves and the stems, and the reproductive parts of the plant, which include flowers and fruits. The shoot system generally grows above ground, where it absorbs the light needed for photosynthesis. The root system, which supports the plants and absorbs water and minerals, is usually underground.
The Root System
The roots of seed plants have three major functions: anchoring the plant to the soil, absorbing water and minerals and transporting them upwards, and storing the products of photosynthesis.
The shoot system: stems and leaves
Stems are a part of the shoot system of a plant. Their main function is to provide support to the plant, holding leaves, flowers and buds. Of course they also connect the roots to the leaves, transporting absorbed water and minerals from the roots to the rest of the plant, and transporting sugars from the leaves (the site of photosynthesis) to desired locations throughout the plant. They may range in length from a few millimeters to hundreds of meters, and also vary in diameter, depending on the plant type. Stems are usually above ground, although the stems of some plants, such as the potato, also grow underground.
Leaves are the main sites for photosynthesis: the process by which plants synthesize food. Most leaves are usually green, due to the presence of chlorophyll in the leaf cells.
The thickness, shape, and size of leaves are adapted to specific environments. Each variation helps a plant species maximize its chances of survival in a particular habitat.
Plant tissues
Plant tissue systems fall into one of two general types: meristematic tissue, and permanent tissue. Cells of the meristematic tissue are found in meristems, which are plant regions of continuous cell division and growth. Meristematic tissue cells are either undifferentiated or incompletely differentiated, and they continue to divide and contribute to the growth of the plant.
Ground tissue carries out different functions based on the cell type and location in the plant, and includes parenchyma (photosynthesis in the leaves, and storage in the roots), collenchyma (shoot support in areas of active growth), and schlerenchyma is the site of photosynthesis, provides a supporting matrix for the vascular tissue, provides structural support for the stem, and helps to store water and sugars.
Vascular tissue transports water, minerals, and sugars to different parts of the plant. Vascular tissue is made of two specialized conducting tissues: xylem and phloem. Xylem tissue transports water and nutrients from the roots to different parts of the plant, and also plays a role in structural support in the stem.
Plant Cell Types
All plants have primary cell walls, which are flexible and can expand as the cell grows and elongates. Some plants also have a secondary cell wall, typically composed of lignin . Secondary cell walls are inflexible and play an important role in plant structural support.
Cells in ground tissue
Ground tissue includes parenchyma, (photosynthesis in the leaves, and storage in the roots), collenchyma (shoot support in areas of active growth), and schlerenchyma (shoot support in areas where growth has ceased).
Parenchyma are the most abundant and versatile cell type in plants. They have primary cell walls which are thin and flexible, and most lack a secondary cell wall. Most of the tissue in leaves is comprised of parenchyma cells, which are the sites of photosynthesis.
Collenchyma, like parenchyma, lack secondary cell walls but have thicker primary cells walls than parenchyma. They are long and thin cells that retain the ability to stretch and elongate; this feature helps them provide structural support in growing regions of the shoot system.
Schlerenchyma cells have secondary cell walls composed of lignin, a tough substance that is the primary component of wood. Schelrenchyma cells therefore cannot stretch, and they provide important structural support in mature stems after growth has ceased. Interestingly, schlerenchyma cells are dead at functional maturity.