Chapter 35 - Vascular Plant Structure, Growth, and Development

Organs, tissues, and cells are organized in a hierarchical structure in plants.

Plants, like the majority of animals, are made up of cells, tissues, and organs. A cell is the most basic unit of life. A tissue is a collection of cells made up of one or more cell types that work together to fulfill a specific purpose.

An organ is made up of many types of tissues that work together to perform certain activities. Keep in mind how natural selection has generated plant shapes that match plant function at all levels of the organization as you study each of these layers of plant structure. We'll start with plant organs because their structures are the most recognizable.

An illustration of a blooming plant. The plant body is split into two parts: the root system and the shoot system, which are linked by vascular tissue (purple threads in this image) that runs throughout the plant. The plant depicted is a stylized eudicot.

A root is an organ that holds a vascular plant in place in the soil, absorbs minerals and water, and frequently stores carbohydrates and other reserves.

The primary root is the first root (and the first organ) to emerge from a germinating seed. It originates in the seed embryo.

It quickly branches to create lateral roots (as shown in the image attached), which can also branch, substantially improving the root system's capacity to anchor the plant and obtain resources such as water and nutrients from the soil.

Tall, tall plants with dense shoot masses typically have a taproot system, which consists of one major vertical root, the taproot, which emerges from the primary root. Absorption plays an important function in taproot systems.

Vascular plants' basic shape reflects their evolutionary history as terrestrial creatures that live and take nutrients from two quite distinct environments—below and above ground.

They must take water and minerals from below the earth’s surface, as well as CO2 and light from above. The capacity to efficiently collect these nutrients may be traced back to the evolution of roots, stems, and leaves as the three fundamental organs.

These organs combine to produce a root system and a shoot system, the latter of which consists of stems and leaves (as shown in the image attached). With a few exceptions, vascular plants rely on both systems to survive.

Roots are virtually never photosynthetic; they die unless they are fed.

Shoots of vascular plants are made up of stems, leaves, and, in the case of angiosperms, flowers. Roots serve as plant anchors, absorb and transmit water and nutrients, and store food.

The major organs of photosynthesis are the leaves, which are connected to the stem nodes. Axillary buds, which grow in the axils of leaves and stems, give rise to branches. Plant organs can be modified to perform particular purposes.

Vascular plants have three tissue systems that run throughout the plant: cutaneous, vascular, and ground. Dermal tissue is a continuous layer of cells that covers the plant's surface. Vascular tissues (xylem and phloem) aid in the transfer of chemicals across vast distances.

Ground tissues perform storage, metabolism, and regeneration functions.

Parenchyma cells are undifferentiated, thin-walled cells with the potential to proliferate; they carry out the majority of the metabolic activities of synthesis and storage. Collenchyma cells have unevenly thickened walls and sustain the plant's young, developing components.

Sclerenchyma cells—sclereids and fibers—have thick, lignified walls that aid in the maintenance of mature, nongrowing plant components. Tracheids and vessel elements, the xylem's water-conducting cells, have thick walls and die as they reach functional maturity.

Sieve-tube elements are alive but highly modified cells that lack many internal organelles; they are fungal.

Roots and shoots become longer as a result of primary growth.

The root apical meristem is positioned near the root's tip and produces cells for the developing root axis and root cap.

A shoot's apical meristem is found in the apical bud, where it gives birth to alternating internodes and leaf-bearing nodes.

Eudicot stems contain ring-shaped vascular bundles, whereas monocot stems have dispersed vascular bundles.

Mesophyll cells are photosynthesis-adapted. Stomata, epidermal holes produced by pairs of guard cells, provide for gas exchange and are important water loss pathways.

The plant body is created by growth, morphogenesis, and cell differentiation.

The major factors of growth are cell division and cell expansion. In a dividing cell, a preprophase band of microtubules dictates where a cell plate will develop. Microtubule orientation influences cell elongation by directing the orientation of cellulose microfibrils in the cell wall.

Morphogenesis, or the production of body shape and structure, is dependent on neighboring cells reacting to positional information.

Cell differentiation, which occurs as a result of differential gene activation, allows cells within the plant to perform diverse roles while having similar genomes.

The location of a plant cell in the growing plant has a major influence on how it differentiates.

Internal or external stimuli may induce a plant to transition from one developmental stage to another, such as from producing juvenile leaves to developing adult leaves. These morphological alterations are referred to as phase changes.

A model system for investigating pattern creation is provided by research on organ identity genes in developing flowers. The ABC theory explains how three types of organ identity genes regulate the development of sepals, petals, stamens, and carpels.