8.1 Plant Organs and Tissues

Like animals, plants contain cells with organelles in which specific metabolic activities take place. Unlike animals, however, plants use energy from sunlight to form sugars during photosynthesis. In addition, plant cells have cell walls, plastids, and a large central vacuole: structures that are not found in animal cells. Each of these cellular structures plays a specific role in plant structure and function.

Plant Organs

In plants, just as in animals, similar cells work together to form a tissue. When different types of tissues work together to perform a unique function, they form an organ; organs working together form organ systems.

Vascular plants have three main vegetative organs: roots, shoots, and leaves.

1. 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.
2. The root system, which supports the plants and absorbs water and minerals, is usually underground. Figure 30.2 shows the organ systems of a typical plant.
3. The leaves of a plant are charged with carrying out photosynthesis, water regulation, gas exchange, and structural support. This multifaceted organ makes up most of our visual perception of a plant.

Plants also have reproductive organs, which carry out sexual or asexual reproduction. In sexual reproduction, a female and male gametes within the flower which result in the creation of a seed. Asexual reproduction subverts the need for pollination and seed dispersal. Instead, plants can experience or perform budding, fragmentation, spore formation (such as in mosses or ferns), and more to create identical genetic copies of themselves.

Figure  30.2   The shoot system of a plant consists of leaves, stems, flowers, and fruits. The root system anchors the plant while absorbing water and minerals from the soil.

Plant Tissues

Plants are multicellular eukaryotes with tissue systems made of various cell types that carry out specific functions: dermal, vascular, and ground

1. Dermal tissue covers and protects the plant
2. Vascular tissue transports water, minerals, and sugars to different parts of the plant
3. Ground tissue serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps to store water and sugars.

Secondary tissues are either simple (composed of similar cell types) or complex (composed of different cell types). Dermal tissue, for example, is a simple tissue that covers the outer surface of the plant and controls gas exchange. Vascular tissue is an example of a complex tissue, and is made of two specialized conducting tissues: xylem and phloem.

Xylem cells, which transport water and nutrients from the roots to the rest of the plant, are dead at functional maturity. Phloem cells, which transport sugars and other organic compounds from photosynthetic tissue to the rest of the plant, are living. The vascular bundles are encased in ground tissue and surrounded by dermal tissue.

Figure  30.3   This light micrograph shows a cross section of a squash (Curcurbita maxima) root. Each teardrop-shaped vascular bundle consists of large xylem vessels toward the inside and smaller phloem cells toward the outside. (credit: modification of work by “(biophotos)”/Flickr; scale-bar data from Matt Russell)