SC.912.L.14.7: Relate the structure of each of the major plant organs and tissues to physiological processes.
Intro
Plants are organisms that are multicellular eukaryotes of the kingdom “Plantae”. There is new evidence that suggests plants may have first appeared as much as 1 billion years ago. Plants producing oxygen, removing carbon dioxide from the atmosphere, removing toxic substances from soil/water are just a few examples of some of the important free services humans benefit from (ecosystem service).
General Plant Anatomy
Roots
The roots burrow into the ground allowing plants to remain anchored to what ever substrate (surface) they are growing in or on. The roots of some plants are so strong that the plant can withstand hurricane force winds.
Another important function of plants is that they allow the the absorption of water and nutrients from the soil. Roots can also serve as a storage organ for plants. Just as animals store excess energy in the form of lipids in their fat tissue beneath the skin, plants can store excess carbohydrates in the form of starch in their roots. A potato that we eat is actually the root structure of the plant, we consume it for the large amounts of starch stored within them.
The roots of some plants allow them to perform a process known as “vegetative reproduction“. Vegetative reproduction is a form of asexual reproduction. It can occur when a piece of a plant is broken off and a copy of the entire plant can regrow from that piece. This can occur with the roots of some plants.
Stems
Stems provide one of the two main axis of the plant. Stems contain two structures called “xylem” and “phloem” similar to veins and arteries in your body. Xylem is a series of tubes in the stem that transport water up from the root to the higher tissues of the plant. Phloem are a network of tube structures that transport some of the products of photosynthesis such as the sugar sucralose.
Leaves
Function/Physiology
Leaves are typically flat broad structures that are attached to projections come from the stem of a plant. Leaves are responsible for controlling gas exchange, regulating water loss, and performing much a plant’s photosynthesis. Because most leaves are flat and broad they have an increased surface area. Surface area is the total area along the entire surface of an object. Just like a wide object will catch more rain drops than a narrow object, a wide leaf will catch more rays of sunlight than a narrow leaf.
Structure/Anatomy
One way modern day plants can be categorized is by the structure of their leaves. Monocot plants have only 1 leaf in their embryological form, the veins of the leaves are also run parallel to each other. Dicot plants have two leaves in their embryological form, and their veins have a branching network structure. Leaves have a layered sandwich like structure.
From the top of the leaf the outer waxy layer is known as the “cuticle” sitting above the “upper epidermis”. The mid section of the leaf the “mesophyll” separates the bottom topside of the leaf from the bottom layer. The underside of the leaf has the stomata and guard cells. The “stomata” is a mouth like opening that allows for the exchange of oxygen, carbon dioxide, and water vapor. To prevent water vapor from rushing out of the stomata the opening can be opened and closed by special cells around the stomata called “guard cells“. Certain plants that have evolved in very hot dry regions will keep their stomas closed during the hottest part of the day to prevent dehydration.
4 Major Plant Tissues
Dermal Tissue
The dermal layer is considered the “skin” of the plant. To prevent gas exchange/water loss the dermal layer is often covered with a thick waxy layer known as the “cuticle“.
Meristematic Tissue
Ground Tissue
Ground tissue is a collection of various tissue types with a variety of functions.
Parenchyma
This ground tissue is responsible for conducting; photosynthesis, food storage, healing and tissue regeneration. Parenchymal tissue contains cells that are generally thin, and flexible with larger central vacuoles. The outer region of the stem, the inner region of the stem (pith), and the mesophyll of the leaves are composed of parenchymal cells.
Collenchyma
Type of ground tissue that acts as support for the plant. The cells of this tissue have thickened cell walls that are still alive. The stringy fibers found in celery are an example of collenchyma tissue.
Sclerenchyma
Sclerenchyma is hardy type of ground tissue, it is often composed of thick cells that may sometime be dead. The prefix “Sclera” means scar, so a useful way to remember that sclerenchyma is tough is to remember that scar tissue feels tougher than normal skin. This tissue is usually found in older regions of the plant that are no longer growing.
Vascular Tissue
Vascular tissue for plants consists of Xylem and Phloem. Just like your vascular tissue; arteries, veins, and capillaries the vascular tissue of the plant is used to transport fluids and nutrients to various parts of the body. The xylem is used to transport water and minerals up from the roots towards the upper parts of a plant’s body. The water exits as vapor through the stomata of the plant via a process called Evapotranspiration or transpiration.
The phloem transports the products of photosynthesis created in the leaves down the body of the plants all the way to the roots. Phloem flows in both directions. It is important to note that the phloem is responsible for bringing sugar to the roots because the roots cannot perform photosynthesis and rely on the other tissues of the plant to create glucose. The phloem also transports glucose to the roots because the roots also serve as a food storage organ.
Plant Reproduction
Angiosperms
Angiosperms are plants that reproduce using flowers and fruits. Any plant that has a flower would be considered an angiosperm! The male portion of a flower the Anther produces pollen which contain sperm cells. The pollen may be blown by wind or carried by a pollinator to the opening on the female part of the flower called the Stigma. The surface of the stigma is coated in a sticky fluid that helps pollen adhere to and get into the opening of the stigma.
When pollen gets into the stigma and travels down the style it will release sperm cells that are able to pollinate the egg cells (Ovule) contained within the ovary of the flower.
Once the ovule is fertilized the ovary will swell, and develop into a budding fruit. This is how most of your favorite fruits are produced by trees.
Gymnosperm
Gymnosperms are an ancient group of plants that produce naked seeds and cone structures to reproduce. Many of these plants possess rounded needlelike leaves instead of the typical broad and flat leaves we commonly see. Evergreen plants like pine trees are examples of gymnosperm plants.
Gymnosperms release their fertilized seeds to germinate in the soil. Each scale on a pine cone could contain a fertilized/ripened seed.
