Plant Structure
The relationship between the distribution of tissues in the leaf and the functions of these tissues is complex and diverse. However there are rules that apply generally to all plants whether they live on land or water. The tops and bottoms of leaves have a waxy cuticle to prevent water loss and evaporation. On the bottom of leaves generally there are stomata. Leaves have big surface area and are thin to facilitate photosynthesis. With a large surface area the leaves can catch more sunlight. The concentration of chloroplasts and photosynthetic organelles are concentrated on the top layer of the leaf. Under this thick layer of chloroplast containing cells there is a spongy layer to allow gas exchange to take in carbon dioxide and release oxygen. In vascular plants there is vascular tissue that collects photosynthetic products and sends water to the leaves.
Over the course of time and evolution land plants have come to adapt in many ways. Terrestrial plants support themselves by means of thickened cellulose, cell turgor and xylem. Plants in the desert have to adapt to conserve water and prevent water loss. In general there are four main adaptations of xerophytes, plants that live primarily in the desert. Xerophytes generally have very small, thin, and few leaves. The waxy cuticle of xerophytes also is generally thick to prevent water loss in the hot dry environment it is generally exposed to. CAM plants take carbon dioxide during the night because if the stomata open during the day the hot weather would drain water from the plant. The CO2 is converted into organic acids and use them in photosynthesis during the daytime when sunlight is available. C4 plants work in basically the same way, but utilize different acids to store carbon dioxide to use in the daytime. Roots of these plants reach deep and spread out to maximize water intake. In such dry environments water is not easy to come by. Stomata are in pits or are surrounded by hairs to prevent water loss.
As with land plants, plants that live in marine environments have to adapt as well. There are two structural adaptations of hydrophytes, plants that live primarily on the surface of ponds and lakes. Hydrophytes live at the surface of aquatic environments. They are spongy and its sponginess allows it to float. Leaves are small and widely spread out to provide a big surface area for maximum sunlight absorption. The roots of hydrophytes usually act to anchor the plant and are relatively simple. The leaves and stems are also flexible and soft in the water.
Aside from Xerophytes and hydrophytes under the category of plants called Angiospermophytes there involves a complex transport system. The root system provides a large surface area for mineral ion and water uptake by means of branching, root hairs and cortex cell walls. This is an evolutionary adaption that allows for maximum surface area and better opportunity for absorbtion of substances vital to plant survival. Roots have tiny hairs on them to help absorb more water. The root hairs are part of the outer most layer of the main root and do not actually contain vascular tissue such as xylem or phloem. Mineral ions are taken into the root by active transport. Roots branch out to spread out and maximize surface area to take in as much water as possible. Cortex cell walls allow for osmosis to occur in the roots because of their permeable properties.
On the topic of root absorbtion of important soil nutrients, mineral uptake in land plants is important as well. The process of mineral ion uptake into roots by active transport is complex. Ions are absorbed from the soil by both passive and active transport. Specific ion pumps in the membranes of root hair cells pump ions from the soil into the cytoplasms of the epidermis cells. Two lines of evidence indicate that active transport is being used: The concentrations of ions inside root cells greatly higher than the concentrations of ions in the soil, so they must be actively transported against the concentration gradient. The active uptake of ions is partly responsible for the water potential gradient in roots, and therefore for the uptake of water by osmosis. Ions diffuse down their concentration gradient from the epidermis to the xylem. They travel up the xylem by mass flow as the water is pulled up the stem. In the leaves they are selectively absorbed into the surrounding cells by membrane pumps.
The process of water uptake by root epidermis cells is no less complex than the process of mineral uptake and involves the movement of water by the symplastic and apoplastic pathways across the root to the xylem. To put things in general, minerals are absorbed from soil through the root surface primarily by root hairs. The water and minerals then move across the root cortex to the vascular cylinder by a combination of the apoplastic and symplastic routes. The uptake of soil solution by the hydrophilic walls of the epidermis provides access to the apoplast, and water and minerals can soak into the cortex along this matrix of walls. Minerals and water that cross the plasma membranes of root hairs enter the symplast. As soil solution moves along the apoplast, some water and minerals are transported into cells of the epidermis and cortex and then move inward via the symplast. Water and minerals that move all the way to the endodermis along cell walls cannot continue into the stele via the apoplastic route. Within the wall of each endodermal cell is a belt of waxy material (black band) that blocks the passage of water and dissolved nutrients. This barrier to apoplastic transport is called the Casparian strip. Only materials that are already in the symplast or enter that pathway by crossing the plasma membrane of an endodermal cell can detour around the Casparian strip and pass into the stele. The transport of minerals that are admitted into the cells within the stele discharge water and minerals into their walls. These walls as part of the apoplast, are continuous within the xylem vessels. The water and minerals absorbed from soil are then ready for transport.
Put how does water move up the xylem against the force of gravity? The answer lies in the transpirational-pull theory. Transpiration can be formally defined as the loss of water vapor from the leaves and stems of plants through the stomata. Water is carried by the transpiration stream, including the structure of xylem vessels, transpiration pull, cohesion and evaporation. Xylem tubes are made of dead cells that have sieve-like ends to allow water flow. Water is a polar molecule so it bonds to other water molecules through adhesion. Transpiration pull works by cohesion-tension theory. When water molecules in the leaves are pulled into the air by evaporation, all the water that is in the xylem tubes moves up the stems towards the leaves because of cohesion and tension. Cohesion is the attraction of water to substances other than water, in this case the xylem walls. Transpiration rate is regulated by guard cells on the plant stomata. Guard cells can open and close stomata to regulate transpiration.
There are many environmental factors that can affect the rate of transpiration in plants. There are abiotic factors, light, temperature, wind and humidity, that affect the rate transpiration in a typical terrestrial mesophytic environment. Depending if stomata is open transpiration increases. Light effects blue-light receptors in the leaves that open stomata by creating a K+ ion gradient and causing the guard cells to absorb water. Hot temperatures cause stomates to close to prevent water loss. Wind accelerates water loss in plants, sucking water out of the plant if stomata are open through evaporation. Humidity has the opposite effect to wind. Humid air has a higher concentration of water in the air than wind and thus the concentration gradient between the plants and the air is not as great. Less water is lost in humid weather.
As with xylem, phloem also plays a key role in transportation of substance throughout the plant. Phloem however does not work using transpirational pull. It instead works with source and asink. The role of phloem in plants lies in the active translocation of biochemicals. Phloem is living vascular tissue that distributes sugar, amino acids, and other organic nutrients throughout the plant. Phloem works by properties of source and sink. Photosynthetic products come from a source then move down the plant to a sink, or target area. Proton pumps do the work that enables the cells to accumulate sucrose. The ATP-driven pumps move H+ concentration across the plasma membrane. Another membrane protein uses this energy source to co-transport sucrose in the cell along with returning hydrogen ions. These sugars are stored in the form of starch in plants.
Plants also have adapted to utilize their environment to the fullest of its potential. For example, angiosperms, or flowering plants utilize insects and wind for pollination and sexual reproduction. Sexual reproduction maximizes genetic recombination allowing more opportunity for evolution and creating a fitter species. Pollination can be formally defined as the transfer of pollen of an anther onto a stigma. Fertilization follows, which is the union of haploid gametes to produce a diploid gamete. Fertilization happens within the ovary of the plant. A gamete is produced. Seed dispersal how the seed is spread out or dispersed after it is mature.
After an angiosperm seed has been fertilized there are a series of metabolic events of germination in a typical starchy seed. After water is absorbed and germination begins, the formation of gibberellin hormone begins in the cotyledon. This stimulates the production of amylase which catalyzes the breakdown of starch to maltose. This metabolism fuels the energy required for growth. But seeds will not germinate unless provided conditions in the environment are suitable. There are several basic conditions needed for the germination of a typical seed. Seeds usually lie dormant until right conditions arise to let them germinate. Typically terrestrial plants are require ample water and sunlight to germinate but each species of plants is different.
In conclusions plants have adapted much over the course of evolution. They have adapted to live on land, in water, in the desert, and have learned how to adapt to their surroundings to maximize its utilization to the fullest potentials. Plants have developed vascular systems to transport minerals and water throughout their systems and have found efficient means of living.
