Structure and Evolution of Plants

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Table of Contents

Phyla of algae
Co-evolution
Life cycle of a moss
Organs and tissues of a plant structure
References

Phyla of algae

Among the different phyla of algae are the following; diatoms which are unicellular and mainly characterized by silica. Most exist singly but can form colonies. They inhabit moist sols, fresh water and even salt water they are yellowish and a times brownish in color. Reproduction is asexually by cell division. Chlorophyta is a phyla made up of photosynthetic organism. They can either be unicellular, multicellular or coenocytic. They are aquatic while a few live on land usually on moist soils or tree trunks. Euglenophyta is made up of unicellular aquatic algae. Some are photosynthetic while others are heterotrophic. They reproduce by longitudinal cell division. They commonly inhabit fresh water. Chrysophyta are also known as golden algae mostly found in freshwater. Their cell walls are made up of cellulose having high levels of silica. This phylum contain chlorophyll pigments a and c. there are situations where they reproduce sexually but they do that usually by cell division. Another phylum is phaeophyta (brown algae). Seaweeds make up a greater proportion of organisms under the phylum. They inhabit the coldest oceans mostly in tidal zones. Some grow in deep waters. The brown color they have is derived from the presence of brownish carotenoids they have in their cell chloroplasts.

Another phylum is the rhodophyta also termed as the red algae. Members of this phylum have a distinct red or purple color derived from accessory pigment, phycobilins. They are multi-cellular and highly branched but lack complex tissue development (Stewart & Rothwell, 1993). The seaweeds are members of this phylum. They are found in oceans but most commonly in warm temperate and tropical regions. They grow in great depths than any other organisms that photosynthesize. Some of the species such as coralline algae produces calcium carbonate important in building reefs. Lastly Cyanobacteria are aquatic bacteria that derive energy via photosynthesis (blue algae). Interestingly, organisms in this phylum can contain chlorophyll a and protein phycobilins responsible for the blue color. However, very few lack the protein but have both chlorophyll a and b; responsible for the bright green color. Cyanobacteria are autotrophic. There are some that need nitrogen and carbon-dioxide as they are capable of fixing nitrogen. They inhabit fresh water, bare rocks, and soils as well as oceans waters.

Co-evolution

Co-evolution is a situation where by two species evolves alongside one another where each is adapting purely based on the selection pressure from the other species (Thompson 1994). I most cases this happens when different species share or have a closer ecological interactions. A typical example is between the honeybees and flowers where the former is dependent on the later and vice versa. Similarly carnivores and herbivores have experiences that have made each other to adapt to strategies to avoid attack while the predator strives to better its skills so that it can capture the prey; lion and impala. Other examples include hummingbirds and ornithophilous flowers, Angraecoid orchids and African moths among others. Evidence to support coevelution includes character displacement, mutual relationship between organisms such as ant-acacia mutualism, mimicry among others.

Alteration of generation is a situation whereby an organism regularly changes from diploid to haploid phases and vice versa. Ideally the concept is of significance to organisms since it enhances of survival. The algae become more adapted to their environment thanks to reshuffling of genes during meiosis.

Water is transported through the plant from the roots via the xylem vessels. The vessel carries water as well as other minerals from the roots via the stem and then to veins in the leaves. This is accomplished through transpirational pull and capillary action.

Depending on their habitat plants have adopted various characteristics to enhance their survival. For instance those in desserts or very dry areas have very long tap roots and root hairs that can be found on surface of the earth to take advantage of dew (Thompson, 1994). They also shade their leaves during certain period to avoid loss of water, some have reversed mechanisms where they open the stomata at night and closing them during the day, others have reduced leave surface area in terms of having spikes. They also have fleshy and succulent leaves useful when storing water. Hydrophytes do have more stomata on top of their leaves to enhance loss of water, they also have the ability of getting rid of water from their cells and their roots are well adapted to such environments.

Life cycle of a moss

Moss plant is mostly monoecious having antheridium and archegionium as male and female ex organs respectively. When archegonium matures it secrets mucilage as well as sugar that will attract antherozoid which will later swim towards an opening of the neck and then enter into the venter. The fusion of an antherozoid with one egg leads to formation of a zygote (oospore). The zygote is diploid and will grow into a sporogonium while inside the venter. It attains nutrients from gametophytes. Similarly spores are haploid and develop inside the capule while sporophytes mature. Once the spores are ripe, the capsule opens up and the spores are released and transported by wind. They then form protenema which will grow into new gametophyte if conditions are favorable (Stewart & Rothwell, 1993).

Organs and tissues of a plant structure

Plants do have three basic organs; roots responsible for anchoring vascular plants, absorbing minerals and water as well as storing nutrients. The stem constitutes an alternating nodes and internodes and the leaves which is the main photosynthetic organ. Tissues in plant include the dermal tissue which is the outer protective covering. Vascular tissue includes xylem and phloem for transportation of water and nutrients. Finally plants have ground tissue which are highly adapted to storing food, offer support to the plant and a times carryout photosynthesis.

Plants collect sunlight through chlorophyll. This is responsible for absorption of the visible wavelength except the green light which is reflected. Plants are able to take up carbon dioxide from the atmosphere through the stomata. Auxin hormone initiates flowering as well as development of reproductive organs, it also enhances growth and development of fruits, auxin also is responsible for apical dominance, initiate root development, enhances healing of both xylem and phloem more importantly auxin is responsible for phototropism, geotropism as well as hydrotropism in plants (Stewart & Rothwell, 1993).

Phototropism has been defined as the tendency of plants and some fungi to grow in a certain direction dictated by the source of light. Plants can either exhibit positive or negative phototropism where they grow towards or away from the source of light respectively. Gravitopism is a term used to refer to the condition where plants orient their roots towards gravitational pull and it is evident when plants grow downward via their roots (Sadava et al., 2008).

References

Sadava, et al., (2008). Life: The Science of Biology. New York: W. H. Freeman & Co., and Sumanas, Inc.

Stewart, W. & Rothwell, G. (1993). Paleobotany and the Evolution of Plants (2nd ed.), Cambridge, UK: Cambridge University Press.

Thompson, J. (1994). The Coevolutionary Process. Chicago: University of Chicago Press.

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Structure and Evolution of Plants

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