Adaptations of Plants in Invasion from Aquatic to Terrestial Habitats
Plants have not always been around on land. For a long time, life was confined to water. The first photosynthetic organisms were bacteria that lived in the water. So, where did plants come from? There is overwhelming evidence that the ancestors of modern land plants evolved in aquatic environments, where they existed and diversified over millions of years. From one group of these organisms, probably ancestors of modern species of protists known as green algae, emerged a new branch on the tree of life. From this branch arose four groups of land plants, including the mosses, the ferns, the conifers, and the flowering plants. These groups are believed to represent a sequence that reflects the evolutionary history of land plants. Evidence also shows that plants probably evolved from a particular freshwater green algae, a protist, namely Chara. The similarities between green algae and plants is one solid piece of evidence. They both have cellulose in their cell walls, and they share many of the same chemicals that give them colour. So what separates green algae from green plants? There are several ways that plants adapted to life on land and, as a result, became different from algae.
As plants evolved from aquatic to terrestrial environments, several obstacles stood in the way. One such obstacle was structural support. In water, organisms are buoyant and the effects of gravity are minimal. Even among larger forms, like kelps, structures with gas-filled vesicles allowed them to float. On land, however, if a plant is to grow tall, it needs to withstand the forces of gravity. What adaptations allowed plants to get tall? Rigid cell walls developed to provide support, as did different types of supportive tissues — the woody tissue layers of trees are good examples. These woody tissues arose from the process of primary and secondary growth in plants. Primary growth mainly involves vertical elongation (increase in height), cellular differentiation and formation of various structures such as primary xylem, primary phloem etc. Secondary growth would mainly be the lateral growth of plants where they increase in girth and develop some more structures such as secondary xylem and secondary phloem. Xylems and schlenrenchyma cells in plants are covered in a layer of strong and tough substance known as lignin which in turn greatly increases support for the plants and reduces the effect of gravity on them. The mosses lack these tissues, and are thus limited to a “lowly” existence on the ground. This type of support is found in the ferns, but is fairly primitive. There are thus few ferns that grow more than a meter above the ground. It is in the conifers and flowering plants that we observe the most well-developed adaptations of this nature
In plants, the embryo develops inside of the female plant after fertilization. Algae do not keep the embryo inside of themselves but release it into water instead. This was the first feature to evolve that separated plants from green algae. This is also the only adaptation shared by all plants.
Another challenge during the transition to land involved bringing sex cells together. In water, sperms are able to swim directly to eggs. On land, this can only happen in moist environments—and this is exactly what happens with mosses and ferns. Other land plants, however, exist away from such environments. What adaptations allow fertilization to occur in these plants? The answer is found in alternation of generations which is the classification of the life cycle of plants into two phases, namely the sporophyte generation and gametophyte generation. The sporophyte generation—the adult generation that produces spores— produces microscopic gametophytes within special structures that provide water and nutrients. The male gametophytes, which form non-swimming sperm, develop within pollen grains. The female gametophytes, which produce eggs, develop on scales (in conifers) or within ovaries (in flowering plants). Pollen is adapted to use wind to transport sperms to eggs, which replaces the need for water.
Continuous struggle for survival of land plants resulted in the evolution of seeds. In aquatic environments, a fertilized egg can develop into an embryo that is never in danger of dehydrating. In addition, the embryo can receive water and nutrients directly from the surrounding environment. The opposite is true on land. On land, an embryo can dry out rapidly due to intense solar exposure and high wind velocity and exists in an environment where water and nutrients exist deep in the ground. Seeds represent adaptations that conquer these challenges. Seeds enclose an embryo in a moist environment with the formation of seed coat developed from intergument. And, tissues within seeds eg. a triploid structure known as endosperm formed as a result of fertilization between sperm cell and two polar nuclei in angiosperms provide food for a developing embryo. Finally, seeds represent a way of dispersing the young of plants away from water as well as away from the parent plant. The seed plants include the conifers and flowering plants. Mosses and ferns, which do not produce seeds, still depend on water for the above functions. Moreover, seeds have the capability to stay dormant until favourable germination conditions are satisfied. This includes various factors such as level of humidity, light intensity, temperature and so on. Unless suitable external conditions are met, the seed will not break dormancy and germinate as to ensure highest possible survival rate are available to the young seedling.
Over time, plants had to evolve from living in water to living on land. In early plants, a waterproof waxy layer called cuticle evolved and developed on the epidermis of plants to help seal water in the plant and prevent excessive water loss by transpiration. However, the cuticle also prevents vital gases from entering and leaving the plant easily. Recall that the exchange of gases—taking in carbon dioxide (CO2) and releasing oxygen (O2)—occurs during photosynthesis, a process where plants utilise carbon dioxide, water (H2O) and photon to produce glucose (C6H12O6) as food through a chain of biochemical reactions in the photosystem.
To allow the plant to retain water and exchange gases, small pores (holes) in the leaves called stomata also evolved. The stomata, controlled by a pair of guard cells can open and close depending on the weather condition. Guard cells controls opening of stomata by the action of osmosis. When it’s hot and dry where transpiration occurs more intensely, the stomata close to keep water inside of the plant. The closure happens as a result of potassium ions (K+) diffusing out of guard cells, thus increasing solute concentration in the exterior of the guard cells. This in turn creates a concentration gradient for water to diffuse from guard cells into surrounding cells. Loss of water in guard cells causes them to lose osmotic pressure and hence become flaccid as well as finally closes the stomata. When the weather cools down, the stomata can open again to let carbon dioxide in and oxygen out. The opening happens as a result of potassium ions (K+) diffusing into guard cells, thus decreasing solute concentration in the exterior of the guard cells. This in turn creates a concentration gradient for water to diffuse from surrounding cells into guard cells. Intake of water into guard cells causes them to gain osmotic pressure and hence become swelled and rigid as well as finally opens the stomata
A later adaption for life on land was the evolution of vascular tissues. Vascular tissues are specialized tissues that transports water, nutrients, and food in plants. In algae, vascular tissues are not necessary since the entire body is in contact with the water, and the water simply enters the algae by osmosis. But on land, water may only be found deep in the ground. Vascular tissues transport water and nutrients from the ground up into the plant, while also carries food down from the leaves into the rest of the plant. The two vascular tissues are xylem and phloem.
Xylem is responsible for the transport of water and nutrients from the roots to the rest of the plant. Xylem basically consists of tracheids and vessel elements. Both vessel elements and tracheids develop a thick lignified cell wall, and at maturity their protoplast will break down and disappear. However, vessel elements also possess perforation plates which are pores between vessel elements that allow continuous stream of water. Movement of water in xylem mainly occurs through transpirational pull mechanism. This process begins as the water in the leaves is evaporated due to transpiration, it creates an lower osmotic concentration in that region. Water from xylem then enters by osmosis, creating an lower osmotic pressure in the xylem instead. As a result, water is pulled all the way up from the roots. The movement of water against gravity is aided further by cohesion and adhesion properties of water itself. Being able to form hydrogen bonds due to partial charges in the molecule, water molecules exert strong attractive forces between themselves during transpirational pull, contributing to their cohesiveness. On the other hand, adhesion of water molecules refers to their ability to stick to the walls of xylem.
In contrast, phloem carries the sugars made in the leaves by photosynthesis to the parts of the plant where they are needed. Phloem comprises sieve elements and companion cells. Sieve elements lack nucleus upon maturity and hence are considered dead cells which relied mostly on companion cells. Companions cells are cells which contain an abundant amount of organelles called mitochondria that provides energy in the form of adenosine triphosphate (ATP) produced by the process of oxidative phosphorylation. This large amount of mitocondria is highly essential in the process of sugar transportation in phloem known as translocation. Translocation transports sugar from the source (site of production of sugar) to the sink (site of consumption of sugar). Sugar in the source were first actively transported into sieve elements using energy produced by mitochondria in the companion cells. This causes solute concentration in the sieve elements to rise and water from xylem diffuse into sieve elements by osmosis as a result of the concentration gradient created. The movement of water into sieve elements increased the osmotic pressure within and generate a pushing force that carries the sugar solution to the sink. Upon reaching the sink, the sugar diffuses into the sink and solute concentration within sieve elements decreases. Water which is higher in concentration in the sieve elements now diffuses back into the xylem, joining the water transportation route once again.
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