Experiment to Compare Stomata Density in Different Dicotyledonous

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Experiment to Compare Stomata Density in Different Dicotyledonous

Aim:  To investigate if stomata density on leaves in different dicotyledonous plants is effected by there country/ eco-system of origin. I will also compare the upper and lower epidermis stomata density to see were it lies.

Information on stomata and Hypothesis Based Upon this Information:

 Diagram 1: Structure of a leaf.

 The lower and upper epidermis along with the stem of a plant may contain stomata. These are openings through which gases are exchanged with the atmosphere and water is lost, this is called transpiration. Carbon dioxide is need in the process of photosynthesis. Carbon dioxide is diffused in through the stomata for photosynthesis and some carbon dioxide is produced through respiration along with the production of water which transpires out.  These openings are surrounded by specialized crescent shaped guard cells, which changes their size and shape to change the size of the stomatal openings. This regulates the gas exchange e.g. open more gas exchange, closed no gas exchange. These guard cells have different stimulus to active or deactivate the openings; light, CO2 concentration, humidity and wind speed. The epidermis is covered with a waxy coating called the cuticle, which functions as a waterproofing layer and which helps to reduces water loss from the plant surface through evaporation. Transpiration (evaporation of water from plant surfaces) happens mostly from the surface openings, the stomata. Stomata transpiration accounts for most of the water loss by a plant, but some direct evaporation also takes place through the surfaces of the epidermal cells of the leaves.

The Leaf is the principal food-making part of a plant. Not all leaves are green; many have additional pigments that produce colours other than green. The shapes and structure of leaves are adapted to the conditions which  they live in.

Stomata represent a hazard to a plant in that they may cause excessive transpiration of water from the leaf. So if a stomata opens a leaf may lose excessive water, particularly if it lives in a dry, hot or windy habitat. Also, if it closes them it may run short of carbon dioxide or oxygen and also limit active uptake of water from the ground. Plants can resolve these problem by reacting to external stimuli and thus not opening them for longer than is necessary or vice versa (see guard cells for more information on stomata opening and stimuli). Stomata density can also be controlled by  plant to limit risk factor of excess water loss. This however does not mean that a plant never loses more water than it can replace from the soil. The basic observation is that plants, not adapted to hot, dry, humid conditions, wilt in hot weather as they lose excessive water (when the stem of a plant lose it’s turgid ness and thus mechanical support of the plant). Plants which live in climates with a moderate humidity (e.g. Britain) are different from those of humid, tropical regions or cold, dry regions. Most leaves have flat blades that expose as much surface as possible to sunlight as to produce as much food as possible, but a plants have adapted their leaves to their environment through evolution. Leaf adaptations:

  • Conifers: are adapted to cold, windy regions which they are native to. They have needle-like leaves that offer a minimum surface to the drying, winter winds. The leaves have one or two veins deeply imbedded in the middle and a layer of strong supporting tissue just beneath the thick and heavily cutinized outer layer.
  • Succulents and cacti: are adapted to arid regions. The leaves are often much more spongy and can retain large amounts of water. They often have a waxy cuticle on both the upper and lower epidermis to minimize water loss. The number of leaves is often reduced to minimize water loss. Stomata opening often only occurs at night to try an minimize further water loss.
  • Tropical forest plants: are adapted to allow excess moisture to run off at the tips, by different shapes and sizes of leaves. The high humidity means low rate of transpiration.

The stomata density thus can also be varied by a plant to suit it’s conditions accompanied by other specializations. Density of stomata on a leaf therefore may be varied by temperature, humidity, light intensity and CO2 concentration. 

For this investigation I choose to use dicotyledonous plants instead of monocotyledon plants. I could not use mixture of plants because monocotyledons plants have rows of stomata and are evenly distributed, whilst dicotyledonous plants have a randomised spread of stomata density. Monocotyledons plants, in general have long point leaves such as iris, maize and wheat plants. They produce a 1 seed leaf and have a number of protoxylem groups in the root. Dicotyledonous plants are 2 seed leaves such as buttercup, sunflower and lime tree. They usually have broad leaves with a vein network. The have s small number of protoxylem groups in the root. Comparing the two types of plant would have caused inaccurate results due to their many differences, making it impossible to compare the two types correctly or with any real judgement.

Guard Cells: Each guard cell contains chloroplasts, unlike the adjacent epidermal cells. The concentration of glucose in the cells changes with the photosynthetic activity and therefore it is the guard cells that regulate the opening and closing of the stoma.

Normally stomata open when the light strikes the leaf in the morning and close during the night. Also, when there is an increase in the osmotic potential, there is a decrease in the water concentration and therefore water moves into the guard cells by osmosis in response to the concentration gradient. As a result the thin-cell wall bulges into the epidermal cell pulling the thick wall with it and therefore opening the pore, making the cells turgid. Thus, when the guard cells become less turgid (loss of water) the pore subsequently closes. The alteration in the size of the stomata occur in response to a variety of the external stimuli such as light, carbon dioxide concentration and water.

The increase in osmotic pressure in the guard cells is caused by an uptake of potassium ions (K+). The concentration of K+ in open guard cells far exceeds that in the surrounding cells. This is how it accumulates:

  1. Blue light is absorbed by  which activates
  2. a  (an H+-ATPase) in the plasma membrane of the guard cell.
  3. ATP, generated by the light reactions of photosynthesis, drives the pump.
  4. As protons (H+) are pumped out of the cell, its interior becomes increasingly negative.

This attracts additional potassium ions into the cell, raising its osmotic pressure.

From a previous experiment, also counting the stomata in the upper and lower surface  area to gather information of where the stomata density lies. The results below had be gathered. This experiment was undertaken in using a 10 X 8 magnification.

From this I worked out that the mean average of stomata on the lower surface area is 1101.31     6 = 183.55. However this is only a very approximant average as there is only a small range of plant results to take and there is mixture of high and low results to thus altering the average. However, in my prediction I will be able to relate my prediction of  the stomata density average, giving me a base of reference.

Plants to be used and Predictions of Stomata Density for Each Plant:

Rhododendrons: About 700 species occur in the Himalayas of northeastern India, Burma, and southwestern China, and an additional 300 or so species occur in New Guinea. They exhibit great variety in size, habit, and flower colour, ranging from small, ground-hugging shrubs to small trees. India temperatures range from 14–31 degrees, with an average rainfall of 662mm up to 1935mm in some area.

  • Due do there natural ecosystem; reasonably hot areas and the large amounts of rain they receive in the rainy season I  would predict that there will be reasonably highly stomata density, and above the average.

Crassula Gollum: This plant originates from South Africa and is a succulent plant (holds water). This plant tends to grow low on the ground in rocky surroundings. They have thick, trumpet like leaves. Green with red/ purple edging. No visible veins, and the waxy cuticle covers the whole leaf. South Africa’s temperatures range from 22 –11 and has an average rain fall of 751mm a year.

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  • As this plant is a succulent, adapted to hot, dry conditions I predict that there will be a low stomata density, in the plants aim to conserve water. The table above also shows a succulent with an average of 31 stomata on the lower epidermis, and thus I predict the stomata count will be below 50 for  both the upper and lower epidermis.  I predict this due to the fact that many succulents have a complete waxy cuticle covering that there will be approximately the same stomata density on both the upper and lower epidermis. The crassula gollum should also ...

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