Plants are a vital part of almost every ecosystem. As autotrophs, they start food chains by producing carbohydrates and other organic molecules that are needed by heterotrophs. They also produce oxygen and absorb carbon dioxide from the atmosphere or from water. When they die, their remains contribute to the humus, which is an important part of soil culture.
Pathway taken by water
The water tension developed in the vessels by a rapidly transpiring plant is thought to be sufficient to draw water through the root from the soil. The water enters the root hair cells and is then passed on to cells in the root cortex. It enters the xylem vessels to be transported up the stem and into the leaves, arriving at the leaf mesophyll cells.
Tissues in the stem and leaf
A typical land plant is made up of three major organs: the roots, stems and leaves. Roots anchor the plant in the ground and absorb water and minerals for plant growth. Some roots also store reserves of food.
The stem provides support for the leaves. It also contains tissues for the transport of materials from the root to the leaf, and for the distribution of the products of photosynthesis from leaves to the growing parts of the plant as shown in Figure below. One of the most distinctive features of a plant stem is the range of different tissues it contains. The outer cortex contain cells that contribute strength to the stem maintaining its erect form. Embedded in the cortex are several tissues, particularly the xylem and phloem vessels, which make up vascular bundles. The central part of the stem is called pith and is composed of spongy cells that provide an area for storage.
Some plants have just a single large leaf, but most plants have many leaves, distributed on shoots or branches that arise from the main stem. Leaves are often arranged in a mosaic to maximise the efficiency of light capture and so that none of them obscures or shades other leaves. The general structure of plant leaves is shown in Figure below. Leaves come in a huge range of shapes and sizes, but all share several common features.
The upper epidermis is covered by an outer cuticle of wax, which lets visible light through, but prevents water loss through the upper surface. The epidermis itself contains a layer of flattened, rectangular, transparent cells. These help protect the leaf from invading pathogens such as fungal spores. Below the epidermis is the palisade layer, containing elongated cells with numerous chloroplasts. The cells are closely packed together to maximise the amount of photosynthesis that can take place. Chlolroplasts often move up towards the upper part of these palisade cells, so they can receive more light. Below the palisade layer, spongy mesophyll cells, also containing some chloroplasts, are loosely packed together with large air spaces surrounding them. Diffusion of gases to and from the stomata can easily occur through these air spaces.
The lower epidermis which covers the lower leaf surface, has guard cells embedded in it, in many types of leaf. Guard cells contain chloroplasts and have an uneven thickening of cellulose in their cell walls. As they take in or lose water they change shape, opening or closing the pore or stoma (plural stomata) between them. This allows carbon dioxide, water vapour and oxygen to pass in and out of the leaf. Stomata on the underside of a leaf are in a shaded position and protected from excess heat, helping to prevent too much water being lost through them.
Leaves also contain vascular bundles, which are continuous with those in the stem. As well as providing support for the leaf, the xylem in the vascular bundles brings water and minerals that are drawn up from the roots directly into the leave. The water is necessary for photosynthesis, and the minerals are necessary for growth.
Functions of vascular bundles
In general, water travels up the stem in the xylem from the roots to the leaves. Food may travel either up or down the stem in the phloem, from the leaves where it is made (the ‘source’), to any part of the plant that is using or storing it (the ‘sink’).
Vascular bundles have a supporting function as well as a transport function, because they contain vessel, fibres and other thick-walled, lignified, elongated cells. In many stems, the vascular bundles are arranged in a cylinder, a little way in from the epidermis. This pattern of distribution helps the stem to resist the sideways bending forces caused by the wind. In a root, the vascular bundles are in the centre where they resist the pulling forces that the root is likely to experience when the shoot is being blown about by the wind.
The network of veins in many leaves supports the soft mesophyll tissues and resists stresses that could lead to tearing.
Conducting structures in a plant
Cortex and pith
The tissue between the vascular bundles and the epidermis is called the cortex. Its cells often store starch. In green stems, the outer cortex cells contain chloroplasts and make food by photosynthesis. The central tissue of the stem is called pith. The cells of the pith and cortex act as packing tissues and help to support the stem in the same way that a lot of blown-up balloons packed tightly into a plastic bag would form quite a rigid structure.
Plants need water and mineral
Plants need to obtain certain raw materials from their environment. The roots of the plant are adapted to absorb both minerals and water from the soil. Water is essential to support the plant, as a reagent in many biochemical reactions and also transport medium. The diagram below shows how water enters the plant through root hair cells.
Minerals have a number of individual functions and together have a great effect on the water potential of the plant tissues. Minerals from the soil are absorbed in the form of ions, for example, magnesium enters the root as Mg2+ ions and nitrogen enters as nitrate NO3– ions. If the soil solution contains higher concentrations of these ions than the root hair cell cytoplasm, the ions can enter by diffusion. However, plants can continue to take up ions even if the concentration gradient is in the wrong direction, that is , if the concentration of the ions is higher inside the cell than in the soil solution.
Plants take in water from the soil, through their root hairs
- At the very tip is a root cap. This is a layer of cells which protect the root as it grows through the soil.
- The rest of the root is covered by a layer of cells called the epidermis.
- The root hairs are a little way up from the root tip. Each root hair is a long epidermal cell. Root hairs do not live for very long. As the root grows, they are replaced by new ones.
Root hair cells, as seen under the light microscope
Functions of root hair cells
- Increase the external surface area of the root for absorption of water and mineral ions (the hair increases the surface area of the cell to make it more efficient in absorbing materials).
- Provide anchorage for the plant.
Passage of water through root, stem and leaf
In the roots:
Water enters root hair cells by osmosis. This happens when the water potential in the soil surrounding the root is higher than in the cell as water diffuses from the soil into the root hair, down its concentration gradient.
- As the water enters the cell, its water potential becomes higher than in the cell next to it, e.g. in the cortex. So water moves, by osmosis into the next cell. Some of water may also just seep through the spaces between the cells, or through the cell walls, never actually entering a cell.
- Water vapour evaporating from a leaf creates a kind of suction, its pressure at the top of the vessels is lower than that at the bottom as water moves up the stem in the xylem, more water is drawn into the leaf from the xylem. This creates a transpiration stream, pulling water up from the root. Mature xylems cells have no cell contents, so they act like open-ended tubes allowing free movement of water through them. Roots also produce a root pressure, forcing water up xylem vessels.
- Water moves from xylem to enter leaf tissues down water potential gradient. In the leaves, water passes out of the xylem vessels into the surrounding cells.
Experiments on the uptake of ions also show that:
- the cells can select which ions enter from the soil solution
- any factor that affects respiration, for example lack of oxygen or low temperature, can reduce the uptake of ions. The diagram opposite shows some results that support these observations.
The explanation of these observation is that the root hair cells use active transport to carry out the selective uptake of ions against a concentration gradient, using energy from respiration.
In the stem:
- Water evaporates out of the top of the xylem.
- This generates a low pressure at the top of the xylem (a mini vaccum).
- This sucks water molecule up the xylem.
- This is called the transpiration pull.
Water molecules are slightly charged (polar). The oxygen atom is slightly negative and the hydrogen’s are slightly positively charged. This means that water molecules tend to stick to each other. Therefore, when the transpiration pull sucks at the water molecules in the top of the xylem, the entire column of water moves up the xylem, not just the molecules at the top.
In the leaf:
Water enters the leaf in the xylem vessels in veins (basically, another name for the vascular bundle). The water moves by osmosis into leaf mesophyll cells, where it evaporates into the air spaces and finally diffuses out of the stomata into the air.
Uptake of salts
The methods by which roots take up salts from the soil are not fully understood. Some salts may be carried in with the water drawn up by transpiration and pass mainly along the cell walls in the root cortex and into the xylem
It may be that diffusion from a relatively high concentration in the soil to a lower concentration in the root cells accounts for uptake of some individual salts, but it has been shown: (a) that salts can be taken from the soil even when their concentration is below that in the roots, and (b) that anything which interferes with respiration impairs the uptake of salts. This suggests that active transport plays an important part in the uptake of salts.
The growing regions of the root and the root hair zone seem to be most active in taking up salts. Most of the salts appear to be carried at first in the xylem vessels, though they soon appear in the phloem as well.
The salts are used by the plant’s cells to build up essential molecules. Nitrates, for example are combined with carbohydrates to make amino acids in the roots. These amino acids are used later to make proteins.
Adaptations to different environments
If plants from different habitats are compared, they may well look quite different. Their roots, stems and leaves serve the same functions, but their structures may be modified to suit their environments. Such adaptations give the plant the ability to survive in a particular climate or enable it to avoid eaten by herbivores, for example.
Some plant have large tap roots that act as storage organs for food reserves. Example of these include carrots, cassava and turnips. Others have swollen roots that can store water. Mangroves produce air roots, which extend above waterlogged soil or water to absorb oxygen. Many cacti have extensive surface roots, which can quickly absorb any rain that falls before it evaporates in hot conditions.
Some plants use swollen, underground leaf bases to form food storage organs called bulbs. These enable plants to survive over winter when photosynthesis may not be possible. Onions and daffodils are examples of plants that produce bulbs. The leaf bases can be seen arranged over one another around a central shoot.
Many plants survive in very arid climates where water loss from leaves would cause problems. They solve this in different ways. Some have very waxy leaves to minimise water loss, while other have reduced leaves or no leaves at all, for the same purpose.
Plants in both temperate and tropical have evolved modified leaves called tendrils as a mechanism for getting their leaves into sunlight, even when they are shaded by surrounding vegetation. Tendrils enable the plant to cling onto other plants or objects, so that it is supported as it grows and climbs up towards the light.
Tubers are stems that grow below ground and are used to store food. Potatoes are stem tubers that store carbohydrate in the form of starch.
Meristems are the growing parts of a flowering plant where cells may divide by mitosis throughout the life of the plant. There are two types of meristem in a dicotyledonous plant – the apical meristems found at the tip of the root and the shoot, and the lateral meristems found in the vascular bundles of the stem. Growth in the main stem occurs at the apical or primary meristem, with fresh tissue forming at the growing tip. This allows a plant to grow upwards from the soil and towards light so that leaves can obtain sufficient light for photosynthesis. In the root, growth of the apical meristem extends the root into the soil.
Many plants also grow at lateral meristems in the vascular cambium. This makes stems and roots thicker, and is known as secondary growth. Side growth may develop from the main stem as shoots or branches to take advantage of favourable conditions and to avoid competition from other plants.
If plants are damaged at the apical meristems, often the first lateral meristem is induced to grow and take over the role of the apical meristem. If a flower on the apical meristem is cut off, this will induce the lateral meristems to switch to producing flowers.
Auxin and phototropism
Plants produce growth-regulating substances that act to control growth and development. These substance are sometimes called plant hormones.
Knowledge of these growth substances was noted by Charles Darwin in a report of his experiments that he published in 1880. He observed that oat shoots grew towards light because of some ‘influence’, which he proposed was transmitted form the shoot tip to the area immediately below. We know that the substances that causes shoots to bend towards the light is auxin, and the response it causes is called phototropism. Auxin is found in the embryos of seeds and in apical meristems, where it controls several growth responses.
Auxin seems to act by loosening the bonds between cellulose fibres in plant cell walls and making them more flexible. The exact mechanism of auxin action is not fully understood. It has been suggested that auxin is redistributed to the side of a shoot tip that is away from a light source. The uneven distribution of auxin allows cell elongation on the shaded side of a shoot, which in turn causes bending towards light.
Plants need support their leaves and flowers and it is often the stem, which connects the roots to those structures,that support them. Support is provided by cellulose cell walls, cell turgor and by thickening certain structures with lingin and other materials.
All plant cells are surrounded by a firm cellulose cell wall, which also contains hemicelluloses and pectin. Over time, some cells may thicken their cell walls with other carbohydrates such as lignin, which provide additional structure around the cell.
Under normal environmental conditions, the large central vacuole in plant cells is filled with fluid containing dissolved minerals. This fluid exerts pressure on the cell wall, so the cell becomes rigid and presses on adjacent cells. Cells in this condition are said to be turgid, and each exerts turgor pressure on surrounding cells. This is sufficient to support leaves and new soft tissue. Its effect is most clearly seen in periods of drought when leaves drop and become soft or flaccid.
But turgor pressure is not enough to support the stem, especially if a plant is tall. Here, xylem tissue in the vascular bundles not only carries water but also provides support. The xylem contains elongated cells, which are hollow, and become thickened with lignin forming a ‘backbone’ to support the stem.
Lignin is a complex substance that is very hard and resistant to decay. Perennial plants like trees lay down more lignin each year, forming wood.
Types of root system
When a seed germinates, a single root grows vertically down into the soil. Later, lateral roots grow from this at an acute angle outwards and downwards, and from these laterals other branches may arise. Where a main root is recognisable the arrangement is called a tap-root system.
When a seed of the grass and cereal group germinates, several roots grow out at the same time and laterals grow from them. There is no distinguishable main root and it is called a fibrous root system.
Where roots grow not from the main root, but directly from the stem as they do in bulbs, corms, rhizomes or ivy, they are called adventitious roots, but such a system may also be described as a fibrous rooting system.
Structure of a typical flowering plant
A young sycamore plant is shown in Figure below. It is typical of many flowering plants in having a root system below the ground and a shoot system above ground. The shoot consists of an upright stem, with leaves and buds. The buds on the side of the stem are called lateral buds. When they grow, they will produce branches. The bud at the tip of the shoot is the terminal bud and when it grows, it will continue the upward growth of the stem. The lateral buds and the terminal buds may also produce flowers.
The region of stem from which leaves and buds arise is called a node. The region of stem between two nodes is the internode.
The leaves make food by photosynthesis and pass it back to the stem.
The stem carries this food to all parts of the plant that need it and also carries water and dissolved salts from the roots to the leaves and flowers.
In addition, the stem supports and spaces out the leaves so that they can receive sunlight and absorb carbon dioxide, which they need for photosynthesis.
An upright stem also holds the flowers above the ground helping the pollination by insects or the wind. A tall stem may help in seed dispersal later on.
The roots anchor the plant in the soil and prevent it from falling over or being blown over by the wind. They also absorb the water and salts that the plant needs for making food in the leaves. A third function is sometimes the storage of food made by the leaves.
Transport in the vascular bundles
- Place the shoots of several leafy plants in a solution of 1% methylene blue. ‘Busy Lizzie’ or celery stalks with leaves are usually effective.
- Leave the shoots in light for up to 24 hours.
If some of the stems are cut across, the dye will be seen in the vascular bundles. In some cases the blue dye will also appear in the leaf veins.
These results show that the dye and, therefore, probably also the water, travel up the stem in the vascular bundles. Closer study would show that they travel in the xylem vessels.
Transport of water in the xylem
- Cut three leafy shoots from a deciduous tree or shrub. Each shoot should have about the same number of leaves.
- On one twig remove a ring of bark about 5 mm wide, about 100 mm up from the cut base.
- With the second shoot, smear a layer of Vaseline over the cut base so that it blocks the vessels. The third twig is a control.
- Place all three twigs in a jar with a little water. The water level must be below the region from which you removed the ring of bark.
- Leave the twigs where they can receive direct sunlight.
After an hour or two, you will probably find that the twig with blocked vessels shows signs of wilting. The other two twigs should still have turgid leaves.
Removal of the bark (including the phloem) has not prevented water from reaching the leaves, but blocking the xylem vessels has. The vessels of the xylem, therefore, offer the most likely route for water passing up the stem.