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9.1 Essential ideas

9.1.1 Transport in the xylem
9.1.2 Transport in the phloem
9.1.3 Growth 
9.1.4 Reproduction

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Book: 9.1 Essential ideas
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Date: Sunday, 27 September 2020, 4:43 PM

9.1.1 Transport in the xylem

You probably recall from Chapter 5, that all plant phyla (except Bryophyta) are vasculated, meaning they have vessel elements that conduct substances through the body of the plant. Water and inorganic solutes are moved through vessels called xylem. Xylem is made of the remains of dead cells that run the length of the roots and stem and branch into each individual leaf as part of a vascular bundle.

vascular bundels

Figure 9.1.1a – Xylem forms a continuous tube through the plant’s superstructure

Xylem vessels are long, continuous tubes composed of non-living cellulose cell walls. They are reinforced by annular rings of lignin, a very tough organic polymer. As a result, xylem tubes also play an important role in supporting plant structure.

lignin

Figure 9.1.1b – Lignin

Lignin reinforces the structure of the xylem vessel, like the wire mesh in a garden hose. Xylem vessels play an important role in maintaining the structure of the plant.

The transpiration stream

Xylem sap moves in an uninterrupted stream through the plant, at an average rate of about 15m/hour or faster. At the end of a xylem vessel, water evaporates into the spongy mesophyll layer of the leaf, where it might become involved in photosynthesis. The majority of the water, however, is not used in photosynthesis and instead is released into the atmosphere through small pores on the underside of the leaf called stomata (singular: stoma).

cross-section leaf

Figure 9.1.1c – Cross-section of a leaf

The evaporation of water through stomata is called transpiration. Transpiration is a more or less continuous process, and is an inevitable consequence of gas exchange in leaves. The opening of the stoma is regulated by two bean-shaped guard cells. As long as there is sufficient water present, the guard cells keep the stoma open to allow carbon dioxide in and oxygen out. With stomata open, the moist mesophyll layer is exposed to drier atmospheric air, and water evaporates. Tension is created by the adhesion of water molecules to cell walls as well as by evaporation. This tension pulls the column of water along the xylem vessels. Water has strong cohesive properties, due to the hydrogen bonds between individual molecules, so the column remains an unbroken chain in the xylem vessel. This known as the cohesion-tension mechanism.

stoma open - stoma closed

Figure 9.1.1b – Swollen vacuoles cause the guard cells to open the stoma pore

Transpiration ensures a steady supply of water for photosynthetic plants. This water must be replaced by absorption through the roots. Water is absorbed through the roots by osmosis, and essentially travels upwards through capillary action. At the root, the osmotic gradient is created by active uptake of mineral ions. As minerals are pumped into the root from the soil, water travels down the gradient to maintain root pressure. In other words, the transpiration stream is pushed up by osmosis in the roots and pulled up by evaporation from the leaves. The combination of these two physical forces accounts for the upward movement of water through the xylem vessel. 

Key questions

  • What is transpiration?
  • Which properties of water allow it to move through xylem?
  • What are the consequences of the active uptake of water at the roots?

Extended essay

  • Some of the best EEs are related to plant science. Plants are easy to manipulate and experimental data is repeatable, yielding reliable results. Why not try to link transpiration rates to climate change (Topic 4) or photosynthesis (Topic 2)?

In the lab

  • Have a closer look at stomata and learn to use an important statistical tool. See: Page 9.2.1 
  • Design an experiment to investigate factors that affect transpiration. See: Page 9.2.2

9.1.2 Transport in the phloem of plants

Phloem is the tissue through which the products of photosynthesis are translocated from the source of production – the leaves – to areas of growth and storage – the sinks, including fruits, seeds, stems and roots. Translocation in the phloem, unlike the transpiration stream in the xylem, is multidirectional, and requires active transport.

Structure of the phloem

Phloem tissue is composed of:

  • sieve tube elements – individual enucleated cells lined up from end to end and connected by sieve plates
  • companion cells – nucleated cells positioned along the length of the sieve tube elements, attached by many plasmodesmata.

phloem elements Figure 9.1.2a – Phloem elements

Source-to-sink transport

  • Sugars are produced through photosynthesis in the mesophyll layers of leaves (source).
  • Following production, sugars diffuse from the mesophyll through the companion cells of the phloem and are loaded into the sieve tube member.
  • Loading in the sieve tube is an active process that requires ATP and a chemiosmotic mechanism using proton pumps and a symport protein.
  • Loading results in a much higher concentration of solute inside the sieve tube member than in the mesophyll.
  • High solute concentration in the sieve tube at the loading points causes water to enter the phloem by osmosis from surrounding tissues.
  • This creates an area of high hydrostatic pressure at the source end.
  • Hydrostatic pressure is relieved at the sink – as sucrose is removed, water exits the phloem by osmosis to surrounding tissues.
  • Companion cells at the sink do not accumulate sugars – they are transported to the growing or storage organs.
  • In this way, plant food always moves from source to sink.
  • Normally, the source and sink are located relatively close together on a plant. For example, lower leaves will supply sugars to growing roots, and higher leaves to flowers and seeds.

overview of translocationFigure 9.1.2b – Overview of translocation

Key questions

  • Define source, sink, translocation.
  • Explain how structure and function are related in phloem.
  • Identify the types of membrane transport involved in translocation.

xylem vs phloem vesselFigure 9.1.2c – Comparison of xylem and phloem vessels

Extended essay

  • Translocation is a process that requires active transport. Think about modified storage organs such as tubers (e.g. potatoes) or bulbs (e.g. onions), which act as either sources or sinks, depending on the season.
  • Is it possible that a potato simultaneously acts as a source and a sink? Which factors influence when a potato acts as a source or a sink? How could you test it?

In the lab

Analyse data collected by aphid stylectomy. See: Page 9.2.4

9.1.3 Growth

  • The areas of growth in a plant are called meristems. Upward and downward growth occurs at the apical meristems, located in the buds of growing roots and stems. These are sometimes called primary meristems.
  • Many plants also have lateral meristems, which are rings of mitotic cells embedded near the vascular bundles. These are responsible for thickening of stems and are sometimes called secondary meristems.
  • The cells in the meristem regions are undifferentiated (stem cells), allowing for indeterminate growth by mitosis.
  • Auxins are a class of plant hormones that influence gene expression and influence growth through tropisms. The best studied auxin is IAA (indole 3-acetic acid) which plays a role in cell elongation and apical dominance.

Plant hormones and tropisms

Plants adapt their growth in response to the biotic and abiotic environment. When there is a change in the direction of growth or orientation of a plant, this is known as tropism. Tropisms occur in response to a number of different stimuli, as shown in the table below:

Tropism

Growth response

Example

Phototropism

Towards light source (positive)

Away from light source (negative)

Sunflowers orient themselves towards the sun (Figure 9.1.4c)

Gravitropism (geotropism)

Shoot grows upwards (positive)

Roots grow downwards (negative)

This occurs even if the pot or plant is inverted (Figure 9.1.4d)

Thigmotropism

Physical manipulation or touch

Vines on a fence (Figure 9.1.4e)

 

Cell elongation and phototropism

  • Auxin, a hormone that stimulates the growth of cells, is synthesised primarily in the apical meristem of the shoot.
  • When a plant is illuminated only on one side, auxin accumulates in the cells on the shady side of the plant, causing them to elongate.
  • As a result, the growing shoot bends towards the light.

Cell elongation in response to auxinFigure 9.1.3a – Cell elongation in response to auxin

Apical dominance

When the shoot meristem is pruned, a plant tends to become bushier. This is a result of apical dominance. Apical dominance refers to the tendency of the apical meristem to inhibit the axial buds from developing into branches.

  • Auxins produced in the shoot meristem travel down the length of the plant and promote elongation of cells.
  • Auxin inhibits the action of another hormone, called cytokinin, whose role is stimulation of the axial buds to branch and develop.
  • The result is mostly upward growth and very little branching (apical dominance).
  • When the apical meristem is cut or damaged, the source of auxin production is removed, and the plant will branch further because of the antagonistic action of cytokinin.
apical dominanceFigure 9.1.3b – Apical dominance
The shoot meristem is intact on the plant on the left but has been removed from the plant on the right.
sunflowerFigure 9.1.4c – PhototropismgravitropismFigure 9.1.4d – Gravitropsim (geotropism)thigmotropismFigure 9.1.4e – Thigmotropism

Key questions

  • Where are meristems located?
  • How do auxins affect plant growth and differentiation?

Extended essay 

IAA (indole 3-acetic acid, an auxin) is commonly available from scientific suppliers and it can be used in a number of experiments on tropisms. Why not try to improve Darwin’s original experiments?

Did you know?

Did you know that one ripe banana can induce ripening in a bunch of green bananas? The chemical responsible is ethylene.

9.1.4 Reproduction

Every plant that makes flowers or fruits reproduces sexually, and many plants rely on animal pollinators to aid in the reproductive process, especially insects and birds. Some flowers are pollinated by wind or water, but animal pollinators account for 85% of the world’s 250 000 known flowering plant species, and it is estimated that up to one-third of our total food supply is pollinated by bees. This means that much of the diversity of flowers is probably a response of natural selection to mutualistic relationships with animal pollinators.

pollinatedFigure 9.1.4a – Flowers pollinated by (left) animals and (right) the wind

Male and female anatomy

Animal-pollinated flowers have conspicuous, colourful petals. They are easily distinguished from wind-pollinated plants, which tend to be white or drab in comparison.

flower anatomyFigure 9.1.4b – Parts of a flower
The carpel is also called ‘pistil’ in some textbooks – be consistent in your nomenclature


The diagram of an idealized flower shows the relative locations of the male reproductive structures (stamen consisting of anther and filament) and the female reproductive structures (carpel consisting of stigma, style and ovary). The non-reproductive structures include the sepal and receptacle. Practise drawing this diagram, paying attention to the relative size and location of the different structures. 

Pollination, fertilisation and seed dispersal

Don’t confuse pollination – the process of transferring pollen from the anther on one plant to the stigma of another – with fertilisation, which is the fusion of a pollen nucleus with an ovule nucleus inside the ovary. Pollen is extremely light: it tends to stick to bristly insect exoskeleton and is deposited on nearby flowers as the insect moves in search of nectar. Pollen and ovules are the haploid gametes of sexually reproducing flowering plants which fuse to form a diploid embryo.

The diploid embryo develops into a seed. Seed dispersal occurs by wind or water, and in angiosperms via the production of a fruit deep in the ovary. Mammals and birds can disperse seeds over very long distances, as the non-digestible remains of the fruit are excreted in faeces.

Control of flowering

Flowering occurs when there is a change of gene expression in the shoot apex, involving a specific protein called phytochrome. Phytochrome is sensitive to light at different wavelengths, and exists in two reversible forms: Pfr (far-red light absorbing) and Pr (red light absorbing). During the day, both forms of the pigment are found in plants at a relatively constant concentration, but overnight, some of the Pfr reverts to Pr. The shift in equilibrium signals to the plant that it is night.

In long-day plants, flowering occurs only when the concentration of Pfr stays above a critical threshold, or when the night length is shorter than the critical night length. Some examples of long-day plants include spinach and lettuce, carnations and clovers. In short-day plants, the critical night length must be exceeded in order for flowering to occur.

short-day plantsFigure 9.1.4c – Short-day and long-day plants


Short-day plants should be called long-night plants, because it is the night length that determines whether the plant will flower or not. This is because Pfr inhibits flowering in short-day plants, so the night length needs to be long enough to allow the phytochrome to revert to Pr. Some examples of short-day plants include chrysanthemums and poinsettas.

A physiological response to a change in day length is called photoperiodism. The control of flowering is an example.

Germination

The formation of a seed is the last stage in the reproduction of plants. Plants invest a lot of metabolic energy in developing elaborate flowers and fruits in order to attract animal pollinators and dispersers.

seed structureFigure 9.1.4d – The structure of seeds


Seeds can stay dormant for long periods of time and are able to withstand environmental extremes. In order for a seed to germinate and develop shoots, optimal abiotic environmental conditions are necessary:

  • water – to activate the metabolic enzymes
  • appropriate temperature – for proper functioning of enzymes
  • oxygen – for aerobic respiration.
germinationFigure 9.1.4e – Germination


Germination occurs in a stereotyped series of events:

  1. Imbibition – water is absorbed and swelling of the seed cracks the testa.
  2. The embryo releases giberellin, a hormone that signals the synthesis of amylase in the aleurone layer.
  3. Amylase hydrolises starches in the endosperm, breaking them up into smaller sugar molecules.
  4. The sugars are consumed aerobically to grow the radicles (root and shoot).

 

germination process

Figure 9.1.4f – Germination process

Did you know?

  • Plants tend not to self-fertilise.
  • Changing leaf colour is another example of photoperiodism.
  • Horticulturalists can manipulate the day length in greenhouses to grow flowers all year round.

Food for thought

Germination does not require light, since energy for the process is stored in starch. That’s why bean sprouts are white, not green!

In the lab

Investigate factors that affect the rate of respiration in germinating seeds. See: Page 9.2.3

International mindedness

Read more about how global food supply is at risk because of dwindling populations of bee pollinators in Packer, L. (2010) Keeping the Bees: Why all bees are at risk and what we can do to save them. Toronto: Harper Collins.