Why Some Fuchsias Are Self-Branching

It's been bothering me. What I said last week about the multi-plant flowering unevenly (see 'Two's Company . . .'). It wasn't quite right. This multi-plant is not flowering unevenly because the cuttings have rooted at different times. It's because two of the plants contained flowering meristems, whereas the third one did not.

Once a plant stem 'decides' to flower, the apical meristem, contained within the terminal bud (growing tip), ceases laying down only dormant side shoots in new leaf axils and begins to produce flowers (or flowers and shoots). At this time, it ceases to be called an 'apical meristem' and becomes instead a 'floral meristem'. However, this change is not obvious; it only becomes apparent once flower buds can be seen. Until this time, it appears like a normal shoot tip, the young flower buds being there but too small to detect. So this is what has happened here. This change into flowering mode had occurred in two cuttings only. The third was a normal stem.

FUCHSIAS LAY DOWN FLOWERS IN DIFFERENT WAYS

SELF-BRANCHING TYPES

The situation is even more confusing in fuchsias because some continue to lay down both flowers and shoots in their leaf axils. 'Genii' (left) lays down flowers (pink) and shoots (lime) at DIFFERENT nodes, whereas 'Hawkshead' (right) lays down both flowers and shoots at the SAME (but not all) nodes. These young dormant axillary shoots, once activated, allow these fuchsia stems to become 'self-branching', thereby developing the shrub vegetatively as flowering continues throughout the summer.

NON-SELF-BRANCHING TYPES

Some fuchsias, however, once flowering has begun, cease laying down side shoots and lay down only flower buds (plus a pair of feeder leaves) in the axils of each consecutive new node. This continues for a few weeks until flowering has finished, at which time vegetative growth resumes, followed by another flush of flowers. Thus, the shrub stops developing during flowering, so must be built up previously. However, there is a plus in that the flowers are more visible (and therefore look more spectacular) because they are not covered by leaves, thereby making these plants more suitable for exhibition purposes. Note that 'Amazing Maisie' (left) produces two pairs of flowers at each node, along with two feeder leaves. This has the effect of making the plant appear more floriferous; it also makes it easier to shape for best effect.

Dicot Stem Cross Section

This is a cross-section (transverse section) of a typical herbaceous dicot stem. Its tissues are laid down in a characteristic manner.

EPIDERMIS
The outer layer, one cell thick, is the 'epidermis'. Some of these cells bear multi-cellular hairs or 'trichomes', which are beneficial for water conservation and defence. Because green stems can photosynthesise, the epidermis also contains 'stomata'. Just like leaves, the epidermis is covered with a waxy layer of cutin, the 'cuticle'.

CORTEX
The cortex comprises tissue located between the vascular blundles and the epidermis, often split into two layers. The outer layer is composed of collenchyma cells whose unevenly-thickened walls give flexible support to the growing plant. And, because it is a living material, can grow as the plant grows. The outermost collenchyma cells may contain chloroplasts, whose chlorophyll facilitates phososynthesis.

Below the collenchyma, the remainder of the cortex consists of unmodified parenchyma cells.

VASCULAR BUNDLES
In dicot stems, these are always arranged with the phloem towards the outside and the xylem towards to the centre. This has been described elsewhere (see 'Vascular Bundle Of Young Dicot Stem, Cross Section')

In dicots, the vascular bundles are arranged in a ring running the whole length of the stem, and connecting with the stele of the root and with the midribs of the leaf veins.

These bundles give much support, especially when a pericyle is present whose tightly-packed, lignified sclerenchyma fibres are extremely tough, forming a bundle, or cap, between the bundle and the cortex.

PITH
This is parenchyma packing material which forms the centre portion of the stem. In some plants, this is missing and the centre is hollow and often filled with water.

See also: 'Cross Section Of Young Dicot Root'

Vascular Bundle Of Young Dicot Stem, Cross Section

This is a transverse section (cross-section) of a typical dicot stem vascular bundle. These are comprised essentially of phloem and xylem tissues which provide both conduction and support. In dicot stems, these bundles are arranged in a ring (see 'Dicot Stem Cross Section') which is separated from the epidermis by cortex tissue. Bundle vascular tissue reaches throughout the whole plant and forms characteristic patterns depending upon whether it is in the root, stem, or leaf. Only in the stem and leaf is it referred to as 'bundles; it forms the 'stele' in the root.

PERICYCLE
Although the xylem tissue's cell walls are supportive, further support is often given to dicot stems by a sheath of sclerenchyma fibres (the 'pericycle', 'bundle sheath', or 'phloem fiber cap') which sits above the phloem tissue. Although not present in all dicot stems, it allows herbaceous stems to flex easily in the wind without breaking.

PHLOEM TISSUE
Consists of sieve tubes, companion cells, phloem parenchyma and phloem fibres (see 'Plant Tissues'.

In vascular plants, phloem tissue carries manufactured food from sites of photosynthesis (mostly in the leaves) to non-photosynthesising parts (mostly the roots). It also carries food products from storage in the roots to any other part where it can be utilised. This two-way traffic (up and down) differs from xylem transport, which is only upwards.

Almost all the sugar is transported in the form of the carbohydrate sucrose (a disaccharide), because this is both relatively inactive and also highly soluble (so can be carried in high concentrations).

Phloem also transports certain processed minerals, particularly nitrogen and sulphur in the form of amino acids, phosphorus as phosphates and potassium ions. Trace elements and growth hormones are also carried.

VASCULAR CAMBIUM
This is a thin ring of cells separating the outer phloem from the inner xylem. It is responsible for secondary growth in older stems.

XYLEM TISSUE
Dead material consisting of thick,lignified (woody) material. The 'metaxylem' is easily identified by the large mature xylem vessels, fibres and parenchyma. The 'protoxylem' is spirally or annually thickened and is closer to the centre of the stem.

Xylem tissue conducts water and nutrients upwards from the roots to the rest of the plant. Analysis of xylem sap has shown that much of the nitrogen is not carried as nitrate or ammonium ions, but as manufactured amino acids. This must therefore have occurred in the roots. Some organic phosphates have also been found.

See also: 'Cross Section Of Young Dicot Root'

How Water Enters A Plant: Apoplast And Symplast Pathways

Water enters plant roots (along a concentration gradient) through fine root hairs (see 'Apoplast And Symplast Pathways In Root A Hair') then makes its way across the root tissues (cortex, endodermis and pericycle (see 'Cross Section Of Young Dicot Root') towards the xylem, that part of the vascular system which distributes water and dissolved mineral salts throughout the plant.

THREE ROUTES OF TRAVEL

Water may travel across the root by one of three pathways:

• apoplast - within cell walls only (unselective)
• symplast - in cytoplasm, through plasmodesmata (selective)
• cell-to-cell (transmembrane pathway) - through vacuoles of each cell (selective)

We have seen that a typical cell comprises an outer cell wall surrounding a partly permeable plasma membrane, enclosing cytoplasm and inner vacuole. We have also seen that the cytoplasm of adjacent cells is connected through 'gaps' in the cell wall called plasmodesmata.

APOPLAST PATHWAY
If once water enters the root hair, it travels from cell to cell, across the cortex, in the CELL WALLS ONLY, it is said to take the 'APOPLAST' pathway. Because cellulose is pervious and non-selective, both water and ALL dissolved minerals may be carried this way. Note that the plasma membrane (separates cell wall from cytoplasm) need not be crossed, not until the endodermis - a defensive ring of cells, one cell deep - is reached. So OSMOSIS need not be involved up to this point, only diffusion.At this point, the impervious Casparian strip blocks progress and forces both the water and mineral salts to cross the plasma membrane into the cytoplasm, where they can continue on the symplast pathway. However, not all mineral salts make it - the plasma membrane is selective and can filter out unwanted solutes.


SYMPLAST PATHWAY
If water passes from the cell wall through the plasma membrane to the cytoplasm, it can then travel from cell to cell through the plasmodesmata in the cell walls. This is called the 'SYMPLAST' pathway. Note that the plasma membrane, because it is partly permeable, can only be crossed passively by OSMOSIS. Passage of dissolved nutrients, because their molecules are too large, are blocked and cannot pass passively; they must cross the plasma membrane into the cytoplasm by active transport, a selective process; osmosis is not involved.

CELL TO CELL
Water can also enter the vacuoles (which are within the cytoplasm), then pass from vacuole to vacuole. This is called the 'CELL TO CELL' or 'TRANS MEMBRANE' pathway.

POINT OF INTEREST
It is commonly supposed that both water and minerals enter the plant cytoplasm as one process. They don't. Each enters independantly of the other; mineral salts can enter a plant even when no water is being absorbed. However, once inside the cytoplasm, minerals travel in solution.

TOXIC VEGETABLES
Although the endodermis prevents toxins and any other unwanted solutes from entering the stele of the plant, and hence travelling to the stems, leaves and fruits, these nasties may be present in the epidermis and cortex of the plant, taken in through the unselective apoplastic pathway. Since the cortex represents the bulky 'food' part of roots, we need to be extremely careful.

Apoplast And Symplast Pathways In A Root Hair

Plants take in water from the soil, along a concentration gradient, through uncutinized epidermal cells of their roots. Near the root tips (where most of the water is absorbed), these cells greatly increase their absorptive areas by extensions called root hairs. As water enters the cell (hair), it can take one of two routes depending upon whether or not it has entered the cytoplasm of the cell. These routes are called the APOPLASTIC and SYMPLASTIC pathways.APOPLASTIC PATHWAY
Because cellulose, the principal constituent of the cell wall, absorbs water like a sponge (think 'cotton wool'), water can travel from cell to cell - along with any solutes dissolved in it - via these cell walls WITHOUT crossing the plasma membrane to enter the cytoplasm of any cell. This is the APOPLASTIC pathway.

SYMPLASTIC PATHWAY
Water can also cross BOTH the cell wall AND the plasma membrane to enter the cytoplasm, then travel from cell to cell via the plasmodesmata in the cell walls. This route is called the SYMPLASTIC pathway. Because the plasma membrane is partly permeable, entry to the cytoplasm must be by OSMOSIS.

NUTRIENT SELECTIVITY

• There is a misconception that nutrients enter root hairs in solution. This is not strictly correct - it only applies to the apoplastic pathway, where the plasma membrane need not be crossed.
• However, in the symplastic pathway, both water and minerals (nutrients) must each cross the plasma membrane by a separate and independent process; one is PASSIVE the other ACTIVE.
• Water passes passively by osmosis.
• Minerals, on the other hand, must pass through the plasma membrane by ACTIVE TRANSPORT, a selective process.
  

See also: 'How Water Enters A Plant: Apoplast And Symplast Pathways'

Cross Section Of Young Dicot Root

This is a typical cross-section of a young dicot root taken in the region of root hair proliferation (see 'Plant Root Growth'), before lateral secondary growth has begun. Hence, the lateral meristematic tissue (cambium) is not yet present.

EPIDERMIS - Developed from the protoderm, it forms a cover, one cell thick, around the root's perimeter. In the region just back from the root tip, cells produce extensions as root hairs to increase the root's water-absorbing potential.

CORTEX - Developed from the ground meristem, these are typical 'fleshy' parenchyma cells with thin walls, large vacuoles and air spaces between cells, facilitating storage of food and water.

In roots of some plants (bog-type plants), some of the cortex cells are lost, producing even more air spaces 'aerenchyma', which allow gases to diffuse WITHIN the plant to and from the root tips.

ENDODERMIS - Surrounds the vascular tissue and can be seen as the innermost layer of the cortex. Unlike the epidermis, each cell wall contains a vertical ring of fatty suberin, called the 'Casparian Strip', making the wall impervious and thus keeping out unwanted solutes.

PERICYCLE - A layer one to several cells thick, between vascular tissue and endodermis. This is modified parenchyma which retains its capacity for cell division and produces the lateral roots which then punch their way out through the outer tissues.

VASCULAR TISSUES - PHLOEM AND XYLEM form an inner stele through the centre of the root, right up to the hypocotyl, where the root joins the stem. The structure then changes into that of a typical stem.

How Fungicides Work

Fungi have no chlorophyll so cannot photosynthesise, and have no vascular system so cannot move water through 'body'; therefore they must live in very close association with plant, usually in a symbiotic or parasitic manner. Here are some brief notes:

What is a Fungicide?

An agent which will kill, repel or prevent, or otherwise mitigate a fungus.
It may be PREVENTATIVE or CURATIVE in nature
It may be naturally derived or synthetic

BIOLOGICAL VS SYNTHETIC

Biological
Soil contains beneficial fungi and bacteria which afford biological control by attacking and destroying pathogenic (disease-causing) fungi.

Synthetic
These are comprised of various chemical treatments concocted by technicians in a laboratory.

HOW DO FUNGICIDES WORK?

• Direct competition
• Antibiosis
• Predation or parasitism
• Induced resistance of host plant
  

PROTECTANT VS CURATIVE

Protectant
Must be present BEFORE infection. Includes Carbamates (includes Mancozeb)

Curative
Tend to work on specific metabloic process. Blocks a part of pathway. Therefore more limited in action. And because they only work at one point, fungus more able to build up resistance.

CONTACT VS SYSTEMIC

How Contact Fungicides Work

• Multi-site activity (therefore effective against a wider range of fungal diseases).
• These fungicides act to kill fungus by overwhelming it with substances which are poisonous to it (e.g. copper).
• These fungicides primarily based on inorganic chemicals such as copper, aluminium, sulphur. Also upon the dithiocarbamates combined with zinc and manganese ions.
• Low risk of resistance build-up.
• Must be present before infection begins

FUNGAL INFECTION PROCESS (from Fungicides_McManus.pdf)

• Inoculation - fungal spore lands on plant surface
• Adhesion - fungal spore exudes a glue so that it sticks to plant surface even in rain
• Germination - fungal spore takes up water and germ tube emerges
• Penetration - germ tube enters plant, either by poking through epidermis or by entering stomata or wounds
• Plant-pathogen recognition - chemical or molecular signals are exchanged between fungus and plant so fungus knows it has infected suitable host
• Infection - fungus invades plant by growing in or between cells, and releases spores from plant surface

TIMING OF APPLICATION

Fungicides must be applied at suitable times to be effective. Contact protectives must be applied before infection. Systemics must be applied at right stage of plant development (eg at bud break).

How Systhane (example of systemic fungicide) Works From IntroToFungicides.pdf

Systhane is a Triazole, which is a sterol inhibitor (DMI). Demethylation inhibitor fungicides (DMI) inhibit the biosynthesis of sterols (enzymes used by fungus) in fungal membranes.

So Systhane works by inhibiting the biosynthesis of sterols (enzymes used by fungus) in fungal membranes.

Fixing A Broken Branch Or Stem

Accidents happen all the time in the garden, nearly always to our favourite plants. Why this should be, I don't know. Another mystery. Anyway, enough said. How do we fix a stem if it breaks? It depends.

• woody perennials (trees, shrubs e.g. forsythia)
• tender perennials (will grow all year but frost-susceptible e.g. geraniums)
• herbaceous perennials (everything else which comes up from roots year after year e.g. lupins, delphiniums)

WOODY PERENNIALS

Branches rarely break away completely from the parent. They usually hang in limbo looking untidy, leaving the dilemma as to whether to bind it back in position or remove it completely and go from there. Some advice:
BIND IT BACK
Broken limbs, not completely severed, are rather like broken human limbs. Setting and binding them back into position early enough will usually work well. Of course, we don't use plaster of Paris on plant stems; we use garden twine, or wire - or even a pin or two - even splints, if necessary. The binding must be tight enough to bring the surfaces back into contact and to provide firm support. This is essential. Remember to remove binding once healing has occurred, or lateral secondary growth (stem thickening) will be impaired.

PROPAGATE NEW STOCK
Not so easy with woody tissue, but new plants can be propagated by layering (not covered yet) or by taking hard wood cuttings, or semi-ripe (even soft wood, in some cases) cuttings. It's a matter of using whatever method you are familiar with that works. These either are or will be dealt with elsewhere (use search box).

TENDER AND HERBACEOUS PERENNIALS

Because there is no secondary thickening (I'll be telling you about this later), these don't heal like woody plants. So they can't be bound and left. So must propagate new stock. Fortunately there are several options:

• shoot tip cuttings (used for tips of non-flowering stems (sometimes will work if flowering))
• basal cuttings (used if stem broken away from base (needs a bit of base plate with dormant buds)
• seeds (used when too late to propagate by other methods. problem that they might not come true, if from hybrid stock)

ROOTING MATERIAL

Garden Soil
Surprising how often pushing a shoot tip into garden soil produces results (e.g. geraniums, fuchsias, dahlias). It's always worth a try. Works very well on broken branches torn away from stem and containing a heel (base of stem includes a little bark from main stem where it was torn away).

Perlite and Water
Gives results for most occasions. Very useful for semi-ripe tissue (starting to turn brown). Seems to work with almost anything. This was described last autumn when used to propagate flowering fuchsias.

Water
Plain water often gives results, especially on woody tissue. It can give brittle roots which are slow to establish when later potted into soil or compost. We used this last year to propagate a triphylla fuchsia. It can even be used on geraniums.

Compost
Although potting compost might be best, just ordinary multi-purpose seems to work quite well, especially if some sand or perlite is added to give aeration. Though, until recently, I have always used plain compost. This method has been described several times, especially propagating dahlias and fuchsias.

MORE ADVICE

This post is very basic. Best to research propagation of your particular plant. However, for what it's worth:

• Always use as sharp a knife as possible (preferably a razor blade or craft knife) to cut a shoot tip cutting below a leaf node. A clean cut helps prevent cell damage, which induces rotting.
• Hormone rooting powder is often useful, though not usually necessary
• Keep cuttings in good light, out of sun and not too hot

Stages Of Plasmolysis In Hypertonic Salt Solution

If cells are exposed to a hypertonic salt solution, water leaves as the vacuoles (which contain most of a cell's water) shrink (see how water enters root hairs), and the cytoplasm gradually shrinks away from the cell walls, leaving empty spaces. This condition is called 'plasmolysis'.

The far left diagram shows a normal cell in a hypotonic solution. After placing in hypertonic salt solution, plasmolysis begins. In its early stages (central photo (sorry, not to scale)), wilting occurs which, in the early stages is reversible by placing plant in plain water. However, as plasmolysis progresses, the cytoplasm shrinks so much that it often causes the roots to shrink also. This stage is fatal - there is no recovery potential.

Note: we also saw this condtion over winter when we looked at cell freezing.

How Hypotonic And Hypertonic Solutions Affect Water Uptake In Plants

Although not so-far described in detail, we have seen that water enters plant roots through the root hairs. Each hair is an extension - up to a length of about 4 mm - of a single epidermal cell (the epidermis is the outermost layer of cells on a herbaceous plant). Because these hairs are so fine, they are able to work their way around individual soil particles and absorb the water surrounding them. This is done by osmosis.

Soil solution is usually HYPOTONIC (contains proportionally more water than solute) to the cell contents of the root hairs, so water moves INTO them by osmosis.

However, problems arise when, for some reason, the soil solution becomes HYPERTONIC (contains proportionally less water), since this causes water to move OUT of the roots hairs and into the soil solution. This is dangerous, since it initially causes wilting, then eventually plasmolysis (when the cell contents pull away from the cell wall) and, eventually, death.

HOW SOIL SOLUTION STRENGTH VARIES
Soil solution strength varies:

• naturally

• induced

NATURAL VARIATION BY DRYING
As the soil and its solution dries, it loses only water, not solute, and becomes more hypertonic to the root hairs

INDUCED VARIATION BY FEEDING
Liquid feeds are a prime cause. If too strong, they can easily be hypertonic to the roots, especially as the soil dries and loses water. Great care should always be taken when feeding. It is so easy to damage a plant. This is why I do not recommend feeding young plants (plants are more vulnerable when young).

Another cause is positioning strong feeds and composts too close to the roots. I always leave a distance of at least 5 cm (2 inches), and even this is hardly enough. I also try to place feed to the side of the roots, if possible. This way, they don't have to grow through the feed - they can take it, or leave it alone.

There is a lesson to learn from all this: FEEDING IS EXTREMELY DANGEROUS AND SHOULD BE AVOIDED WHENEVER POSSIBLE

What Is Osmosis?

Osmosis can be considered as a specialised form of diffusion and is the means by which, in plant systems, water (the solvent) moves across a partly-permeable membrane (the plasma membrane of the cell) in an attempt to equalise solution concentrations on each side of that membrane.

A solution consists of the solvent (water) plus the solute (e.g. sugar or salts).

As you can see, only water can diffuse through the partly permeable membrane because the pores are too small to allow any solute through. Hence, to equalise concentrations on each side of the membrane, water passes from the side with the highest proportion of water (the most dilute, or 'hypotonic' side) to that with the least proportion of water (the most concentrated, or 'hypertonic' side). Consequently, the volume of this hypertonic side increases.

If allowed to continue unhindered, each side of the membrane will eventually become equal in concentration, or 'isotonic'. However, this rarely happens in plants, since the cell wall exerts a backward pressure called the 'turgor pressure'. Once the turgor pressure equals the osmotic pressure, an equilibrium is reached and water stops crossing the membrane.

The key thing to remember with osmosis is that it involves the passage of WATER only, from a relatively dilute (greater proportion of water) solution to a more concentrated one, across a partly permeable membrane (plasma membrane of the cell in plants, but could also be cellophane or pig's bladder in experiment).

In plant root cells, the surrounding soil solution usually contains a relatively dilute concentration of mineral salts to that of the cytoplasm of the root cells (i.e. it is hypotonic to the root cells), so water enters the root by osmosis. I have often heard it said that feed enters plant roots by osmosis or that it moves around the plant by osmosis. This is untrue. Remember, only WATER MOVEMENT is involved.

How Can Cuttings Survive In A Sealed Propagator?

After partially venting the fuchsia cuttings in the coffee jar propagator for a couple of weeks , they should be hardened enough to remove the lid. As you can see, after removing the diseased dead flower buds there is now no sign whatsoever of any botrytis. The cuttings do seem to be growing slowly, though I am not yet sure they will all survive. However, what I cannot understand is that, given what we have seen about the compensation point, how can cuttings even survive for any length of time in a sealed propagator?

PERMANENT COMPENSATION POINT
When both photosynthesis and respiration proceed at the same rate, there is no sugar left over for plant development (i.e.growth). In a sealed container, photosynthesis (which produces oxygen and uses carbon dioxide) and respiration (which produces carbon dioxide and uses oxygen) must reach an equilibrium point where each complements the other with the necessary gases with which to function. In other words, a sealed propagator must be permanently at compensation point.

This could explain the lack of development of the fuchsia cuttings over the ten weeks they were sealed in the propagator. Any development could well have occurred during the time taken to reach this gaseous equilibrium, or compensation point. Of course, we cannot ignore the fact that temperatures were very low, so even if gas concentration levels were adequate for growth, it would be much reduced anyhow. However, things would be very different in summer when temperatures are higher. This might be why venting seems necessary at this time.

Supercooling Helps Plants Resist Frost

When a solution's temperature falls below freezing point without ice crystals forming, it is said to be supercooled. Some plants are able to do this, and so can withstand freezing temperatures (many fish can do this as well). We saw recently how plants can avoid ice damage by removing fluid from the inside to the outside of their cell walls, where it can freeze between the cells without causing damage.

This article not only illustrates supercooling, it also shows how the latent heat of fusion released by this liquid as it turns to ice raises the temperature to the extent of 80 calories per gram of ice formed (This also illustrates the usefulness of spraying crops with water on the evening before a frost).

It also mentions that plants induce this supercooling by producing such compounds as sugars, amino acids and other solutes; also that plants produce antifreeze proteins and other low molecular weight compounds, which 'provide freeze tolerance by inhibiting ice crystal growth and the nucleation of ice crystals' (crystal structures form around a nucleus - usually a solid one such as dust (though obviously not inside a cell)).

Should We Dig in the Dark?

I know the ground is frozen solid, but for those of you with some digging still to do in weedy soil, here's something to consider:

As mentioned elsewhere, many seeds are sensitive to the quality of light for germination; this helps them grow in ideal positions. Some species require an open site; others prefer company. By assessing the quality of light (i.e. its constitution), they are able to determine their position.

How do they do this? They use a pigment called 'phytochrome', which is capable of determining the proportions of red and far-red light available. Leaves filter out red light whilst far-red is filtered much less, hence decreasing the red:far-red ratio. Those seeds requiring an open position germinate when there is a high proportion of red light available, and vice versa.

Many garden weeds germinate following light stimulation, often for just a few seconds. Digging in the dark would deprive exposed red-light-requiring weed seeds of this stimulus and hence hence reduce germination up to four-fold (since they would have the impression that they had landed in shade).

Before you rush for the spade handle and the flashlight, please bear in mind that this is only an idea, not - I think - one to be taken too seriously. . . although I don't know . . .?

Watch Sun on Frozen Evergreens

We have recently seen how plants can survive hard frosts by reducing the water content in their cells. And if things get really bad, the roots can freeze as well. The plants react as they would in drought: they shut down and begin to wilt, as seen here with this frozen fatsia, which has dropped its leaves down by its side awaiting the thaw.

This is a particularly dangerous time for many evergreens, especially young ones with tender tissue and shallow roots. A drying wind could leave them unable to replace any water which might be subsequently lost, thus causing scorching. Strong morning sun is also a problem on such plants as camellias which might have similar difficulties in frosty weather.

How Plants Lose Heat

The best way to protect plants against frost is to understand how they can lose heat (note that they lose heat, not gain cold), then take appropriate action. There are three main ways:

Convection
Conduction
Radiation
Evaporation


CONVECTION
Associated with air currents moving around the plant and soil (think of putting on a jacket to keep out the wind). Most plants (and objects) have a layer of still air close to their surface, which acts as insulation. Air currents remove this. Some plants have leaf hairs, which help matters since they help trap this layer of air. Although the main function of this is to cut down water loss, it also helps insulate the plant. (and if the hairs are light in colour, they also aid cooling by reflecting back the sun's rays).

CONDUCTION
Associated with heat loss through making contact with a surface with a different temperature (think of touching cold metal with a warm hand). Since the ground is invariably warmer than the air on a frosty night, this usually warms the plant.

RADIATION
Even bodies in a vacuum can gain and lose heat by radiation, provided the sides of the vessel are clear; no air or contact is necessary. Dark objects are the best (or worst) radiators; light ones vice verse. As recently mentioned, fleece (because it is white) cuts down radiant heat loss (by reflecting back any heat trying to escape this way). We also saw in the summer that black pots could cause compost temperature to rise so much that roots could literally boil. Covering the pot with shiny aluminium foil helped prevent this and reduced temperatures considerably in the tomato pots. Foil will work the other way in winter.

EVAPORATION
Plants cool themselves in summer by 'perspiring' water through the pores (stomata) in their leaves. The act of turning water at 100 deg C to vapour at 100 deg C uses up much energy and takes this as heat from the plant, thus cooling it. The heat needed to do this is called 'latent heat of vaporisation'. The process of plant water loss is called 'transpiration'.

Water acts in the opposite way at freezing point, when it turns from liquid to ice - it GIVES UP HEAT to the plant surface and thus lowers the freezing point slightly, often just enough to prevent damage. Market gardeners spray their crops before nightfall when spring frosts are forecast.