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The glaucous
green or blue colour of many species of cacti is
due to the deposition of a layer of wax crystals on top of the outermost
layer of the "skin" (epidermis) of the plant, the cuticle. This
layer of wax (called epicuticular wax) is deposited
on top of the cuticle cells shortly after these cells are formed at the apex
of the cactus stem. Because this layer of wax is produced only on young cells
and is not replaced later, as times passes this layer of wax is slowly
removed in plants exposed to the elements - that is the reason why young stem
growth is always more glaucous and greyish or bluish than older parts of the stem.
This layer of epicuticular wax has several
functions: it serves to reinforce the waterproof quality of the cuticle so
that the plant does not lose water through the epidermis; it functions in defense,
forming a physical barrier that resists penetration by virus, bacteria and
other disease organisms, such as the spores or growing filaments of fungi;
and lastly it serves to reflect part of the excess solar radiation (sunlight)
that the plant receives.
Perhaps this last property is the most important to explain the blue color of
many Pilosocereus species. But before we can discuss that, first we
need to remember the relationship between plants and light.
The sun emits a range of different radiations, and light is just but one of
the many kinds of radiation which are emitted by the sun. The radiations are
emitted by the sun in the form of pulses or waves, and the distance between
the crest of two consecutive waves, termed a wavelength, is how we identify
the different kinds of radiation. Wavelengths vary in length from nanometers
(one billionth of a meter) to several meters.
The wavelength indicates how fast the pulses are of a given type of
radiation, and thus how much energy it carries: short wavelengths are more
energetic, while long wavelengths carry less energy. The difference has to do
with the amount of pulses in which a given radiation is emitted in a given
unit of time: a radiation with a short wavelength will have many pulses
emitted during a given time, while a radiation with a long wavelength will
have less pulses emitted in the same period of time. Because of this, more
energy per unit of time is transmitted by radiations with short wavelengths
than radiations with long wavelengths.
The sum of all kinds of radiation emitted by the sun is called the solar
spectrum, and light is nothing but the spectral range of solar radiation
which is visible to the human eye, and which ranges from about 380 nanometers
to 780 nanometers. For humans, this range of radiations is translated in colors
as follow:
range of radiation between 380 & 450 nm: violet
range of radiation between 450 & 495 nm: blue
range of radiation between 495 & 570 nm: green
range of radiation between 570 & 590 nm: yellow
range of radiation between 590 & 620 nm: orange
range of radiation between 620 & 750 nm: red
Plants absorb light radiation though a special pigment, the chlorophyll, and
convert this light energy into chemical energy through a process called
photosynthesis - which is basically the process in which plants manufacture
their own food using the light of the sun, water and carbon dioxide.
Thus, plants need to absorb light in order to live. However, plants do not
absorb visible light through the whole of its range, but there are two peaks
where absorption is at a maximum: between about 400 to 500 nm which roughly
corresponds to the blue color of the spectrum, and between 600 to 700 nm
which roughly corresponds to the red color of the spectrum. The range that is
not absorbed, from about 500 to 600, correspond to the green color of the
spectrum; because this range which is not absorbed by the plants is
reflected, this is the reason why plants appear green to us. The range of radiations which are useful for the plants to
perform photosynthesis are termed Photosynthetically
Active Radiation, often abbreviated PAR.
Now let�s go back to the third function of the epicuticular
wax in cacti, which is to reflect part of the excess solar radiation
(sunlight) that the plant receives. The green cells in plants can process a
limited amount of light through photosynthesis at any given time, and the
excess light radiation can be damaging to cells and tissues, specially the
more energetic radiations of shorter wavelengths. The same thing occurs in
humans regarding ultraviolet (UV) radiation, which is the sun radiation of
wavelengths below about 400 nm. Moderate exposure to UV radiation is healthy
because it stimulates the skin to produce Vitamin D3, a vitamin
which plays an important role in the maintenance of many organ systems in our
bodies; on the other hand, prolonged exposure to UV radiation can cause skin
cancer.
Plants that live in areas with high intensity of sunlight cannot absorb all
the light that they receive, and need to devise ways to prevent damage to
their tissues by the excess sunlight. Normal leafy plants usually have leaves
that live for a short period of time only; if a leave is damaged by excess
radiation the plant drops this leaf and simply grows a new one, thus
basically avoiding the problem. Xerophitic plants
that live in dry areas lose their leaves periodically during periods of
drought.
Cacti however do not have this luxury: their epidermis has to live and
perform photosynthesis for many years. Thus, in order to protect their
epidermis from damage due to excess sunlight, the cacti evolved different
mechanisms: some decided for a partial shade strategy, and developed a high
number of ribs which partially shade each other, or a dense cover of spines
which partially shade the stem, or long hairs like those of Espostoa
or Oreocereus which perform the same shading function. Other cacti and
many succulent plants with long-lived leaves, like for example Echeveria
and Dudleya of the Crassulaceae, evolved to
produce a very thick layer of epicuticular wax in
their epidermis, which reflects the excess solar radiation. This layer of wax
or farina as it is sometimes called usually confers a greyish
to bluish color to the epidermis of the plants, and the reason for the bluish
color in particular is because blue, violet and ultraviolet are in the short
wavelength range of the spectrum and thus more energetic and therefore
potentially more harmful to the plants.
In short, the advantage of the glaucous blue color
of many of the Pilosocereus species is to reflect excess solar
radiation that would otherwise be harmful to the plant, and the reason for
the color blue in particular is because this color is composed of radiations
of shorter wavelengths, more energetic and potentially more harmful to the
plants.
Fruits of Pilosocereus and other columnar cactus species are glaucous green or bluish when they are unripe and the
reason for the color is also a dense cover of epicuticular
wax. Usually this protective wax cover is lost and the color
of the fruits change when they are ripe.
The fact the Pilosocereus need more warmth during the winter does not
have anything to do with their color - these plants grow in warm areas in the
Americas,
and are not used to grow in low temperatures.
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From a photographic point of view in
filters, lens coatings etc., I understood the color you see are the colors
they reflect or hold back, the other part of the spectrum passes through. I
presume therefore the glaucous blue color
our eyes see on cacti is the exact part of
the spectrum the wax coating is reflecting,
while allowing the rest through?
Exactly, the color that you see on the surface is the
color that is reflected by this surface, all other colors passes through and
are absorbed. A black object absorbs most of the light it receives, while a
white object reflects most of the light it receives. However, there is seldom
absolute reflection or absolute absorption, and materials of one color mostly
reflects all the light in the wavelength of that color, but still absorbs
some of that color as well.
The cacti with a blue epidermis do not have a pure blue color but more of a
grayish blue or sky-blue color, and the reason for
this is due to the fact that the color we perceive in these cacti is a
mixture of many different wavelengths being reflected, from green to shorter
blue and violet wavelengths. Actually it is the same effect that makes the
sky blue - the shorter wavelengths are mostly reflected by the upper layers
of the atmosphere, while the longer wavelengths mostly pass through.
But the cacti with blue epidermis still absorbs part of the blue wavelengths
that it receives, in spite of reflecting a good part of it, and uses the
light in these blue wavelengths for photosynthesis.
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Is this characteristic for all cacti, or
just the ones that get scorching light?
Not all cacti produce this layer of wax in the
epidermis, some have evolved other ways to cope with the excess light, for
example by growing lots of spines or hairs that shade the stem, or many ribs,
etc.
Some have avoided the problem altogether by growing in forests as epiphytes.
But many columnar cacti produce this layer of wax to a greater or lesser
extent - it is not always blue, sometimes the wax is whitish and the plant
looks a greyish green color = glaucous.
Not only cacti produce this layer of wax - actually, not only succulents
produce it - many other plants do the same, and its not only to protect from
the sun, but also to protect from disease organisms as virus, bacteria and
fungi by forming a physical barrier to prevent them entering the epidermis,
and also to render the epidermis of the plant waterproof. For example, the
common cabbage has a very thick layer of epicuticular
wax on its leaves:
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