|
How they
live, manifacture food, reproduce, adapt to the environment
Growth factors
The elements that determine plant development (heat, water, light,
nourishment) are strictly interdependent: if one of them increases, also the
others must increase, and vice versa, of course within determined limits.
For instance, if a plant is watered too much, the light, the temperature and
the nourishment must be also increased. But since every plant has a
characteristic equilibrium, which for each factor varies among a minimum
limit, a maximum limit and an optimal condition, the more are the factors at
their optimal level, the less are influent those that don't reach it.
Nevertheless, if some factors are below the minimum level, the whole
vegetative activity stops, because the plant can’t make use of any other
available elements (Giusto Liebig’s minimum law). It’s necessary to point
out that for a lot of plants these factors have a large variability range,
so they result enduring (so-called easy plants), while others are not at all
(difficult plants). So it’s very important to know the climatic conditions
of the plants’ native habitat, even if it’s not necessary to recreate these
conditions, which would be almost impossible.
Heat:
is the factor which determines the growth, since it conditions the vital
functions, regulating their intensity. In cultivation, in winter especially,
the plants are kept at slightly higher temperatures that in native habitat,
because of the difficulty in getting very low humidity values. Cultivation
guides determine the minimum temperature values so that the more cold
sensitive species don't suffer. The ideal temperature is 25-35°C (73-77F°) in summer;
temperature values near 40°C (104F°) are tolerated, well knowing, yet, that above
50°C (122F°) the vegetable cells begin to die if the heat is not associated with
good ventilation. Rest, at low temperature and humidity (except for some
sporadic cases), with little or no watering, is essential for cactaceae
flowering.
Water:
is the circulating solution, integral part of the living matter and source
of equilibrium in presence of micro-organisms, essential element for the
vegetable nutrition, since it allows the mineral salts absorption. It’s
absorbed by plants from the ground and, to a smaller extent, from air.
There are more plants dying from excessive water than from lacking of water; and
it’s easier to recover a plant in the second case, rather than in the first
one. The prolonged water excess makes the empty spaces among the earth
particles occupied, causing roots asphyxia and stem rotting.
If you learn to observe plants, these show you when it’s time to water them,
after the rest period, presenting their apical zone deeper green coloured.
To prevent calcium accumulation in the ground, it should be always used
rainwater collected, for instance, from the roof and preserved in a dark
place, to avoid algae growth. Watering must be performed with a good soil
soaking in the evening, during summer, and in the first morning at the
beginning of season and in autumn.
Light:
is the essential element for
chlorophyll
function, i.e. the formation of organic substances from carbon dioxide and
water, using light energy. In other words it is the
process in
which plants store sunlight energy which will be released at the proper time
to activate
numerous
and complex chemical reactions.
But not
all the succulents need light in the same measure, some species like to stay
in the bright sunlight for the whole day, others prefer sunlight shielded by
mats or nets, imitating what grass and bushes do in habitat; finally, others
like the shade and the high humidity, as Epiphyte, native to the forests.
But also sunlight loving plants can be burnt if they don’t get accustomed
gradually, as our skin at the seaside or in the mountains, during summer.
Nourishment:
all the living beings need it; the plant draws mineral salts dissolved in
water from the ground and carbon dioxide from air. Ventilation is of prime
importance in the making of living substance and in the inside temperature
regulation. But draughts must be avoided. Fertilizers must be used diluted
to half the strength recommended on the label and, however, their contents
must have an elements ratio not much different from one part of Nitrogen,
two parts of Phosphorus, four parts of Potassium, diluted to one-thousand
strength.
Liebig’s law is valid not only for the growth factors, but also for nourishing
elements, so if a fertilizing element is low, the whole vegetable production
suffers from it. Plants, without any exception, must not be fertilized
during the vegetative rest, nor after a transplanting.
For further information consult the page Cultivation guide
Pests and diseases:
can be the cause of a growth stop at first and can, subsequently, bring
plants to death, so it’s necessary a careful check to avoid them.The most
common pests are the mealy bugs (“mealies”) which attack the stem and the
roots (“root mealies”). A way to control widespread mealy bug attacks is
sprinkling and soaking with a 2 part per thousand solution of specific
insecticides (based on diazinon, for example). Other dangerous pests are red
spider mites which can be killed by systemic insecticides, miticide, and by
misting the plants.
Among cryptogamic diseases, the most serious damages are caused by the rottenness
and by the mould, in presence of soil with too much organic substance and
excess of humidity. If the attack originates from the roots, generally there
is no remedy, because the illness will be already spreaded to the stem;
instead, if it is at neck level, it is possible to try to remove the sick
part, disinfect it with copper sulphate, wait for cicatrization, and finally
it can be treated as a cutting. The prevention, carried out with systemic
fungicides, is fundamental.
Against animal and vegetable pests, should be enough performing two liquid
applications with the products described above, at the beginning and at the
end of the growth season. Personally, I think it’s good also to perform a
treatment, in the first December, mixing a fungicide and a parasiticide
broad spectrum dust.
For further information consult pages Pests and
diseases
(under translation).
It’s better to keep separate summer growing plants from winter growing
plants (some not acclimatized southern hemisphere species).
 Flowering
Is
the inimitable tool that Nature has produced to perpetuate the species, and
that so much joy and satisfaction can give to whom, like us, cultivate
succulents for passion.
The flower is essentially constituted by:
-
the
calyx, above the stem; it protects, from the outside, the delicate
flower organs. It is composed of sepals, which are called tepals
if they have a colour different from green;
-
the
corolla, that is composed of petals often showy to attract
insects;
-
the
androecium, male organ with stamens; it is composed of the
filament and the anther which contains pollen grains;
-
gynoecium,
the female organ; it is composed of the ovary which contains
ovules; upward the ovary ends with the pistil, constituted by the
style and the stigma.
Flowering
is also sign of the maturity of the plant and it is reached at very
different ages from a species to another on the base of the information that
the plant receives from its genetic patrimony. So we have Rebutia that
blooms after few months from the seeding, but it has a brief life, while for
the Carnegiea it is necessary to wait 40-50 years, but it has a life of over
one century. Melocactuses bloom in correspondence of the
cephalium
appearance that also puts an end to the growth, while some species must
reach a determined height. For the Agave, Aeonium tabulaeforme, Sempervivum
the flowering marks the near plant death (monocarpic species). In
cultivation a genetically mature plant could also not to bloom for a whole
series of negative factors as the lack of balance in growth factors or for
some illnesses. To bloom, a lot of species must have observed a determined
day length and must have been subjected, for a given period, to low
temperatures, followed by higher temperatures (thermal shock). This
technique is often used by gardeners to force ornamental plants. Various
studies and experiments make to think that an increase of auxin conducts to
an increase of the female flowers in the monoecious plants, contrarily to
what happens with gibberellin. Some hormonal treatments have also allowed
the sex inversion in the dioecious plants. Most plants ensure cross
pollination (heterogamy) facilitating it with opportune mechanisms like:
bringing stamens and pistils to maturation in different times, or
positioning them so that the pollen cannot come into contact with the
stigma.
Discovered the secret of the flowers!
How is transmitted the information that tells all the flowers of a certain
variety that it is time to bloom, so that they can pollinate each other
allowing the species to perpetuate? This is the question that all the
botanists asked for about seventy years. Some has spoken of not well defined
"messengers", some of a hormone (perhaps a gibberellin) some, more recently,
of equilibrium between nitrogenous substances and carbohydrates. All these
theories have never found a scientific confirmation. Recently a team of
Japanese, Swedish, German and American scientists have given a sure answer
to the question, using the molecular biology, which enabled the reading of
the genes.
The
"sensors" present in the plant are able to recognize the temperature and the
duration of the day or of the night, so that when conditions are proper, a
gene produces a protein called Ft (flowering locus T); this
protein is carried through the lymphatic vases (phloem), up to the bud top,
where another gene recognizes the protein and starts the flowering
mechanism. This is a great discovery, which can also bring remarkable
advantages in repopulation. Gardeners use to induce some plants to bloom
through the "forcing" technique, that consists in artificially altering
illumination and temperature in the greenhouse, but in an empirical way and
without knowing "why" and "how" the phenomenon happens.
The pollination
When a grain of mature pollen is deposited on the mature stigma of a flower of
the same species, it germinates, swells, and creates the pollen tube;
this grows inside the style crossing it up to the ovary locule, going
toward an ovule to which it sticks, penetrating in the micropyle.
In the meantime the generative nucleus of the pollen forms two spermatic
cells that are placed at the end of the pollen tube; one of these
penetrates the embryonic sack and it connects with the egg cell; the two
nuclei merge: this is the fecundation that immediately starts the
embryo formation.
The other spermatic nucleus enters the primary endosperm, it merges with
the two polar nuclei and forms a reserve of nutritive tissue inside the
seed, constituted by one or two embryonic leaves called cotyledon.The
ovary increases its dimension coming to form the fruit, while the ovules
will become seeds.
The artificial fecundation.
Men use this knowledge to artificially intervene to get more beautiful, more
resistant and more productive breeds.The flower from which it’s wanted to
get the fruit is chosen; with special tweezers the immature stamens are
removed (castration), to avoid self-fecundation; the flower is protected by
a gauze bag; when the stigma appears completely open, shiny and viscous,
which are maturity signs, it is fecundated with mature pollen (not
agglutinated) from the chosen flower; the bag is inserted again; a label
with the date and the name of the mother and the father plants is applied to
the flower. This first generation is called F1, and usually it is not
remarkable, so often other crossbreeding follow.
In Nature succulents pollination is performed by "pollinators" constituted by
bugs, birds, bats which often are not present in cultivation; nevertheless,
by the wind, some hybrids can be produced through unwanted crossbreed
between different species and different kinds, that would never happen in
the origin places, because of the distance. Who wants to maintain race
purity can protect flowers with a tight weave gauze or can cap with it the
whole plant, if its dimensions allow it.
The fruits
Origin from the ovary transformation, they have the pericarp outside and, inside,
the seeds constituted by fertilized ovules. They can assume different forms,
according to the family or to the species. So we can have the berry, the
achene, the follicle, the capsule, the drupe, confining to the most common
forms in succulents.
The berry, typical of the cactaceae and of a lot of others succulents, can
be pulpy or dry, dehiscent or indehiscent. Generally it has a fleshy
pericarp and a membranous epicarp.
The achene, typical of Compositae and of Moraceae, is a dry fruit,
indehiscent with coriaceous pericarp which wraps the seed without sticking
to it.
The follicle, typical of the Crassulaceae, Apocynaceae, Asclepiadaceae is
a fruit constituted by a single carpel able to open in longitudinal sense
and to free the seeds endowed with "pappus", transportable by the wind.
The capsule, characteristic dry dehiscent fruit, can assume different
forms and behaviours: with three lobes as it happens in the Euphorbiaceae;
able to open with the rain and to close with the dry as it happens in the
Mesembryanthemaceae; with some locules as in the case of a lot of Aloaceaes.
The drupe is a fruit with pericarp pulpy outside and woody inside; it is
typical of a lot of succulents belonging to the families of the
Anacardiaceae, a lot of Arialiaceae, Apocynaceae and Burseraceae.
The seed and its germination
The seed is composed of a wrap inside which there is the embryo that is a sketch
of the future plant. Once mature it enters a phase of rest (latency) up to
when it will find proper conditions of temperature, humidity, illumination,
oxygenation. A well conformed embryo, with enough reserve substances, will
bud giving life to a new plant. The reserve substances result so much
optimal as best are vegetative conditions in which the mother plant is
during the reproduction. Another fundamental element is the morphological
and physiological maturity of the seed that usually is reached with fruit
maturation, which, in some cases, coincides with the dehiscence (opening).
The phase of latent life, according to the species, can last some months, years
and even a few centuries; after that, the embryo dies. The reason of this is
in the composition of the reserve substances, and therefore in the faster or
slower oxidation of the fat substances. This process can be slowed down
preserving the seeds in a dry environment and at low temperatures.
Some seeds must wait for the disappearance of some inhibiting chemical substances
to bud; among the necessary external factors, already mentioned, we remember
water, which starts the whole process and is also needful to break the
teguments of the hard and impermeable seeds. The seed, during swelling,
needs oxygen so that the metabolic process can begin. Also the temperature
has an important role (it is enough to think about vernalization),
necessary for the seeds of the so-called cold plants. For many seeds
also the light has an important role even if an illumination of few luxes
and for short times is enough. The seeds that need light to germinate are
called photoblastic and they constitute
around the 70% of the species; those that escape it, on the contrary, are
called aphotoblastic.
The possibility that the plants have to colonize a determined region depends on
the dispersion both of the fruit and of the seed, performed by the wind and
the birds.
The dormancy, besides in the seed, is also manifested in other vegetative organs,
typically in that territories where exists a seasonal nature, where are
periodically present conditions adverse to the metabolism. The causes must
be found in cold and in lack of rain, for which the apical buds become
impermeable and the plants minimise transpiration becoming the more
resistant to cold the smaller is the water content in tissues. When
conditions are favourable, the seed absorbs water, it swells, it breaks the
tegument, the radical meristem is activated, the radicle comes out
and it penetrates in the ground because of the positive geotropism; shortly
after the apical meristem of the young stem does the same: the plumule
comes out from it, because of the negative geotropism, and it directs upward
searching the light. Root-hairs are soon formed and the seedling
starts to absorb through them. The seedling doesn't have true green leaves
yet which could produce organic matter, so its nutrition is provided by the
substances accumulated in the seed. A normal apex is formed only in presence
of a suitable illumination. With scarce light the seedling has difficulty in
stretching, and the extension of the young stem is abnormal. With the start
of the photosynthetic process the young seedling becomes autonomous and
grows according to the information contained in its genetic patrimony.
Control of the life cycle
The life cycle of a plant is the consequence of complex interrelations between
the genetic information and the environment. At each stadium of plant
development, there will be one or more hormones able to regulate its
activity.
Abscisic acid
(ABA) is an inhibitor able to activate itself in concentrations of
one part on 5 million; it supervises the dormancy of the seeds and the buds,
as it happens to the seeds of desert or to those of the cold regions, that
only a strong rain or strong frosts, can remove respectively. The winter
sleeping buds of a lot of plants contain high levels of this substance,
which decreases with the springy awakening; but, perhaps, the principal
function is helping plants to store the water in aridity periods and making
them more resistant to freeze. Also the action of leaves and the fruits
falling is associated to it.
Gibberellins
are hormones present in the apical and subapical meristems of young stems
and leaves; they stimulate embryo germination, are able to make the dwarf
plants grow in an impressive way and to promote their flowering; their
carriage is operated by the vascular bundles without a remarkable
accumulation which could produce damages. About sixty gibberellins
(identified with the letters GA followed by a number) are known; but just
some of them are present in each individual. They are very similar but
plants are able to distinguish them and to react in an abnormal way to some
and to be insensitive to others.
Auxins,
said vegetable growth hormones, react in different ways according to their
concentration and to the organ to which are applied; it seems they can make
the radicle to direct downward and the young stem to direct toward the
light, in opposite directions, because of that they also have a control on
the phototropism. It supervises the apical dominance, so an apical bud in
growth phase suppresses the development of lateral buds in the same stem;
they promote the ramification and the
initiation of root growth,
as happens in cuttings: stem cells contain all the necessary information to
form the lacking parts of the plant. They stimulate
the production of ethylene for the growth of new adventitious roots
and for the inhibition (or promotion) of leaves and fruits abscission
(fall).
Ethylene
is the only gaseous phytohormone; at the
concentration of one part on 6.000.000, it provokes seedling deformation, it
stimulates leaf abscission, it interrupts
dormancy of seeds and apical buds, it accelerates fruits, flowers and leaves
ripening.
Cytokinin is a chemical substance which regulates the cell division; it can delay
the process of organs aging, intervening on trophism; it is present in the
roots, in the seeds and in the fruits.
It is thought that Auxin and Cytokinin stimulate lateral buds growth
(branching) in consequence of the apical bud topping (apical dormancy).
Auxin and Cytokinin interaction coordinates the balanced development of the
aerial and underground parts. Auxin and Gibberellin control the process of
differentiation between the carrying of sap (phloem vascular
tissue) and the carrying of water and mineral salts (xylem vascular
tissue).
A pollinated and fecundated flower is able to produce a fruit, only if Auxin and
Gibberellin stimulate ovary cells to multiply and to grow. In autumn the
fruits and the leaves, under the action of hormones among which Auxin (IAA),
maybe Cytokinin, and Abscisic acid, undergo a process of senescence that
ends with the formation of an abscission layer at the stalk base;
enzymes action destroys this layer and allows fruits and leaves to fall. The
plant slows down its metabolism, the new buds enter dormancy; all is ready
to face winter, waiting for the awakening that will come when the
temperature will raise and the days will draw out.
The plant recognizes the season on the basis of an element that it considers
stable: the length of the night. So long-day plants bloom when the day
reaches one determined length; short-day plants bloom when it goes down
under a determinate value, considered critical; day-neutral plants, instead,
bloom accordingly to their reached maturity, independently on the hours of
light. So that this mechanism works it is necessary that the plants can
measure passing time, and they do it with a kind of internal biological
clock, also without any signals given by the environment (circadian
rhythms); the light is measured by a pigment-hormone called
phytochrome, which is of blue colour and which can induce seeds
germination, etiolation, chlorophyll synthesis and buds dormancy; it is
present on the leaves and in the seeds and in smaller measure in all the
other vegetable parts. The circadian rhythms are fed by the respiratory
process and they are also observable in Succulents, as in the flowers
opening and closing of Kalanchoe blossfeldiana and in the carbonic
dioxide emission of Bryophyllum fedtschenkoi.
Daily cycle.
To survive in hostile environments succulents has contrived metabolic
processes different from those of all the other plants.
Cycle C.A.M.
(Crassulacean Acid Metabolism). During the night, succulents, with open
stoma,
uptake
endogenous
carbon dioxide
and store it
as malic acid
through a
particular enzyme (PEPC). During the day, carbon dioxide is released by the
malic acid and the stoma are closed to avoid water loss caused by heat;
anyway, the light can activate photosynthesis and the connected sugar
production, with formation of oxygen and consumption of carbon dioxide.
Carbon dioxide used by CAM plants for the photosynthesis is both atmospheric
and internal, coming from the respiration. This cycle is typical of
Agavaceae, Aizoaceae, Asclepiadaceae, Asteraceae, Bromeliaceae, a lot of
Cactaceae, Crassulaceae, Cucurbitaceae, Didieraceae, Euphorbiaceae,
Geraniaceae, Labiatae, Liliaceae, Oxalidaceae, Orchidaceae, Piperaceae,
Portulacaceae, Vitaceae.
C3 cycle.
Also said Calvin cycle is constituted by a continuous series of reactions
that convert the carbon dioxide into carbohydrates by fixation, during the
dark phase of the photosynthesis. The first product of carbon dioxide
fixation is a molecule with 3 carbon atoms, so plants that use it are said
C3. This process causes a carbon dioxide loss with a damage in growth as
greater as brighter is the light intensity.
C4 cycle.
Some tropical plants have developed another effective way to capture carbon
dioxide through its pre-fixation, followed by a transfer to Calvin cycle
(C3). These plants fix the carbon dioxide forming malic acid through an
enzyme which doesn't bind oxygen and from whose reaction is obtained a
compound with 4 carbon atoms (C4).
Plant growth.
In the Angiosperms we distinguish root and aerial part. The first fixes the
plant to the ground, it absorbs water and mineral salts, it stores excess
sugar, it distributes water, salts, sugar and hormones to the whole plant.
The aerial part attends to the photosynthesis, materials transport, the
reproduction, the hormones synthesis. Plants grow for the whole life,
nevertheless height growth occurs in the tall part, while the developed
parts don't grow in this sense. It is the primary growth realized by
meristematic cell division, where a cell remains as it is, while the other
forms the permanent parts of the plant. Secondary growth takes place by
division of lateral meristem cells which attend the development in width.
Plant death.
Some plants live for a season, others for millennia, but the natural death
is an ineluctable destiny in multicellular plants because of the
differentiation of the somatic cells which perform determined functions and
of the following degenerative process. Cells senescence is determined by
toxins accumulation which conducts to the subcellular damage of the
organelles, which die, also causing the subject death. In each stadium of
vegetable life exist old cells, new cells, more or less diversified cells,
and dead cells.
Death cause in the annual plants must be searched in the lack of lasting organs
capable to withstand the winter stasis. Some grassy plants, with underground
reserve organs, are able to survive even if the aerial part dies. A lot of
plants, which in their natural habitat are considered perennial, behave as
annual if cultivated in other environments.
 Adaptation
to the environment
Plants grow in every part of the world, showing to be able to tolerate a
large range of climatic conditions. Nevertheless plants with a determined
habitat are certainly not able to survive in a completely different
environment, because each individual has developed certain characteristics
that make it proper for its local climatic conditions. To reach these
results the plants had to modify both the physical structure and the
physiological and biochemical mechanisms.
When plants left waters where they lived, to colonize the dry land, they had to
face and to solve a lot of problems: developing a support structure;
preventing the excessive water loss allowing, at the same time, gaseous
interchanges; protecting the delicate reproductive organs; improving the
mechanisms of adaptation to variable climatic conditions, in relationship
with seasonal nature; inventing a system for carrying nourishing mineral
substances through the various parts of the organism; assuring the
reproduction in the most proper period; giving to seedlings protection and
nourishment. This has involved big changes to metabolism and has made
necessary the acquisition of sensory structures and the creation of a
precise internal biological clock. Besides the time the plants are able to
measure gravity, temperature, light, they feed, they breathe, they fight
infections and in some cases they form symbiosis with mushrooms and
bacteria. They can also be regenerated in various ways using single cells,
indicating that each of them contains the genetic information to
reconstitute whatever part of the plant. The vegetable cell represents the
"brick" with which are built all the organs that allow the development of a
new individual, by cellular differentiation and cellular division.
Plants live in competition both with the environment and with the other
vegetables for conquest of light, of space and of nourishing substances,
necessary elements to their survival.
Alliance among plants, bugs and birds.
The most known mutual symbiosis is that among pollinator bugs and
entomophilous plants. Insects activity is fundamental for seeds production
of many vegetables; in exchange the vegetables furnish pollen and nectar.
The same do hummingbirds (Ornithogamy) and
bats (Chiropterogamy), for example towards
saguaro, whose flower, opening during the night and for a few hours, could
not be fertilized otherwise.
Another symbiosis is that between ants and Myrmecodie (Rubiaceae), where the ants
find shelter and nourishment in caudex cavities, protecting in exchange the
plant from the attacks of harmful phytophagous species. A particular type of
symbiosis is also that offered by the galls, that is from an abnormal
development of cells or tissues caused by a parasite that could be a
nematode (roots), a bacterium, a mushroom, a mite or an insect. The gall can
develop not only on the roots but also on buds, leaves, inflorescences,
fruits. The relationships between insects secretion and plant tissues have
not been clarified yet, what is certain is that the secretion is injected by
females together with the egg or from the same larva through the salivary
glands.
|
|
