Chapter 2: Physiology of Plants
To achieve good results, a home grower must know about plant physiology.
Plant physiology is the part of biology which is concerned with the way
plants grow and flower. In this chapter, the priciples of plant physiology
are discussed. With the growth and flowering of plants, it involves a select
combination of light, air, and water. For light, it's about sunlight for
outside growing, a combination of sunlight and artificial light for greenhouses,
and just artificial light for inside growing. For air, the amount of carbon
dioxide (CO2) is of principal importance. Water performs various functions.
Plants need water (H2O) for the growth process, but also to transport other
2.2. Principles of growth
Plants change CO2 and H2O into glucose under the influence of light. Glucose
is the chemical building block for the structure and sturdiness of the
plant. From glucose, the plant makes cellulose, the material which gives
plants their fibrous structure. (Glucose is, in fact, stored light energy).
The chemical process in which carbon dioxide and water are converted into
glucose is called photosynthesis (from the Greek 'photos' = light, and
'synthesis' = to compose). Chlorophyll, which also gives plants their green
color, is indispensible for this process. If all the conditions are right,
the following chemical reaction occurs:
6CO2 + 12H2O = C6H12O6 (glucose) + 6O2 (oxygen) + 6H2O
We can deduce a number of things from this formula. To get one part glucose,
we need six parts CO2 and 12 parts H2O. It would seem that less water is
necessary. When we look at the chemical formula, six parts water are also
produced next to the 6 parts oxygen, and 1 part glucose. However, research
has shown that in the chemical process, 12 parts water are needed. The
'excess' water is used in the intermediate steps. The water does not re-appear
until the end of the process. CO2 is a gas in the atmosphere. There must
always be sufficient carbon dioxide available, otherwise, plant growth
will reduce. Everyone knows that plants need water From CO2 and H2O, not
only glucose, but also oxygen is made under the influence of light, by
the plants with the help of chlorophyll. For plants, Oxygen is a by-product
of growth. For people and most animals, it's the primary condition of life.
This is a good combination. In fact, in their metabolism, animals do the
converse of what plants do. They convert glucose and oxygen into carbon
dioxide and water to be able to move, and to allow the heart and lungs
to work, etc. CO2, a gas which is exhaled by people, can again be used
by plants for photosynthesis. It can be thought of as a cycle. The glucose
made by plants is an energy source for the plant. Some processes, such
as the intake of water, require energy. Next to that, glucose forms the
building material for all kinds of other processes with which the plant
lets all its specific properties show. It would go to far beyond the pupose
of this book to look into all those chemical processes. For the reader
of this book, it's about getting good results growing cannabis at home
A plant cannot grow without light, air (which contains CO2), water, and
various nutrients. The chemical process in which CO2 and H2O are converted
into glucose and oxygen under the influence of light is called photosynthesis.
When we look at this process a little closer, it actually involves two
different chemical reactions. The first is called photolysis. In photolysis,
water is broken down into oxygen (O), and hydrogen (H). Both light and
chlorophyll are necessary for photolysis. This is called the light response.
The second chemical reaction is called the dark response As the term suggests,
no light is necessary for the dark response. With dark response, carbon
dioxide is converted into glucose, with the help of the hydrogen produced
during the light response. The distinction between the light- and dark
reaction is of interest to the cannabis home grower in order to gain insight
into the manner in which the plants must be illuminated (and sometimes
kept in darkness). The plants grow optimally only when a good balance is
found between the light and dark reactions.
2.3. Osmotic processes
With osmosis, we mean the processes in which water and nutrients are absorbed
by plants. Osmosis is based on the principle that the plant's walls permit
some materials to pass through, and others not. Cell walls are semi-permeable.
An example: when we place a bladder with a sugar solution in a tank of
water, the bladder swells. The sugar solution attracts the water. The more
sugar in the solution in the bladder, the more water will be absorbed,
and the pressure in the bladder will rise, but don't try this at home!
Among other things, osmosis provides for the sturdiness in plants' cells.
So much water is taken in that the plant cells become saturated, and the
stalk and the leaves stand upright. If too little water is in supply, the
plant cells give off the water; slowly, but surely. The strength is lost,
and the plant wiltsAnother way for a plant to lose its sturdiness is for
osmosis to work in the reverse direction. If there is too high a concentration
of materials in the water fed to the plant, the plant will not absorb water.
It will release water, and become less sturdy. An example is the addition
of too high a dosage of fertilizer to plants. With over-fertilization,
plants dry out and burn . . . A second important function of osmosis is
the 'hitch-hiking' of salts (nutrients) together with the water that, through
osmosis, ends up in the plant cells. Nutrients are necessary to allow certain
growth processes to take place. The salts also cause various kinds of plants
to develop various properties. That brings flowers, fruit, and fragrances
to mind. In general, plants need the following materials in a water solution:
- nitrogen, phosphorus, and sulphur for the construction of cells; - magnesium
to manufacture chlorophyll; - potassium, calcium, and magnesium for osmotic
processes; - water for growth, for the transport of nutrients, and for
sturdiness; - iron, boron, copper, manganese, and zinc as building materials.
Most of the nutrients for plants are sufficiently present in our ordinary
tap water. But not all The law of minimums plays a great role in the feeding
of plants. Material that is present in too small a quantity is a limiting
factor on the plant's health. So-called 'deficiency disease' appears when
a plant does not receive one or more nutrients. For example, a shortage
of iron causes rather white leaves, while a shortage of nitrogen causes
reduced growth and yellowed leaves. 'deficiency disease' involves not only
the direct effect (an unhealthy plant doesn't grow well), but also impaired
resistance. If needed materials are lacking, the chance for infection by
moulds and vermin increases. We will discuss plant diseases more extensively
in a later chapter. In order to raise healthy plants, we need further amplification
of the materials which, by nature, appear in our water. This involves primarily
nitrogen (N), phosphate (P), and potassium (K). A formulated combination
of these materials is available in shops, and is called 'NPK solution'.
We differentiate the different nutrients in order of importance. We call
the most important the primary nutrients; - the NPK combination just mentioned.
The secondary nutrients follow; namely magnesium (Mg), and calcium (Ca).
Finally, there is a group of micro-nutrients, also called trace elements.
Sulphur (S), iron (Fe), manganese (Ma), boron (B), zinc (Zn), and copper
(Cu) belong to this group, among others.
2.4. Intake and transport of materials
Water, and the nutrients dissolved in it (salts), is absorbed through the
root hairs of the plant. The condition of the soil plays an important role.
Hard dirt allows little space for water to reach the root hairs, a looser
soil has much more space, while rockwool substrate can guarantee a good
water supply. Root hairs are very important. When they don't work well,
the plant receives too little water and food. Growth is retarded. Root
hairs are very sensitive; they can easily be damaged by exposure to air
and light. Moreover, you can ruin them by careless transplanting, or just
by exposure. The intake of water and nutrients requires energy from the
plant, so oxygen and glucose are necessary. Ultimately, temperature is
a limiting factor. Even if you take care to provide sufficient water and
nutrients, the growth of the plant will be impeded if the ground temperature
is too low. This is one of the reasons why most plants outside grow very
slowly during the winter. The transport of water and nutrients insures
that these materials end up in the leaves. Two forces are responsible for
this: the suction power of the leaves, (they lose moisture by evaporation,
causing suc-tion to occur), and so-called root pressure. Root pressure
can be observed when we cut off a branch of a tree in the spring. Moisture
comes from the 'wound', and we call this the plant's sap. The suction force
of the leaves depends on the evaporation of water through the leaves. Stomata
are responsible for this evaporation process. The stomata can open and
close. Next to the evaporation of water, they provide principally for the
intake of carbon dioxide (CO2) from the air. They also issue the oxygen
which is produced. In the previous paragraph, we have seen that plants
lose their sturdiness if they lose too much water. The stomata dispose
of a mechanism to prevent that: they can close. Generally, a stoma will
be open if there is light, (thus providing for CO2 intake, and for optimal
suction power of the leaves), and closed if it's dark (when no CO2-intake,
or evaporation is necessary). If the air is extremely dry (dry, hot, mid-summer
days!), the stomata can also close during the day. For stomata to work
properly, a clean surroundings is necessary, since a stoma can become blocked
with dirt particles. Sufficient potassium (nutrients!) are also needed.
2.5. Factors influencing the growth of plants
We conclude this chapter with a summ-up of the principal concerns for the
optimal growth and flowering of plants. The following factors are the most
important ones: - the correct temperature; - the correct CO2 content in
the air; - the correct light intensity, with the correct wavelength of
the light; -the correct amount of water and nutrients - the right soil;
- (for cannabis growers) the right seeds or cuttings/clones; - 'green fingers'
In the second part of this book, we discuss which materials you need for
growing at home. We take a deeper look into the different factors which
influence growth and flowering. Summing up this comes down to an optimal
control of climate.
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