Plant Growth

In way of apology for a lack of words in my August monthly report, I have decided to write one of my mid month specials.  For those of you that have been to our caravan, you will know that we had a conifer hedge planted 2 years ago to define our rear boundary.  Until about two or three months ago, these shrubs looked very much like they did the day that they were planted.  Everyone told me that they grow substantial root systems before switching to growth above ground.  I have recently been reading about plants as part of my research for my book.  I previously knew very little about plants and have never read a gardening book in my life.  I still haven't, what I researched was the anatomy and physiology of plants along with their evolution.  I decided to apply my new found knowledge to my hedge; here is the story.

Plants grow through photosynthesis where they collect energy from the sun and use this energy to convert carbon dioxide which they breath and water which they suck up through their roots and convert them to carbohydrates.  The move and store these products through channels around the plant called phloem.  The waste product for this process is oxygen.  If it wasn’t for the plants and the algae that preceded them, no multi-celled animals could exist.  Sometimes these carbohydrates are assembled into cellulose and starch and moved down to the roots through the phloem for storage, such as carrots and onions.  Sometimes they store it in tubers such as potatoes etc.

The formulae for this conversion is;

H₂O + CO₂ + energy -----> CH₂O + O₂

The formulae for a carbohydrate is, as its name suggests, water with a carbon in front of it.  However, the simplest carbohydrate, sugar requires at least 3 carbon atoms and therefore 3 water and carbon dioxide molecules as the ingredients for the conversion.  Most sugars that plants and animals manufacture for growth have 5 or 6 carbon atoms arranged in a circle.  Animals and plants also make fats and proteins but we shall ignore those here.  The vast majority of plant growth is by the manufacture of sugars.  You would think that the easiest way to make the CH₂O from the H₂O + CO₂ would be to take the C from the CO₂ adding it to the H₂O leaving the O₂ but things don't always happen the way you would imagine.  It turns out that the O₂ comes from two water molecules instead.  The actual process of photosynthesis is incredibly complicated and I don't believe that we fully understand it, otherwise we would be using it to make energy and food.

In the summer, you cut your grass and discard the cuttings.  this might be done twice a week when conditions are right (lots of sunshine and rain), so a substantial amount of material is removed from your lawn, year in year out.  Why doesn’t your lawn slowly sink down as material is removed?  If the mater for growth, is coming from the ground then this would happen.  As we have seen, the material of plants are made from carbon dioxide and water.  The carbon dioxide comes from the air and the water comes from the sky, very little material comes from the ground.  Some material does come from the ground but only a tiny amount for nutrients.  Inside every raindrop that has ever fell is a tiny piece of dust.  The water vapor needs something to nucleate around when forming a raindrop and mostly it is dust that provides this service.  All these dust particles from all the raindrops is easily enough material to replace the nutrients drawn into the plants.  Imagine that, all the material in a tree comes from the air, tons of it.  Thin air.

Unless an element converts into another element by radioactivity, the total quantity of each element remains the same whatever chemical reaction it is involved in.  There will always be the same amount of oxygen whether it is part of a water molecule or a carbon dioxide molecule, or indeed, a sugar molecule.  There is only a tiny percentage of carbon dioxide in air so the plant has to accumulate it constantly to be able to make its sugars for growth.  It does this by breathing the air in, removing the carbon dioxide, adding the oxygen and breathing it out again.  Of course a plant cannot respirate in the same way as animals would.  Plants have tiny holes, usually in the underside of their leaves, called stomata.  Although they are only tiny openings, they open up into, relatively huge caves in between the plant cells.  All of the carbon dioxide entering these stomata is removed from these caverns, creating a concentration gradient for carbon dioxide.  This concentration gradient causes a physical partial pressure that attracts more carbon dioxide into the stomata.  Of course, something has to go out to make more room for the carbon dioxide, this is the oxygen waste product.  The oxygen is pushed out by a similar concentration gradient between the inside and outside.  As the oxygen is released from the cells as waste, the concentration of oxygen becomes greater inside therefore causing a partial pressure and movement to the outside where the gradient is lower.  Oxygen and carbon dioxide are not the only gases that move through the stomata, water vapor does too.  Water evaporates around the cells, inside these stomata causing a higher concentration inside than outside, so there is a movement of water vapor to the outside as well.  This causes a negative water pressure inside the plant which sucks up more water from the roots.  Thin water channels called, Xylem also help to move water from the roots to the leaves ( by capillary action), where it is needed for the reactions to take place, but this provides no where near as much force that is required in all but the smallest of plants.  This only works if water that has fallen onto the leaves is evaporated to draw more up from the roots.  Nothing is gained if only water that has been drawn up is evaporated to draw more water up.  After all, every time it has ever rained, the leaves get wet as well as the ground where the roots are.  This would seem like a good time to evaporate the water from the leaves to draw more water up into the leaves.  Most mornings the leaves also get wet from condensation.

In all plants and animal cells, we have tiny organs called organelles in each of our cells.  One of these organelles is called a lysosome which is a membrane bound bubble that carries nutrients, waste and water around our cells.  Each plant cell has a huge lysosome that occupies almost all of the volume within the cell.  As plants don't have a physical structure like bones or shells, they get their rigidity by pumping water into these lysosomes causing a strong pressure that gives each cell its firm structure.  We call this turgidity.  As the cell pumps water into this lysosome it makes the cell turgid.  When flowers or plants don't have enough water they become limp and droop (flaccid), this is because they loose their turgidity.

Although a plant needs one water molecule for each carbon dioxide molecule for its reactions to make sugars, the plants actual requirement for water molecules is far greater in reality.  It needs some to pump into the lysosome to provide turgidity and some to evaporate to provide the pressure to draw more water up from its roots.  When new growth takes place, the cells divide and the new cells grow until they too can divide.  Each new cell needs lots of new water to fill its lysosome.  To get this water up to the new growth it must evaporate more to provide this pressure.  Until the plant has all the elements that it requires for new growth it will stay dormant only providing the water it has so sustain the cells it already has.  Only when it has more than it needs to sustain its current cells, will it initiate new growth.

As I said earlier, plants also make a small amount of proteins and they also make hydrocarbons (fats) for their membranes and other things.  Proteins require quite allot of nitrogen and the cell membranes require a small amount of phosphorus.  Lots of other elements are requires in minute quantities before they can initiate new growth.  Although about 78% of air is nitrogen, this is not normally available for plant or animal growth.  The nitrogen in air moves around in pairs of nitrogen atoms making nitrogen molecules that have three strong bonds joining them together.  Only a few bacteria are able to split these bonds and separate the nitrogen into single atoms as they are required by life for growth (nitrogen fixing).  Fertilizers provide the nitrogen and other elements required for growth in the form that can be used.

Getting back to my conifers.  I decided that they were not growing because they required nutrients to initiate new growth.  I bought some plant feed that has to be dissolved in a watering can and sprinkled over the plants.  This should be done once a week but I did it every day I was at the caravan.  I cant provide any carbon dioxide, but I can provide the water.  I set the sprinkler hose up so I could easily water them and watered the roots and the leaves several times a day.  I felt it particularly important to water the leaves because they need to evaporate water from the leaves to be able to draw new water up from the roots.  The results were amazing; the conifers shot up from the day I started my program.  New growth usually looks a much lighter colour than the old growth and all my plants got lighter and lighter.  My horticulturally knowledgable neighbor spotted me watering the leaves in the sunshine and told me not to, or it would scorch the plants.  He said that most gardening books would tell me this.  Good job I don't read the gardening books.