Roots wonderful roots. Anatomy

Posted: May 24th, 2021 - In: Care guides

 

Let’s talk about roots. Some may find this a little strange but I find growing roots just as satisfying if not more so than growing foliage. I know they’re not as big or as shiny as a philodendron leaf nor are they as colourful as a begonia, (actually they can be…) but without a fine set of roots you won’t have much of a plant to admire anyway. I love them so much that I grow almost all of my plants in clear pots just so that I can see them.

There are different types of root system employed by substrate dwelling plants: the first is a fibrous or mass and identifies a plant as a monocot and the second is a tap which identifies a plant as a dicot.  Both are basal root systems and are located underground. Mass roots are made up of lots of different strands that head off in all directions in search of moisture and grow directly from the stem of the plant. A tap root is a single usually quite thick root that will drive down into the soil directly from the stem and will have very fine lateral roots that run off the tap. A fine example of a tap root is a carrot!

Another type of root that we all encounter if we are growing Aroids are adventitious roots, these are produced at any other point of the plant than that of the basal root system. For example they may emerge from the stem, leaves or the nodes of your plant and will wind their way around looking to anchor your plant to some sort of substrate or for feeding. Many of us will utilise moss poles that allow the adventitious roots produced at each node, and sometimes at the internodes, as the plant grows vertically to anchor themselves to and provide structure to the plant. Although these anchor roots are produced at the node, they are different in structure to the feeder roots. They have a more fibrous structure with a narrow central cylinder that gives them more strength to withstand the rigours of life.

 

Feeder roots will be thicker with a much larger central cylinder to allow for the transport of nutrients. The downside of this added thickness and wider cylinder is that these roots are much more fragile and so you will tend to find them growing under the substrate  in areas where they are protected. This would make sense, as feeder roots of Aroids are reported to be negatively heliotropic (grow or turn away from the sun) and positively hydrotropic (grow towards moisture) Goebel & Sandt (1930).

 

 

In the family Araceae, roots can be found growing along surfaces, under the ground and can be found hanging free in the atmosphere. When we are speaking of Aroids, any root that is in contact with a substrate; be that soil, a host tree or rock are termed terrestrial. Once a root starts to descend into the atmosphere they are termed aerial. A good example of this are the brown roots that you observe heading off in all directions from a Monstera deliciosa. Once a root becomes aerial there are significant changes in its morphology and physiology specifically a change in the outer layers of the root structure to allow for greater water retention and an ability to photosynthesise. Further detail can be found in a paper published in the Annals of Botany, lead author Arinawa Liz Filartiga.

Let’s begin by looking at the basic structure of a Terrestrial root and how the different parts function.  Each root produced by your plant has what’s called a root cap on the very tip. This is a hard layer of cells that are designed to push through the soil and around debris protecting the softer cells that form the rest of the root. Think of it like a crash helmet; it can absorb the odd bash and scrape while keeping the soft tissue inside nice and cosy.  Another cool thing about the root cap is it has a type of cell called a statocyte which contain a substance called statoliths which can detect the pull of gravity. This substance will settle in the direction of gravity which ensures the root is always growing in a downward direction when it is not blocked by a solid object. This is called gravitropism and its why if a plant is knocked over, the roots will always grow downwards regardless of the new orientation of the plant. I think its really cool!

The second section of the root sits just within and behind the cap and is called the ‘region of division’ and is comprised of specialised tissue called apical meristem. The root cap and apical meristem are the only part of the root that is actively pushing through the ground.  Cells are thought to divide every 12-36 hours in this area; each new cell is a bit like a building block waiting to be allocated a job.  Apical meristem is one of three types of meristem (tissue) which can differentiate into different cell types. (Differentiation is the process in which a cell changes from one cell type to another. Usually, the cell changes to a more specialized type). This process of division (growth) and differentiation only occurs in meristem tissue.  The term “Apical” is a description of growth occurring at the tips of the plant, both top and bottom. For example, this includes root tips and buds on new leaves. Another example of where Meristematic cells are located that we are all familiar with is the axilary buds.

Next is the ‘region of elongation’. This section is about 10mm in length and is just behind the region of division. This area of the root is responsible for the elongation of the cells that are produced in the meristem.  This section of the root  is responsible for growth, where cells will become several times longer and often thicker, and at the same time the tiny vacuoles within the cell merge to become one large vacuole. In case you’re wondering what a vacuole is..  Vacuoles are storage bubbles found in cells. Vacuoles might store food or any variety of nutrients a cell might need to survive. They can even store waste products so the rest of the cell is protected from contamination. Eventually, those waste products would be sent out of the cell. 

The final section is the ‘region of maturation’. Remember those cells produced in the meristem that were all just building blocks waiting to be allocated a job? Well here is where that happens: cells mature or differentiate into the various distinctive cell types of the primary tissue found in the root. Here the growth is restricted to a thickening of the root and you will also find the very fine hairs or protuberances appear.  These hairs (which are my most favourite part, all fluffy and soft looking 😊) are there to absorb water and minerals and anchor the root to the soil. Interestingly, the hairs are not made up of cells but are in fact tubes that sprout from the cells on the surface of the root (epidermal cells). In effect, they are like straws sucking in all the goodness your plants need to survive. Here is a crazy fact brought to you from Stern’s Introductory Plant Biology:  typically you will find more than 38,000 of there per square cm! A single rye grass plant occupying less than 0.6 cubic metres of soil was found to have more than 14 billion root hairs with a surface area of a football field. Don’t ask me how they managed to count them, it’s not a job I would have liked! But it’s an amazing fact never the less.

Unsurprisingly these hairs are extremely delicate and will break with the slightest movement. This is why you should only repot your plant occasionally and when you do you should pop it in the shade for a few days allowing them to regrow. Even exposure to light can cause them to shrivel and die which is why you tend to see them only growing on the side of the root that is not exposed to light in the case of adventurous roots and more specifically anchor roots. Many of the Aroid plants we love to keep have adapted these roots so that they are able to grow in the air due to the very high humidity and low light levels. The primary function of these roots is the exchange of moisture and oxygen.

I hope that you enjoyed this little meander down the wonderful world of roots, I have tried to the best of my ability to be as accurate as possible but if I have made any errors please feel free to comment about them below and I will be happy to make any amendments, I am by no means a botanist only an enthusiastic amateur. I have used a variety of resources in this post, I have links to anything I have quoted directly from.