Photo credit: Garden Drum
How leaf and stem cuticles work
Sometimes life is so busy that it is hard to see, well, ‘the wood for the trees’. A good case in point is that microscopic waxy outer layer of the herbaceous parts of plants called the cuticle – so easily missed and, for so long, suffering all the assumptions that have been made about it.
What is the cuticle?
The cuticle is an outer layer of waxy type compounds (cuticular waxes to be precise) plus a product called cutin. Both of these are hydrophobic – repelling water – which is essential for them to fulfil their role in plants. Plants want to keep as much water inside them as possible; with no water they die, just like us.
The cuticle lies outside the epidermis (the outer cellular layer of all herbaceous parts of plants). It used to be thought of as a relatively inert layer but as happens so often with science, this was a false assumption. It is dynamic and very often interacts closely with the epidermal layer itself, almost as one unit.
Its appearance varies greatly, as we all know, from the glossy leaf on ivy to the matt finish of broccoli. Not all cuticles are equal and there is as much variation in their makeup as there is in leaf shapes.
Why was a cuticle essential for plants to survive on land?
First, we must know that like us, the ancestors of our plants had to move from their protective environment in oceans and waterways on to the harsh environment that existed on the land.
This would be like going from a cosseted warm house to living in the open in Antarctica, a big leap into the unknown. Suddenly, plants could lose water, there was harsh UV radiation, temperature extremes, and they had weight – they had become ‘obese’ – well not really but you get the picture, gravity was real.
It was at this time that the cell walls all plant cells have, began their development to what we see today. The organisms we’re talking about were the green algae (Charophycean to be precise) and the cell walls they developed not only gave them protection, but structural support as well.
The biggest issue mind you, was still how to deal with water loss from the plant parts suddenly now exposed to the air; a completely new phenomenon for these new land dwellers. Some say this was one of the most important developments in plants and without it, of course, movement on to the land would have been impossible.