What is sap?
Sap is the name given to the fluid which circulates in all parts of plants and which can, in some way, be compared to our blood, because like it, sap is essential to life. Like the blood in our body, the sap circulates in vessels which wind through the trunk, the branches, the twigs, the leaves, etc. Nothing escapes it! Sap is essentially composed of water, but not only that. Depending on its composition, there are two types of sap: raw sap and processed sap.
- Raw sap : It consists mainly of water and vital nutrients for the plant, such as potassium, nitrogen, phosphorus and magnesium, which are drawn from the soil by the roots, then redistributed by the process of circulation of the sap, which we will talk about later, in all plants.
- The sap produced : It is rich in sugars resulting from photosynthesis and other organic compounds. Because it depends on photosynthesis, a chemical reaction which results in the transformation of water and minerals into sugars under the effect of light, the sap produced is produced in the leaves, then transported, too, through the different parts of the plant to allow it to grow and store energy.
The complexity of the vascular system
As we have seen, sap, whether raw or processed, circulates in the tree. To ensure this circulation, plants have a fairly complex vascular system which includes two types of specialized tissues: xylems and phloems.
The xylems
Responsible for transporting raw sap, the xylems are small channels located just under the bark of the tree. Raw sap flows through these channels from the roots to the top of the tree and plays an important role in the plant’s photosynthesis and transpiration. It helps maintain the internal pressure which promotes the development of the plant and contributes to the cooling of the tree.
Phloems
Just as essential, the phloems are the vessels which allow the transport of the produced sap, that is to say the sap produced in the leaves from photosynthesis. They therefore ensure the transport of this sap from the leaves to absolutely all parts of the tree, from the buds to the roots, via the branches, the trunk, the fruits, etc. In this network of vessels, the circulation is done in all directions, which ensures the distribution of energy resources everywhere, but also the storage of nutrients.
The roles of sap
We have said it many times, sap, both raw sap and processed sap, is vital for plants. But if it is, it is because it fulfills three essential functions:
- Raw sap ensures plant survival providing them with the water and minerals they need to survive.
- The sap produced allows the plant to develop, because it is she who feeds him. It provides it with the sugars it needs to grow, create new branches and new leaves, defend itself against attacks, etc.
- The union of raw sap and elaborated sap guarantees fruiting or flowering in order to perpetuate the species and therefore has an important role in reproduction.
How does the sap rise?
As raw sap circulates, it rises up the tree and you may wonder how this liquid rises through the plants. The answer to this question lies in three mechanisms that will come into play to contribute to the movement of the sap. Here are these three mechanisms briefly:
- Osmotic pressure : The thrust exerted by this pressure causes the circulation of water from environments which are more concentrated in minerals, such as the soil and the roots, towards the less concentrated areas which are the leaves.
- The phenomenon of capillarity: It also comes into play and allows the spontaneous diffusion of sap into porous tissues and canals.
- Evaporation: Caused by heat, it causes the transformation of part of the water present in the sap into steam. This results in a difference in pressure in the xylems which will act like pumps and push the sap upwards.
The process of sap circulation
We have seen the different physiological mechanisms that allow the rise of sap, but let us now discover the process of sap circulation. Two types of sap, two different processes:
- Raw sap : It starts from the roots and is transported to the leaves through transpiration. By evaporating, the water present in the leaves creates a water deficit. This negative tension allows raw sap to be sucked through the xylems. In addition to this, the root pressure generated by the active suction of minerals from the soil helps increase internal pressure and push water and nutrients upward.
- The sap produced : Its distribution is by an osmotic pressure gradient in the phloems. We mentioned the presence in the phloem of sugars produced by photosynthesis in the leaves, but it is these sugars which will create a concentration which will attract water from the xylems. This is how the sap produced will be carried into all parts of the plant. In addition to transporting essential nutrients, it plays an important role in regulating the internal hydration of the plant, allowing it to enjoy balanced growth, but also to adapt to environmental conditions.
Factors that affect sap circulation
Different environmental factors have an impact on the circulation of sap:
- The light : It is a crucial element since it plays an important role in photosynthesis. However, without adequate photosynthesis, a plant cannot develop normally, since it is this which allows the production of the elaborate sap which has the mission of nourishing the plant concerned. This is why it is necessary to provide adequate light to the plant to allow it to benefit from optimal photosynthesis.
- Temperature : It also has a significant impact on the circulation of sap. Indeed, when temperatures are high, transpiration of the leaves is greater, which promotes an acceleration of the rise of raw sap. Conversely, low temperatures will slow it down, or even stop it.
- humidity : It plays an important role in that when humidity is high, transpiration is reduced and raw sap is slowed down, while low humidity will increase it and accelerate its circulation.
This explains why, in winter, when light is low, temperatures are low and humidity is high, certain plants go into rest.