The Vascular System of Cannabis Plants
Plants don’t have a heart, yet they move water from roots to leaves using evaporation and microscopic forces strong enough to lift moisture several meters upward.
Learning basic terminology and functions of cannabis plant structures will help growers understand how to properly maintain their crop. Most of the water the plant takes up is evaporated from the underside of the leaves through the stomata. This internal movement of water and nutrients is made possible by the plant’s vascular system, which functions as the transport network connecting roots, stems, and leaves.
Understanding the Xylem and Phloem Systems
Vascular plants include flowering plants, such as Cannabis. These are distinguished in part by their use of specialized cells known as xylem to raise water and nutrients from the roots up to the leaves, and phloem, which transports sugars and other organic compounds produced during photosynthesis from the sites where they are made to the areas where they are stored or used. The xylem and phloem systems run throughout the plant, branching so that every cell is within reach.
How Transpiration Moves Water
The xylem allow plants to distribute water internally via transpiration. There are small openings in the outer skin of the plants called stomata; most of these are concentrated on the undersides of leaves. These openings allow for carbon dioxide (CO₂) to enter for use in photosynthesis and enable oxygen (O₂) and water vapor (H₂O) to exit.
At the top of the xylem system are the guard cells. They surround the stomata and either swell to open or deflate to close. They open when light and moisture are conducive to growth and close during drought, high heat, or darkness. This is why supplemental CO₂ is generally only used during the “lights-on” hours in an indoor garden.
Cohesion and Adhesion: The Water Chain
Water moves through the plant much like a chain. Because of adhesion (sticking to the sides of the vessels) and cohesion (water sticking to itself), water in narrow tubes forms a continuous column. As water evaporates through the stomata, it pulls the rest of the chain up the stem through vessels made from dead xylem cells.
The sugars produced in leaves don’t stay there—plants actively redistribute energy through the phloem, sending nutrients to roots, growing shoots, and even developing flowers depending on their needs.
Root Function and Osmotic Pressure
Before water reaches the Casparian strip—a waxy bottleneck within the root that controls the uptake by the plant’s vascular system—it enters the roots via osmosis. Since the water in the soil typically has a lower mineral concentration than the root cells, water passes through the semi-permeable cell membranes. This osmotic action creates a small amount of ‘root pressure’ to assist in moving the water up to the rest of the plant, although this force is smaller than the capillary action and other forces drawing water up through the plant.
Root hairs extend their surface area to absorb nutrients. Many growers utilize beneficial mycorrhizal fungi to extend this reach even further. These fungi trade minerals (especially phosphorus) for carbohydrates produced by the plant, creating a symbiotic relationship that exploits small cracks in soil particles.
- Roots end in a root cap that protects the rapidly reproducing meristem cells pushing it deeper in the growth medium.
- Root hairs grow along the roots in order to extend their range and increase surface area. They absorb nutrients by direct contact.
- The reach of root hairs can be strengthened through the use of beneficial fungi, which allow them to enter and exploit smaller cracks in soil particles.
- Transpiration draws the water from the soil up from the root hairs to the roots, through the plant via the xylem and out through the stomata openings.
Translocation: The Role of the Phloem
In contrast to the xylem, the phloem consists of living cells that transport sugars, hormones and other organic compounds to the roots for storage, and this process is known as translocation. This sugar solution, or “sap,” moves through sieve cells via osmosis. Unlike the one-way street of the xylem, phloem can move sap in either direction depending on the plant’s needs—sending energy to the roots for storage or upward to flowering sites for bud development.
Sugars are added to the sap where they are made or stored and removed at the location where they are needed (the ‘sink’).
Being acquainted with the basics of a Cannabis plant’s vascular system can help growers to understand how a plant takes up nutrients and water and uses sap to transport sugars and organic molecules.
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