The Cannabis Vascular System

Vascular plants include flowering plants such as cannabis, and 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 to transport sugars and organic compounds produced in photosynthesis around the plant from sources where they are made, to where they are stored or used.

Vascular plants include flowering plants such as cannabis, and 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 to transport sugars and organic compounds produced in photosynthesis around the plant from sources where they are made, to where they are stored or used.

The xylem allows plants to take up and distribute water internally via a process known as transpiration. There are small openings in the outer skin of the plants called stomata. Most of the stomata are concentrated on the undersides of leaves. These openings allow for carbon dioxide (CO2) in for use in photosynthesis, and allows the oxygen (O2) and water vapor (H2O) out. Most of the water the plant draws in from the roots will ultimately be evaporated through these stomata openings.

At the top of the xylem system are the guard cells. They surround the stomata openings and swell to open, or deflate to close the openings. The guard cells allow the stomata to open when light and moisture are conducive to growth, and to close in times of drought, high heat, and darkness. This is why supplemental CO2is generally only used during the lit hours of an indoor garden (unless a continuous release system is used where stopping and starting the CO2 flow is problematic). Since the stomata has to be open for CO2 uptake, it makes sense to restrict CO2 enrichment to times when the stomata are open. 

Xylem cells run throughout the leaves and stems, supplying water and minerals to cells in need as well as to the stomata. In flowering plants these xylem cells are known as vessels. Water enters the xylem pathway from the root system by passing through cell membranes at the Casparian strip. The Casparian strip is a waxy bottleneck that forces the water to pass through a cell membrane before entering the xylem pathways.

Before it reaches the Casparian strip, water first passes from the surrounding soil into the roots via osmosis. Since the water in the soil has a lower mineral concentration than the water in the roots cells, water will pass through the semi permeable cell membranes. This osmosis 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 and other forces drawing water up the plant.

Root hairs absorb nutrients by direct contact. Roots end in a root cap that protects the rapidly reproducing meristem cells that push it deeper into the growing media. Root hairs grow along the roots to extend their range, and increase surface area.

Root hairs can extend their reach by use of beneficial fungi. Mycorrhizal fungi (if present) trade minerals (especially phosphorus) to the root hairs in exchange for carbohydrates produced by the plants. This benefits the plant since the fungi both extends the reach of the root hairs, and is able to enter and exploit smaller cracks in soil particles.

In a nutshell, transpiration draws the water from the soil up from the root hairs, to the roots, through the plant through the xylem, and out through the stomata openings.

In contrast to the xylem; the phloem is a series of cells that run through the plant that transports the sugars made in photosynthesis in a process known as translocation. The sugar solution is known as sap which moves through small holes in the ends of sieve cells by osmosis. Unlike xylem, which are dead cells that use passive physics to move water in one direction only, the phloem are living cells that move the sap in either direction depending on need. They can transport sugars, hormones, and other organic compounds to the roots for storage, or from the roots to flowering sites for seed development. Sugars are added to the sap where they are made (or stored) and removed at the location where they are needed (the sink).

Gentle Readers, being at least acquainted with the basics of a cannabis plant’s vascular system can help growers understand how a plant takes up nutrients and water, and uses sap to transport sugars and organic molecules.