The Vascular System of Flowering Plants
Being acquainted with the basics of a flowering plant’s vascular system can help gardeners understand how a plant takes up nutrients and water, and uses sap to transport sugars and organic molecules.
Vascular plants include flowering plants, and are distinguished in part by their use of specialized cells known as xylem that bring water and nutrients from the roots to the leaves.
Vascular plants also use a system of specialized cells known as the phloem that transport sugars (mostly sucrose) and organic compounds produced in photosynthesis around the plant from sources where they are made, to sinks where they are stored or used. The xylem and phloem systems run in parallel throughout the entire plant, branching as needed so that every cell in the plant is within a few cells reach.
The xylem allows plants to take up and distribute water internally via a process known as transpiration. There are small openings in the epidermal (outer skin) layer of plants called stomata. Most of the stomata are found on the undersides of leaves, although they are also spread throughout much of the rest of the epidermal layer as well. These openings allow for carbon dioxide to be absorbed for use in photosynthesis, and allow the plant to vent off oxygen and water vapor. Most of the water the plant absorbs though the roots will ultimately be evaporated through the stomata openings.
An important facet of the xylem system is the function of guard cells. Surrounding the stomata openings, the guard cells swell to open the passage, or deflate to close it. 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 CO2 is 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 have to be open for CO2 uptake, it makes sense to restrict CO2 enrichment to times when the stomata are open. In normal growth conditions, when a guard cell is exposed to light, it will eject positively charged hydrogen atoms, creating a negative charge in the cell. This negative charge is used to draw positively charged potassium ions into the cell.
As the concentration of potassium ions increases in the cell, osmosis will draw in water to try to equalize the dilution, consequently swelling the cell and opening the stomata. This reaction is part of why potassium is a needed nutrient for proper stomata function and healthy plant growth.
In a nutshell, transpiration draws water from the soil up from the root hairs to the roots, then through the plant through the xylem, and out through the stomata openings. The transpiration of the xylem system makes use of some of the physical properties of water. When discussing using water to transfer force (hydraulics), it is common to think of using water to push with, but water can also be used to pull.
A siphon can be used to raise water over obstacles, and a single drop of water between two plates of glass can bond them together with surprising strength. The evaporating water vapor that exits through the stomata allows the plant to raise water internally. This is done passively and makes use of a couple of the special attributes of water – adhesion and cohesion. Water has a tendency to stick to the sides of a container (adhesion), and it has a tendency to stick to itself (cohesion). Because of these two properties, water in narrow tubes form a continuous chain of water molecules.
Water evaporating through the stomata pulls the chain of water up the stem to replace the lost fluid through many long narrow hollow tubes made from dead cells known as the xylem, mentioned earlier. While the top of the chain is lost to evaporation, the rest moves up to fill the void.
Small apertures or pits provide a path for water to rise from one xylem cell to the next. These small apertures help keep the water clean and help localize vapor locks from air bubbles. The path is one way—water rises from the roots to the leaves. At night, when the stomata close, the chain of water is held in place until they open again and the cycle continues. Transpiration increases with higher temperatures, which is why plants require more frequent watering in summer heat. Keep in mind that if temperatures are too high, the stomata will close to conserve water, and transpiration may stop.
Xylem and the Casparian Strip
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 can take up minerals by direct contact, absorbing nitrogen as ammonium or nitrate, phosphorus as phosphate, potassium, calcium and the rest of the mineral nutrients. 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 plants since the fungi extend the reach of the root hairs and are able to enter and exploit smaller cracks in soil particles.
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.
Cells that make up the phloem are known as sieve elements. Unlike xylem— 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 needs. 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). Maple syrup, for example, is made from the result of sugars created by photosynthesis moving through the phloem to storage in the roots to weather the winter, which then in the spring rises through the xylem (where it can be tapped, cooked down and put on pancakes).
Gentle readers, being at least acquainted with the basics of a flowering plant’s vascular system can help you understand how a plant takes up nutrients and water and uses sap to transport sugars and organic molecules.