The evolution of a vascular system was needed for trees to be able to grow without physically touching water by transporting and storing it within the organism. In fact, it is said that a tree might only be limited in terms of how high it can grow by how far it is able to push the liquid up its veins.
Not all plants use these vein-like highways (bryophytes like moss do not), but almost every plant in your garden does have a vascular stem including ferns, cone-bearers and flowering plants. There are two types of veins that these plants use:
Xylem tissues are responsible for moving water and minerals from the roots to the rest of the plant.
-They transport water in a unilateral system; that is water only travels up from the roots to the leaves and does not descend again.
-When they mature, the cells will die inside the tubes which will become hollow connecting tunnels, hence xylem can be a non-living tissue.
-They occupy the centre of the vascular bundle.
Phloem tissues are responsible for moving food and nutrients around the plant.
-They transport water and nutrients in the form of sap in a bilateral system that moves up and down from leaves to the roots and back.
-They are made up of living tissue.
-They surround the xylem in the vascular bundle.
Dicot vascular bundles of xylem and phloem are arranged in a ring, whereas monocot bundles are sporadic. Diagram via Plants Grow Here.
The organisation of the vascular bundle can differ in plants, but there are some main similarities. Dicots have one vascular bundle (with phloem surrounding xylem) in each branch and stem, causing tree rings as they grow which may be seen when the branch is cut. Monocots have multiple vascular bundles growing up the stem, which is why palm trees don’t have tree rings.
Generally, monocot roots just have a single ring of xylem surrounded by phloem (like dicots), unlike their stems that have bundles haphazardly situated throughout.
Monocot and dicot root vascular systems are more similar than their stems. Diagram via Plants Grow Here.
All vascular plants grow out longer, but not all can grow thicker. Some plants in the dicot and gymnosperm groups grow new cells laterally in order to grow wider, which allow for greater support and a greater volume of nutrients and water to be moved for tall trees and shrubs.
This “secondary growth” is done with a “cambium”, which is a thin layer of stem cells (meristematic tissue*) that divide to create new cells that runs the whole length of the stem. The main cambium that plants with secondary growth have is the vascular cambium, which sits between the xylem and phloem and grows new cells inwardly and outwardly, eventually so that the immature vascular bundles meet and become rings.
Some secondary-growth-producing plants (not all) have an additional cork cambium, which is a meristematic ring of cells around the outside of the phloem and create epidermal tissue, like cork and bark. Some other plants have a unifacial cambium, which create only xylem cells on the inside of the phloem ring.
Dicots start out with many bundles separated in a ring formation, then as secondary growth occurs the secondary xylem and phloem meet and form complete rings. Diagram via Plants Grow Here.
Rarely, “anomalous secondary growth”, which differ from the usual ways of performing secondary growth can occur. Palms are one such example, which don’t have a cambium but can still thicken up to a limited extent to give support for their frond crowns.
Knowing about vascular systems is important to how we approach plants. Every time we cut a plant, we leave the veins open to pathogens that can take advantage and enter the plant’s internal systems.
We must avoid strangling or “girdling*” our plants with secondary growth by leaving rope or wire around the trunk, or allowing the roots to encircle the trunk, to ensure that our plants can use the highways they’re creating.