Roots:Underground and Overlooked

Unbranched roots of a leek (monocot)

All roots are not created equal. A general definition of a root is a part of the plant body that bears no leaves and therefore no nodes. There are two basic types of root systems, tap root systems and fibrous root systems.  Tap root systems develop a primary root that gives off lateral roots. The main tap roots anchor the plants to the ground and have the ability to reach water that is locked deep in the soil giving them an advantage in drought conditions. Tap root systems can occur only in dicots. Fibrous root systems are made of clusters of thin fiber like roots that are shallow and spread horizontally. They are unable to provide the stability that tap roots provide. Fibrous root systems occur in monocots but also common in dicots.

The roots of an onion are an example of a fibrous root system.  Solanum mauritianum is another plant that has a very conspicuous and well defined fibrous root system.  Ambrosia acanthocarpa is a native southern California plant that has a well developed tap root system.  Because of it's deep and well developed tap root, it is often still green when everything else is dried up in late summer. While there gradations many plants will fall into one of these two categories possessing either a tap root or fibrous root system.

The roots of angiosperms(flowering plants) can be differentiated with little effort between monocots and dicots. Monocot roots do not fork but continue out in a more or less straight direction until they terminate growth. The diameter of the root that is first established by the root cap is the permanent diameter in monocots. If they expand in diameter it is very little, if at all. A classic example of monocot roots can be seen in the onion where all the roots are the same diameter. As compared to monocots, the roots of dicots can grow continuously in diameter and length, taking on the classic look of tree roots. In dicots, root branching can occur anywhere and form new roots.

Dicots have root systems that can range from having very fibrous roots, as seen in marigolds, to having very deep taproots, as seen in pecan and mesquite trees and alfalfa. The word phreatophyte is often used to describe plants which have very deep taproots that are able to reach deep into the ground to absorb water in the phreatic zone or zone of saturation. This term often refers to plants known as"pump plants" In Greek, Phreat means "a well". I do not know if the term phreatophyte has ever been applied to Pecans trees by the scientific community. However, from my own observation, they can tolerate considerable drought. Dicots also produce lateral roots which grow in a smaller diameter than the primary roots. These are called secondary roots. Even smaller roots may grow from these secondary roots and these are called tertiary roots. From the tertiary roots and sometimes secondary roots, roots hairs may grow which are only the width of a single cell and sometimes only as long as one or a few cells.

 Root hairs are derived from the tertiary or secondary roots. Root hairs are very delicate and cannot be exposed to ordinary air without shriveling up. Root hairs are the primary structures for absorbing water. All the other sizes of roots have a cortex that is often quite impervious to water leaving the root hairs as the primary source of water uptake. Once they die they can no longer function and new root hairs must re-grow to pick up the water. This is why it is important to keep root hairs wet when transplanting. The best time to expose roots is in the morning when the humidity is high or when there is a light rain or heavy mist. Otherwise it is important to keep the root hairs misted until they are safely tucked away into the ground.

The roots of gymnosperms(conifers, such as pines) undergo similar developmental processes as the dicots. I have made an interesting observation regarding the roots of pines which may or may not occur in other gymnosperms. Where they are overlaid with concrete or similar hard material they frequently form a circular pad-like structure that presses up against the hard material. These structures are one half to three inches in diameter and their function is unclear to me at this time.

 Among the smallest of the root-like structures we see come from ferns. Ferns form attachments to the ground that are commonly referred to as roots but technically these structures do not quite fulfill the definition of a root as seen in angiosperms. These structures are commonly called rhizoids. Rhizoids are slender root-like filaments that perform similiar functions as roots in ferns, mosses and other gametophytes. However, with regard to ferns, the word root is used loosely because there is some question as to whether ferns form true roots as they are defined by higher plants. Some authors have chosen to use the term rhizoids (being root-like) instead of roots.

Over the years I have made many interesting observations about roots. For instance, on many plants , especially young plants, the early growth will supply a swollen area below ground called a lignotuber (i.e. Mirabilis jalapa, the four o'clock ). A lignotuber is a starchy swelling of the root crown. It offers protection to the stem from being burned or eaten by a predator. The lignotuber will sprout again and again ensuring plant survival. Other examples of lignotuber include several species of Eucalyptus such as Eucalyptus citriodora also known as lemon scented gum. Many, many plants produce lignotubers.

Starch performs an important function in plants.   Sugar, being soluble, is moved down into an area to be stored and converted to starch. There it remains throughout the winter. For example, in a potato, the mature tuber is starch that has been converted from sugar by the removal of one molecule of water from one molecule of sugar. This starch allows the tuber to store valuable energy for next years growth, whereas if  energy was stored as sugar the least injury to the plant would allow the sugar to be lost due to it's high solubility. In short the storage of starch allows a plant to retain energy in the form of insoluble starch. 

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Enlarged storage area on the rots of Alstoemeria sp.

There are also enlargements that form down in the ground on the roots that act as storage for water and nutrients with no ability to form sprouts. It is important to note that these enlargements are strictly part of the roots and not lignotubers as previously discussed. Two examples of this include Asparagus sprengeri and Alstroemeria both in the Lily order. In some plants or trees, large buttresses or large burls which are portions of the tree that has grown in a deformed manner, serve a function not unlike a lignotuber. The buttress will sprout buds from the base of the tree if the tree is killed. It is common for people to say they are going to prune a tree to force new buds to grow. However, the concept of forcing a new bud to grow erroneous. The top growth is constantly sending down signals to suppress the buds below the ground from growing. If the dominant growing part of a the tree dies or is destroyed by fire, for example, then the buds below ground are allowed to sprout since they are no longer being suppressed by the growing plant.

Strangler Fig showing anastomosis

Another root form we see are aerial roots. They are found mostly in trees in tropical regions. They arise spontaneously  from the trunk or from the branches near the crown. They grow downward until they reach the soil and continue growing into the soil. It is reasonable to think that these new roots could be replacing, in time, older and often diseased roots. However, this is only a conjecture. A well known example of this root structure is the strangler fig tree which sends roots so numerous down the trunk of the tree that they will anastomose, or fuse together when they come into contact with one another as result of normal growth. The process of anastomosis forms a cylindrical tube around the tree. In time the supporting tree will die and decompose and the aerial roots are so well developed in this cylindrical fused structure that they will hold up the tree for some time. The decaying portion of the tree will provide nutrients for the remaining fig.

There is no end to noteworthy information that can be written about the roots of plants. I have mentioned some of the many interesting facts about roots but there are many more to be dealt with. I am continuously interested in and compiling new information on the morphology and behavior of roots and I am always in touch with other careful observers.  From time to time, I will share new observations on this blog.