master the garden

part three botany basics: roots
Bloedel Reserve photo l. fowler

In this third installment of botany basics – roots are discussed. Perhaps the most important system of any plant – is the root system. They absorb nutrients and moisture, anchor the plant or tree to the soil, support the stem or trunk, and store food. In some plants the roots are also used for propagation – and when they fail…the result can be catastrophic and result in the plant death and in the case of large trees pose a hazard to life and property.

“It is in the roots, not the branches, that a tree’s greatest strength lies.”

Matshona Dhliwayo
Large Pinus Newhalem February 2019 photo l. fowler
root structure

meristematic growth

Meristems are clusters of cells at the tips of roots and stems that actively divide, producing new cells as a basis for continued growth. In most vascular plants the root and stem elongation is solely the result of the formation of these cells and takes place in the apical meristem. Growth from the apical meristem is called primary growth – in roots, it produces both the root cap and the cells responsible for the initial root diameter and root elongation. In stems, the apical meristem accounts for growth in stem length and the formation of buds (primordia).

Additional meristematic growth including: lateral meristems (secondary growth), marginal meristems (leaf development), intercalary meristems (located above nodes), and basal meristems (found at the base of leaves) will be covered in following installment of Master the Garden.

zone of elongation

Most growth in roots and stems is restricted to a localized zone of cell elongation located immediately behind the apical meristem.

root cap

root hairs (these can be torn off or dry out when a plant is transplanted

zone of maturation

Beginning at the first root hair is the zone of cell maturation where the root cells begin to differentiate into special cell types.

root tissues

In roots, the basic organization is in layers of cylinders. With roots exhibiting only primary growth, the outermost cylinder consists of the dermal layer which is made up of epidermal cells.


The outermost layer of the cylinder and these cells form a boundary between the plant and the external environment. The epidermis serves several functions: it protects against water loss, regulates gas exchange, secretes metabolic compounds, and (especially in roots) absorbs water and mineral nutrients. Epidermal cells include several types of cells that make up the epidermis of plants. Although they serve a number of important functions, their primary role is to protect from a variety of biotic and abiotic stressors and as a protective mechanism these cells are situated close together to prevent water loss.


They next layer of the cylinder is a layer of ground tissue called the cortex and it is made up of parenchyma cells. The next layer is the endodermis and it is the inner most layer of the cortex. The casparian strip, a layer of a waxy substance called suberin that has been embedded into the endodermal cell wall and prevents the passage of water and other molecules through the endodermis. The right inside of the endodermis is the pericycle, a layer of meristematic cells that functions in the formation of lateral roots, the vascular cambium, and the cork cambium.

AP Biology

The cortex cells are responsible for moving the water from the epidermis to the vascular tissue or xylem and phloem, and to store food. There are also root hairs which are unicellular extensions in the epidermis that function in the uptake of water and other nutrients.

Many roots have a naturally occurring, mutual relationship (symbiotic) with certain fungi. Mycorrhizae, a special class of fungi, grows along with the roots and helps with water absorption.

Bozman Science – Plant nutrition and transport
types of roots

There are two major types of roots:

  • primary root (tap root)
  • lateral root (secondary root)

If the primary root ceases to elongate then lateral roots develop and become a fibrous root system. Plants will naturally form a fibrous root system (grasses) and often if the tap root is severed* it can encourage the plant to form a fibrous system of roots.

*This technique is used by nurseries on trees with tap roots to form a fibrous compact root system that enables successful transplanting.

into the weeds…

I think of trees as the ‘top of the food chain’ in the Plant Kingdom. Tree structure and growth systems represents those of most plants and is often more visible. I use tree planting guidelines and principles when installing all my landscapes.

When planting trees, there inevitably are questions about roots and root growth. Problems that can and will occur with tree roots are usually the result of poor tree selection and placement – but to understand the above you have to understand a little tree biology.

First and foremost trees, like all plants, are living organisms and must grow and become larger to survive. Branches, trucks, and root systems become more complex as they expand. The following excerpts from my favorite book – Up by the Roots by James Urban – covering the six of the seven principles and most important areas of tree biology and root systems.

1. Trees Grow

“…any design that ignores this principle, trees systems must grow, will fail when the tree bumps into a restraint. Nature regulates the ultimate size the tree (or plant) can attain, but urban trees die long before they reach their natural limitations.”

2. Leaf-to-Root Balance

“Trees, like all plants, must maintain a balance between the surface area of the tree that can photosynthesize (mostly leaf area) and the surface area of the tree that can take up water and chemicals (its absorbing roots). When one part of the system gets out of balance, hormonal regulators within the tree adjust the other.”

“If the roots are cut, damaged, or have no room to grow, branches die, twigs do not grow as long, and the tee produces smaller/or fewer leaves.”

3. Horizontal Roots

“To survive, tree roots need oxygen and water, both found in the upper layers of the soil….Tree roots grow much faster then branches and have been measured at up to 10 feet or more per year. The need for oxygen and the rapid growth of roots mean that a tree’s root system is often horizontal in structure and will cover far more territory then its crown. The extended roots will intertwine with the roots of adjacent trees.”

4. Aggressive, Opportunistic Roots

“Roots tend to opportunistic and aggressive in finding and exploiting resources. They create complex, and dense mats in favorable conditions and long, less dense systems in poorer conditions.Roots also tend to grow in the most favorable environment available.”

“Tree roots can exert great force on objects. If a root grows under or beside and object and later finds an area of good growing conditions, the expansion of that root can move or break objects of great weight and strength. These root qualities cause damage to paving, curbs, and walls if not understood.”

5. Fragile Roots and Strong Roots

“When preserving existing trees or preparing sites for new trees, it is important to consider all the factors that can damage tree roots. While roots can be aggressive and opportunistic, the smallest roots, those that start the growth process and take up water and nutrients, are quite fragile and short-lived. They are easily killed by soil compaction, cutting, too much water, too little water, extrees of heat and cold, and many chemicals are toxic to plants, including chemicals produced by other plants*.”

*Allelopathy is the production of plant inhibiting chemicals by one plant to regulate the growth of others in its vicinity.

6. Dynamic Structure

“A tree’s structure is conceptually a vertical cantilever, which resists the dynamic forces of wind and gravity in the crown. The root plate anchors the trunk, which is cantilevered from its horizontal position.”

root growth

The quality and distribution of a plants roots influences the plants ability to absorb nutrients and moisture from the soil. Most plants feeder roots (absorbing) are located in the first 12 inches of soil – and the soil environment – balance of fertility, moisture, and air spaces – in this area has a greatest influence on the vitality and vigor of a plant.

Other factors are also important to root growth:

  • plants in water saturated soil will not grow well and eventually will die due to lack of oxygen
  • compacted, dense soil will restrict or stop root growth
  • container grown plants are restricted by the amount of soil available for growth and can also be susceptible to damage from cold due to lack of insulation from soil volume
  • roots grow downward and laterally and root systems often go beyond the drip line.

glossary of terms

Anther – The pollen sac on a male flower

Apex – The tip of a root or shoot

Apical dominance – The tendency of an apical bud to produce hormones that suppress growth of buds below it on the stem

Apical meristem – the growth region in plants found within the root tips and the tips of the new shoots and leaves. Apical meristem is one of three types of meristem, or tissue which can differentiate into different cell types. Meristem is the tissue in which growth occurs in plants.

Axil – The location where leaf joins the stem

Bolting – plants produce a flowering stem in a natural attempt to produce seeds as a means of survival when under stress.

Cambium – A layer of growing tissue that separates the xylem and phloem and continuously produces new xylem and phloem cells

Chlorophyll – The green pigment in leaves that is responsible for trapping light energy from the sun

Chloroplast – A specialized component of certain cells; contains chlorophyll and is responsible for photosynthesis

Cold hardy – generally measured by the lowest temperature a plant can withstand

Cortex – Cells that make up the primary tissue of the root and stem

Cotyledon – The first leaf that appears on a seedling. also called a seed leaf.

Cuticle – A relatively impermeable surface layer on the epidermis of leaves and fruit

Dicot – having two seed leaves

Herbaceous – vascular plants that have no persistent woody stems above ground

Epidermis – The outermost layer of plant cells

Fibrous roots – a network of feeding lateral roots found on most plants

Guard cell – Epidermal cells that open and close to let water, oxygen and carbon dioxide pass through the stomata

Internode – the space between nodes on a stem

Lateral root – roots that branch from larger primary roots

Marginal meristems – the meristem located along the margin of a leaf primordium and forming the leaf blade. The apical meristem or growing tip, is a completely undifferentiated meristematic tissue found in the buds and growing tips of roots in plants.

Meristem – Specialized groups of cells that are a plant’s growing points.

Meristematic zone – located at the tip of a root and manufactures cells: it is an area of cell division and growth

Mesophyll – A leafs inner tissue, located between the upper and lower epidermis; contains chloroplasts and other specialized cellular parts (organelles)

Monocot – having one seed leaf

Mycorrhizae – symbiotic association between certain fungi and roots of a plant

Node – an area on a stem where a leaf, stem, or flower bud is located

Ovary – The part of a female flower where the eggs are located

Petiole – The stalk that attached a leaf to the stem

Phloem – Photosynthate-conducting tissue

Pistil – The female flower part; consists of a stigma, style, and ovary

Primary root – originating at the lower end of a seedlings embryo and continues to elongate downward. It may or may not persist into plant maturity, and has limited branching – it is called a tap root

Respiration – the process of converting sugars and starches to energy

Root cap – group of cells protecting the apical meristem at the root tip

Root hairs – delicate, elongated epidermal cells that occur in a zone behind the root’s growing tip with the function of increasing the roots surface area and absorptive capacity

Root plate – That part of the root system (excluding the small outermost roots) needed to keep a tree wind-firm.

Stamen – The male flower part; consists of an anther and a supporting filament

Stigma – The top f a female flower part; collects pollen

Stoma (pl. stomates, stomata) – tiny openings in the epidermis that allow water, oxygen, and carbon dioxide to pass into and out of a plant

Style – The part of a female flower that connects the stigma to the ovary. Pollen travels down the style to reach the ovary, where fertilization occurs

Tap root – see Primary root

Transpiration – the process of losing water (in the form of vapor) through stomata.

Turgor – Cellular water pressure; responsible for keeping cells firm

Vascular tissue – Water, nutrient, and photsynthate-conducting tissue (xylem and phloem)

Vegetative structures – The vegetative (somatic) structures of vascular plants include two major organ systems: (1) a shoot system, composed of stems and leaves, and (2) a root system

Xylem – Water and nutrient-conducting tissue

Zone of elongation – located behind the meristem. Cells in this area increase in size through food and water absorption. As they grow, they push the root through the soil

Zone of maturation – located directly beneath the stem. Cells in this zone become specific tissues such as epidermis, cortex, or vascular

suggested reading

Botany for Gardeners, Capon, B – 2bd edition (Timber Press, Portland, OR 2004)

Plant physiology, Salisbury, F.B. and Ross, C.W., 4th edition (Wadsworth Publishing Company, Belmont, CA, 1991)


  1. Oregon and Washington Master Gardener Handbook, 2001

self test

  1. Describe the different types of supporting roots found in trees. List the major parts of a fibrous root.

Roots that grow along the soil line are known as lateral roots. Roots that grow outwards and then straight down are called sinker roots. Some trees begin life with a tap root that grows directly downwards from the seed, but most lose this feature by the time they are mature. Oblique roots will grow at a 45° angle downwards to assist in anchoring of the tree. Fibrous roots consist of a root cap which covers the expanding meristematic tissues beneath. Behind the meristem is the region of elongation, where growth occurs; followed by a region of differentiation, where root hairs diverge out at 90° from the parent root.

2. What is the major function of root hairs?

To absorb water and minerals from the soil

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