Plants with adventitious roots

How Deep Are the Roots of Garden Vegetables?

What’s the secret to growing a healthy, vigorous plant this season?

Hint: It doesn’t start with what you see above ground.

Root depth is a topic that isn’t often considered when we think about growing in containers, building raised beds, or planning an irrigation system for our garden. But knowing how deep the roots of your plants reach is one of the most important pieces of the puzzle, especially if you’re working with limited space.

We tend to visualize our plants growing up or out, but before we transplant that first seedling, we need to know how deep they’ll go beneath the surface as well.

Why Does Root Depth Matter?

Most plants will grow within the space you allow them. They’ll survive with a minimum of soil depth, but they’ll thrive if you give them as much room as possible for their roots to branch out and breathe.

In fact, if you’re a container gardener, this plant study found that simply doubling the pot size allowed plants to grow 43 percent larger. So forget those 5-gallon buckets for your indeterminate tomato vines — you want them in half-barrel planters (at least 15 gallons) or larger for a good harvest.

Just like the saying goes — Feed the soil, not the plants — you want to focus on building strong roots, not just healthy stems and leaves. Roots that grow deep down in the soil are better able to anchor plants in the ground, maximize their water uptake, and pull in more nutrients and trace minerals.

If you’re getting a garden bed ready for planting, knowing the root depth of the plants you want to grow can help you determine how extensively to prepare the soil.

For example, shallow-rooted plants like lettuce may do better in soil that’s high in clay and doesn’t drain well. This may seem to go against common gardening wisdom, but after dealing with heavy clay soil for nearly a decade in my old garden, I’ve realized there’s a time and place for it.

Since the roots of lettuce and other leafy greens stay close to the surface, they like the extra moisture, tolerate less frequent watering, and only need nutrients in the top 2 to 3 inches of soil, reducing the amount of soil prep you have to do every season — a simple top dressing will work for these types of plants.

On the other hand, heavy-feeding, deep-rooted tomatoes require rich, loamy, well-draining soil, so they benefit from aged compost and plenty of amendments dug down at least 12 inches where the bulk of their root mass is concentrated. (Learn how to grow bigger and better tomatoes with my tried-and-true tricks.)

If you install raised beds over concrete or gravel, you’ll want to know the root depth of the vegetables going in, since they will guide how high you need to build the sides of your beds.

Raised beds built over grass or dirt, however, typically don’t need to be more than the standard 8 to 12 inches in height because the roots can sink into the subsoil (assuming your beds are open on the bottom).

Root depth is also helpful for hydrozoning, especially if you live in a drought-prone area. Hydrozoning is the practice of grouping plants with similar water needs together in order to conserve moisture and irrigate more efficiently.

This means you’ll cluster all your shallow-rooted plants in the same bed so you don’t inadvertently overwater them, and group deep-rooted plants together in another bed (on their own irrigation line) so you don’t underwater them.

Hydrozoning also comes in handy if you like to interplant your crops by growing beneficial flowers and herbs among your vegetables, or planting quick-growing crops (like radishes) between rows of long-season crops (like broccoli).

It’s Not Just How Deep, but How Wide

While we usually think of roots as growing downward, it’s important to remember that roots grow laterally as well, and to account for that in our garden beds and containers.

For example, a cucumber plant sends down a single tap root 3 to 4 feet deep. The majority of its roots, however, extend outward about 2 feet and are concentrated just below the soil surface.

You’ll also want to factor in the final height of the mature plants, as insufficient soil volume in a container can make them too top-heavy. Always follow the spacing recommendations on your seed packets and plant tags to ensure you give your plants plenty of room to grow.

Soil Depth Requirements for Common Garden Vegetables, Fruits, and Herbs

Some vegetables, like spinach and spring radishes, have very shallow roots and don’t need more than 4 to 6 inches of soil to grow successfully. But the smaller the planter, the more often you’ll need to water. Just keep in mind there’s no need to plant these varieties in containers deeper than 12 inches, as you’ll only be wasting soil and space.

On the other hand, plants like lemongrass (which clump and multiply rapidly) and tomatoes (which grow adventitious roots along their stems) will grow as large as you let them, so giving their roots more room to roam will produce a more robust harvest.

Shallow Rooting
(12 to 18 Inches)
Medium Rooting
(18 to 24 Inches)
Deep Rooting
(24 to 36+ Inches)
Arugula Beans (pole) Artichokes
Basil Beans (snap) Asparagus
Blueberries Beets Beans (lima)
Bok choy Cantaloupes Burdock root (gobo)
Broccoli Carrots Cardoon
Brussels sprouts Chard Okra
Cabbage Cucumbers Parsnips
Cauliflower Daikon Pumpkins
Celeriac Eggplant Rhubarb
Celery Muskmelons Squash (winter)
Chives Peas (shelling) Sweet potatoes
Cilantro Peas (snap) Tomatoes
Collard greens Peas (snow) Watermelons
Corn Peppers (hot)
Endive Peppers (sweet)
Fennel Rosemary
Garlic Rutabagas
Ginger Sage
Jerusalem artichokes Squash (summer)
Kale Turnips
Kohlrabi
Leeks
Lemongrass
Lettuce
Mint
Mustard
Onions
Oregano
Parsley
Potatoes
Radishes (spring)
Radishes (summer)
Radishes (winter)
Scallions
Shallots
Spinach
Strawberries
Tarragon
Thyme
Turmeric

More Ways to Prepare for Planting This Season:

  • Checklist for Your Spring Garden Tune-Up
  • A Fall Garden Checklist for Maximizing the Season and Winterizing Your Yard
  • Soil Solarization in Raised Beds
  • Edible Ground Covers for Vegetable Gardens
  • How to Use Days to Maturity on a Seed Packet to Help You Be a Better Gardener
  • How Much to Plant in a Vegetable Garden to Feed a Family
  • Brewing Compost Tea for Better Plant Health
  • Common Gardening Mistakes and What I’ve Learned From Them

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In this article we will discuss about:- 1. Definition of Adventitious Root System 2. Typical Adventitious Roots 3. Modification.

Definition of Adventitious Root System:

Roots that grow from any part of plant other than the radicle or its branches are called adventitious roots (L. adventitious— extraordinary). They branch like the tap root. A mass of adventitious roots along with their branches constitute an adventitious root system. Adventitious root system may be underground or aerial. They generally develop from stem nodes, intermodals, leaves, etc.

Horizontal stem of creepers often develop adven­titious roots from the nodes (e.g., Grass, Wood Sor­rel). Branch cuttings and leaf cuttings (e.g., Rose, Sugarcane, Tapicca, Sansiviena) develop adventitious roots when placed in soil. In Coleus, the cuttings develop adventitious roots on being partially immersed in water. Hormones also induce development of adventitious roots.

Typical Adventitious Roots:

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Fibrous Roots:

They are underground roots which arise in groups from the nodes of an horizontal stem (e.g., Grass, Fig. 5.10). The main roots are of equal length. They give off small branches. Both the main root and their branches are thin and thread-like. Therefore, they are called fibrous roots. The fibrous roots do not penetrate deep in the soil. They remain near the soil surface and are called surface feeders.

Modifications of Adventitious Roots:

Storage of Food:

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1. Fleshy Adventitious Roots:

The adventitious roots become thick and fleshy due to the storage of food.

They are of several types depending upon the shape and place of the swollen part:

(i) Tuberous Root or Single Root Tubers:

(ii) Fasciculated Fleshy Roots:

The swollen roots or root tubers occur in clusters. In Dahlia they lie at the base of the stem (Fig. 5.11 B) while in Asparagus the fasciculated fleshy roots occur at intervals on the normal roots (Fig. 5.11 C).

(iii) Palmate Roots:

(iv) Nodulose Roots:

(v) Moniliform or Beaded Roots:

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(vi) Annulated Roots:

These thickened roots possess a series of ring-like outgrowths or swellings, e.g., Cephaelis or Psychotria (Ipecac, Fig. 5.12 D).

Mechanical Support:

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2. Prop or pillar (Fig. 5.13):

They are thick pillar-like adventitious roots which grow from and support heavy horizontal branches of Banyan tree. Initially the roots are aerial and hygroscopic. They become red in the moistened state. Root caps are present at their tips.

As the roots reach the soil, they become thick and pillar-like. The main trunk of the tree often becomes indistinguishable. Its death will not affect the growth of the tree because the crown is supported and nourished by prop roots.

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A Banyan tree (Great Banyan Tree) growing in Indian Botanic Gardens, Howrah (Indian Botanical Gardens, Kolkata) has 1775 prop roots. Its main trunk has decayed. The crown of the tree has a circumference of 404 m.

The tree is over 200 years old. The largest Banyan tree grows in Thimmamma Marrimanu village of Anantapur district in Andhra Pradesh. It is spread over an area of 5.2 acres. Two other famous trees are at Adayer (=Adiyar) in Chennai and Ketohalli village near Bangalore. Rhizophora a mangrove plant also possesses prop roots on which lenticels occur.

3. Stilt Roots (Brace Roots):

They are short but thick supporting roots which develop obliquely from the basal nodes of the stem. In Sugarcane, Maize, Pennisetum and Sorghum the stilt roots grow in whorls. After penetrating the soil, they develop fibrous roots which hold the soil firmly to provide support to the long and narrow jointed and unbranched stems (culms) like the ropes of pole or tent (Fig. 5.14).

Additionally they allow for better absorption of water and mineral salts. In Screwpine or Pandanus odoratissimus the stilt roots develop only from the lower surface of the oblique stem to provide support. Being one sided, they are also called prop roots. The supporting roots of Pandanus bear much folded multiple root caps (Fig. 5.15).

4. Clinging or Climbing Roots:

These are non-absorptive adventitious roots which are found in climbers. They may arise from the nodes (e.g., Tecoma, Betel), intemodes (Ficus pumila) or both (e.g., Ivy). The clinging roots penetrate the cracks or fissures of the support.

Vital Functions

5. Assimilatory Roots:

They are green roots which are capable of photosynthesis. In Trapa (Water Chestnut, vern. Sanghara, Fig. 5.17) the green assimilatory roots are submerged like other roots.

They develop from the stem nodes and are highly branched to increase photosynthetic area. Photosynthetic roots are also found in Tinospora (vern. Gilo, Gillow, Gurcha, Fig. 5.18). They are like green hanging threads which arise from the stem nodes during the rainy seasons and shrivel during drought.

6. Haustorial or Parasitic Roots:

The roots occur in parasites for absorbing nourish­ment from the host. Hence, they are also called sucking roots or suckers. Cuscuta (Dodder, vem. Amarbel or Akashbel, Fig. 5.19) has nongreen stems and scale leaves. It does not have any connection with the soil. The parasite sends haustorial roots into the host (e.g., Duranta, Zizyphus, Citrus, Acacia, Clerodendrum).

They make connections with both xylem (water channel) and phloem (food channel) of the host absorbing both water and food (Fig. 5.20). The partial parasite of Viscum (Mistletoe) is green. It sends a primary haustorium into the host from which secondary haustoria arise making connections with the xylem channels of the host for absorbing water and mineral salts only.

7. Epiphytic or Aerial Roots (Hygroscopic Roots, Fig. 5.21):

The roots occur in epiphytes (plants liv­ing on the surface of other plants for shelter and space only; hence also called space para­sites). Epiphytes bear three types of roots — clinging (for fixation), absorbing (for absorb­ing mineral salts and moisture from dust col­lected on bark) and hygroscopic aerial or epiphytic.

The aerial or epiphytic roots are thick, irregular and hang down in the air. They do not have root caps and root hair. Instead they possess a covering of dead spongy tissue known as velamen. With the help of velamen, the epiphytic roots are able to absorb water from moist atmosphere, dew and rain, e.g., Vanda, Dendrobium.

8. Floating Roots (Root Floats, Fig. 5.22):

They occur in Jussiaea (= Ludwigia). Here a number of adventitious roots arise from each node. Some of them store air, become inflated, project out of water, make the plant light and function as floats. The root floats help the plant in floating on the surface of water. They also help in gaseous exchange (hence also respiratory roots).

9. Reproductive Roots:

These adventitious roots are generally fleshy and develop adventitious buds. The adventitious buds can grow into new plants under favourable con­ditions. Such roots are called reproductive roots, e.g., Sweet Potato (vern. Shakar Kandi, Fig. 5.23), Dahlia.

Plants rely on roots for their supply of nutrients and water but not all roots are the same. Typically when asked to picture a plant root system we might think of the tap root with ‘lateral’ roots branching off this central, ‘primary’ root. However there is another whole class of roots which are also important for plant development and resilience. These are adventitious roots and they form from any other non-root tissue such as stems or leaves. This general definition separates adventitious roots from primary and lateral roots but within this general heading there is even more complexity as there are subgroups of adventitious roots formed both as a stress response and during normal development. A few examples are illustrated in Figure 1 and include roots that form on nodes of strawberry runners as part of normal development, roots that form on stems when plants are flooded and also include the roots that form on the base of cuttings such as used in nurseries to multiply plants.

Fig. 1. Examples of adventitious root types. This figure highlights a few examples of the diversity of adventitious roots (A-C) show types of adventitious roots that form during normal development including. (A) those potentially established in the embryo; (B) the dominant root system of monocots including maize crown roots (yellow) and brace roots (orange) and on (C) eudicots such as strawberry. Panels (D-F) show adventitious root development under stressed conditions. (D) Arabidopsis under low or no light (used as a model for adventitious root regulation); (E) flooding can induce adventitious roots from either nodal or non-nodal stem positions; and (F) wounding such as taking a cutting induces de novo adventitious root development. Primary and seminal roots are depicted in white, first order lateral roots in blue and second order laterals in pink. This figure is adapted and reproduced from Steffens and Rasmussen (2016) Plant Physiology 170:603-617 www.plantphysiol.org Copyright American Society of Plant Biologists.

The reason that these diverse adventitious roots matter is that we are under pressure to grow more food with fewer inputs and with more extreme weather events. We are also under pressure to preserve biodiversity and protect endangered plant species; work which is often supported by extensive cutting propagation programs. Adventitious roots are involved in all of these cases.

Take cereal crops like wheat, maize and rice – each of these depend on nodal adventitious roots called crown roots for their nutrient uptake. The development and growth of these roots can respond to changing nutrient or water availability.

In some regions floods are going to occur more frequently and adventitious roots are one way that many plants cope with flooding. Selecting varieties better able to rapidly induce adventitious roots may reduce crop losses due to flooding.

Propagation of elite varieties of fruit, forestry or ornamental plants is dependent on cutting propagation – which depends on adventitious root development on the cutting bases. This technique is also used for propagation of endangered species. A famous example of this is the Wollemi Pine which grew when dinosaurs roamed the planet. In 1994 only a handful of individual trees were found in an isolated valley of the Blue Mountains, Australia. Since then as a result of cutting propagation, the Wollemi is now grown in parks and gardens around the world.

There is now evidence that each of these adventitious root types is regulated in a different way. If we are to improve nutrient efficiency, or resilience to extreme weather events, then understanding precisely which root type we are studying and exactly how it differs from other adventitious roots is absolutely crucial. For this reason we highlight some of the key aspects of adventitious roots to which we should pay attention. We also use three more detailed case studies (flooding, nutrient responses and cutting propagation) to highlight some of the similarities and differences across these categories.

Bianka Steffens1 and Amanda Rasmussen2
1Plant Physiology, Philipps University, 35043 Marburg, Germany
2Division of Plant and Crop Science, University of Nottingham, United Kingdom

Publication

The Physiology of Adventitious Roots.
Steffens B, Rasmussen A
Plant Physiol. 2016 Feb

Plant Roots

When plants evolved from marine species and made their way onto land they had to overcome many challenges to survive and thrive on land. One significant difference between water based environments, such as lakes and oceans, and living on the land was the separation of nutrients. Plant roots were a key adaptation for plants on land to allow access nutrients and water stored in soil.

Living in water, plants are able to collect CO2, water and other nutrients straight from the surrounding water. On land however, these essential nutrients are separated between the atmosphere and the soil. Over time, plants evolved two main systems to collect sufficient nutrients from the two different environments.

Plant roots, found almost entirely underground, is their way of collecting water and nutrients essential for growth and survival. The roots of a plant perform a range of services that are essential to the survival of any land plant; they absorb water and nutrients from the soil, help to anchor the plant to the ground and often store large quantities of food.

Fibrous and tap roots

A plant’s root system can be either fibrous or have a distinct tap root. Many dicot plants have a main root known as the tap root which has many lateral roots growing from it. By having a thick tap root that grows deep into the soil, the plant gains extra anchorage to the ground. Some plants species have extra thick tap roots such as carrots and parsnips that store large amounts of nutrients.

Fibrous root systems, common in monocots, have many very thin roots spread out under the surface and form a mat of roots underground. Keeping their roots close to the surface means they may lose some stability provided by the deep tap root, however the large number of roots firmly secure the plant to ground. Plant species with fibrous roots such as grasses are also great at stabilising the soil and preventing erosion. The extensive root system provides excellent exposure to nutrients and water in the soil.

Root hairs

Plant roots are important for securing the plant to the soil but the majority of nutrient uptake occurs near the tip of the root. Here a large number of root hairs grow; very fine roots with large surface area to volume ratio. By having a large surface area and low volume it increases the efficiency of absorption of minerals and water.

Micorrhizae

A common feature of almost all root systems is what is known as mycorrhizae. A mycorrhizae is a relationship between the roots of a plant and fungi where both species usually benefit. The plant supplies the fungi with a constant source of sugars and the superior absorption abilities of the fungi help to provide the plant with an increased supply of water and nutrients.

Carbohydrates produced through photosynthesis in the plant’s leaves and stem are transported down to the root tissue and the fungi. The fungi are far smaller than any root hair and therefore have a much smaller surface area to volume ratio and are much more efficient at absorbing nutrients such as nitrate, ammonium and phosphate. Mychorrizal partnerships are most beneficial in nutrient poor soils.

Adventitious roots

It is common knowledge that roots grow below ground, but in certain cases, plants will grow roots above ground from stems and even leaves. Plant roots such as these are labelled adventitious, a term used to describe a structure that grows in a strange place.

The banyan tree is a great example of a species with adventitious roots, which have a large proportion of their roots above ground. As it begins its life in the branches of a host tree, the young banyan tree germinates and grows its roots down to the soil, often wrapping itself around the host tree on the way down. Many mangrove species also grow adventitious roots from stems to provide support in an environment of constantly changing tide height and mangrove fruit germinate on the tree and begin to grow an adventitious root while they are still connected to the parent plant.

Although roots can come in many forms and develop in a number of different ways, in nearly all circumstances their purpose remains constant and were developed as a mechanism to collect nutrients and water in a land based environment.

Last edited: 18 January 2016

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Adventitious roots are specialized roots that develop from a non-root tissue part of the plant, such as the stem, branch or leaves, rather than the root. Adventitious roots are formed either as part of the plant’s normal development or in response to stressful conditions. These roots are ecologically important as they allow plants to adapt to environmental stress, and humans utilize adventitious roots for food production and the vegetative propagation of plants. Adventitious roots can be differentiated into various types, which are described below.

1. Adventitious Roots As Storage Organs

Adventitious roots as storage organs are specialized to perform the function of food storage. These types of roots are thick and fleshy, and can further be subdivided into different categories based on the location and shape of the swollen parts.

a) Tuberous Roots/Single Root Tuber

An enlarged fleshy root that is modified to store food is called a tuberous root. It is not a true tuber, which is a modified stem tissue. In such plants, only a single such root exists.

Example: sweet potato

b) Fasciculated Fleshy Roots

When swollen or fleshy adventitious roots occur in a cluster, they are called fasciculated fleshy roots.

Example: Dahlia

c) Palmate Roots

As the name suggests, such fleshy roots appear like the palm.

Example: Orchis

d) Nodulose Roots

Nodulose root swellings occur at the tips of the root and appear like nodules.

Example: turmeric

e) Beaded Roots

Just like a beaded necklace, this type of fleshy roots exhibit swellings or nodules at definite intervals.

Example: Indian spinach

f) Annulated Roots

In annulated roots, the fleshy root has a series of ring-like grooves.

Example: Cephaelis

2. Adventitious Roots That Provide Mechanical Support

Adventitious roots of some plants are modified to provide structural support to the plant so that it can grow and spread further. The different types of such supporting roots are as follows:

a) Prop Roots

Prop roots are adventitious roots that arise from the aerial horizontal branches of certain trees. They function to provide extra support to the plant. Initially, these roots are aerial and hygroscopic in function, with root caps protecting their tips. However, they grow vertically downward to touch the ground and penetrate the soil, and become thick and pillar-like. The death of such roots does not result in the death of the plant, as other roots are supporting the plant as well. In such plants, the main stem often becomes indistinguishable from the rest of the plant. Occasionally, the main trunk of a tree will die but the rest of the tree continues to survive with the help of prop roots.

Example: banyan tree

b) Stilt Roots

Stilt roots grow obliquely from the basal nodes of the stem, and then touch the ground and penetrate the soil. In monocots, such roots arise in whorls from a few basal nodes on the stem. Stilt roots provide mechanical support to the plant, allowing rapid vertical growth. They also enhance the absorption of water and minerals from the soil by increasing the surface area for absorption.

Example: sugarcane, Rhizophora Pandanus

c) Buttress Roots

In certain large trees, plank-like roots develop at the stem base on the surface of the soil. These roots are also called ballast roots, as they act like ballasts by supporting the plant.

Example: Terminalia and Salmalia.

d) Climbing Roots

Some plants with weak stems often use other plants or objects as support to reach greater heights so that their leaves are exposed to sufficient sunlight. These plants often develop adventitious roots that grow aerially from the nodes or internodes of the stem, and then twine and clasp around a support to allow the plant to reach greater heights.

Example: Betel wine

e) Floating Roots

Floating roots are adventitious roots that develop from the nodes of aquatic plants that float. These roots act as a floating device that keeps the plant afloat. They are filled with air, and are therefore spongy and inflated. They also allow gaseous exchange and are called respiratory roots.

Example: Jussiaea

f) Contractile Roots

Contractile roots are adventitious roots that develop from the base of a bulb or corm, or other underground and subaerial modified stems. These roots swell or contract to push or pull the modified stems to the correct depth in the soil.

Examples: Crocus, Lilium

g) Clinging Roots

Like climbing roots, clinging roots also allow plants with frail stems to attain greater heights. Such roots enter the crevices of the support to fix the plant to it.

Example: Orchids

3. Adventitious Roots Performing Other Specialized Functions

a) Reproductive Roots

In some plants, adventitious roots can be used in the vegetative propagation of plants. Such roots bear adventitious buds that can give rise to new plants when conditions are favorable.

Examples: Sweet potato, Dahlia

b) Assimilatory Roots

Roots modified to perform photosynthesis are called assimilatory roots. They are green roots that are often highly branched to increase the photosynthetic area.

Examples: Water chestnut, Tinospora

c) Saprophytic Roots

Saprophytic adventitious roots are associated with fungal hyphae, either ectomycorrhizae or endomycorrhizae. Such plants usually grow in humus when roots are infested by fungal mycelia, which form a mantle on the root. The mycelia aid in the absorption of food solutions from the soil that is utilized both by the host plant and the mycorrhizal fungus. Roots in such plants are usually underdeveloped. Tips stop growing and root hairs are mostly absent.

Examples: Sarcodes, Monotropa

d) Parasitic/Haustorial Roots

Some plants live on other plants for their nutrients and water supply. The roots of parasitic plants penetrate the stems or roots of the host plant, either only up to the xylem or even up to the phloem, in order to absorb the necessary water, minerals, and organic food.

Example: Mistletoe

e) Epiphytic Roots

As the name suggests, epiphytic roots are found on epiphytes or plants that grow on other plants. In such plants, the roots are hygroscopic and hang freely in the air. They have modified epidermal tissue called velamen that performs the specialized function of absorbing moisture from the air. Some of these roots are also greenish due to the presence of chlorophyll and perform carbon assimilation.

Examples: Dendrobium, Venda

f) Root Thorns

In some plants, adventitious roots are modified to form thorns that are hard and pointed. Such roots might function to defend the plant from being uprooted by animals.

Examples: Acanthorhiza, Iriartea

    Adventitious Roots and Shoots

The buds located at the nodes on a plant’s stem, if “activated,” will produce more stems…right? Usually, this is the case-stem tissue begets stem tissue and root tissue begets root tissue. But of course things are never that neat and tidy!
Think back to that ivy vine sitting in a jar of water. When we “root” a cutting like that, we’re hoping that root tissue will form along the submerged stem. And remember our example of the violet plant? When sprouts form at nodes along the creeping stems, both shoots and roots are produced on the stem tissue.
Botanists have a term for this phenomenon. Roots that arise from the stem (or any other non-root tissue) are called adventitious roots. Adventitious shoots, as you might guess, are shoots arising from non-stem tissue, usually roots.
Now let’s look at some asexual means of propagation and see where adventitious roots and shoots play a role.
Asexual propagation by underground structures. Many adaptations for asexual propagation involve underground plant parts. In some cases the structures are true roots, but most are modified underground stems.
Most of the underground plant parts that are important in asexual propagation also function as food storage structures. This type of food reserve is especially common among herbaceous plants in temperate regions, whose aboveground parts die back at the end of each growing season.
Tubers. Who among us hasn’t stored potatoes for too long, or improperly, sothat the “eyes” begin to sprout? It is unfortunate for the chef, but a great botany lesson for your students! The common potato isn’t a root at all: it’s a tuber-the swollen tip of an underground stem. The “eyes” on a potato tuber are nodes from whichnew plants, both stems and roots, can sprout. Most gardeners plant “seed potatoes,” which are pieces of potato (or small whole potatoes) that have at least two eyes. As the plant grows, it uses the food reserves in the “seed.”
Production of tubers is one way that a plant can store carbohydrates during the growing season. Then at the end of the growing season, the aboveground portion of the plant dies back. The new tubers overwinter, and sprout again in the spring.

Tuberous roots and stems. Although often grouped together with tubers, botanically these are different structures.

Tuberous roots are enlarged secondary roots; because they are root tissue, not stem tissue, they have no nodes or buds. Two familiar examples of plants with tuberous roots are sweet potato anddahlia. Tuberous roots canact as storage structures, allowing the plant to survive a dormant period. In the spring, the new, adventitious shoot growth uses the food stored in the roots, and the root shrinks and eventually disintegrates. The plant produces new tuberous roots each season.
Sweet potatoes are usually propagated from slips-adventitious shoots produced when the sweet potato is placed in a moist location.

Like tubers, tuberous stems are swollen sections of stem; however, unlike tubers, they don’t occur at the tips of underground stems. Rather, they occur on the main stem, often just below the soil level. Tuberous stems generally have a vertical orientation with vegetative buds produced on the upper end and roots on the lower end. Examples include tuberous begonia and cyclamen. Unlike tuberous roots, tuberous stems are perennial, and continue to enlarge every year. If you have begonias, for example, you can dig up the tuberous stems, store them, and replant them year after year.

Rhizomes. A rhizome is an underground stem that grows horizontally at or just below the soil surface. Rhizomes can be categorized into two types: the thick, fleshy, compressed structure typified by the iris, and the slender, elongated underground stems of lily-of-the-valley. Because they are stems, rhizomes have nodes and buds along their length; new plants can arise at these nodes. Other examples of rhizomatous plants include bamboo, sugar cane, and many grasses. Quackgrass and poison ivy are two troublesome plants that spread via rhizomes.

Bulbs. A bulb is a modified stem that occurs in many familiar monocotyledonous plants. The bulk of a bulb consists of concentric layers of bulb scales that enclose the immature vegetative or flowering shoot. An onion is an example of a bulb that most students may have “dissected” in the kitchen. Tulip, daffodil, amaryllis, lily, and hyacinth are some of the more common flowering bulbs.

Plants with bulbs generally reproduce asexually by forming daughter bulbs from buds occurring between the fleshy layers.

Corms. A corm is a swollen leaf base enclosed by dry, scalelike leaves. In contrast to the layered scales of the bulbs, the bulk of a corm is solid, with distinct, if compressed, nodes and internodes. Familiar examples of corm-producing plants are gladiolus and crocus. As with the other structures that we have discussed, corms provide food for emerging shoots. The corm shrinks, and during the growing season another corm is formed at the top of the old one in preparation for the following season.

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