Botany Three - Stems

Stems develop from the Plumule and Epicotyl of the Seed embryo and are part of a plants shoot system, they are its main vertical axis and give support to the leaves and reproductive structures. Generally, they grow away from the soil (negatively Geotropic) and towards the light (positively Phototropic) They also provide the channels through which the plant moves water and soluble nutrients from the roots to the upper parts as well and moving the products of photo synthesis from the leaves to other parts. Young stems can themselves be green and photosynthetic. In some case they can also act as food stores. Though usually above ground there are exceptions like Potatoes whose stems are underground and whose tubers are swollen stems. Stems are incredibly diverse, they may be herbaceous (soft) or woody, plain, or intricately branched, as short as millimetres or as long as hundreds of meters.

A main characteristic of stems is presence of Nodes separated by Internodes. Branches, Leaves, Roots, and flowers are Exogenous (originate in the outer Cortical region) and attach to the stem at Nodes. If present, Stem hairs are usually multicellular. In the case of Leaves there may be a special stem termed the Petiole (Leaf Stalk) between the Stem and the Leaf. In the space between the Petiole and Stem, the Axil, Axillary Buds occur and may give rise to a Branch or a Flower. The top (Apex)of the Shoot bears the Terminal or Apical Bud which contains the Apical Meristem.

Stem Anatomy

The anatomy of a Stem is in large part very similar to that of a root, where appropriate I’ll reference to the relevant section above to save repetition. In comparison to roots they have a much thinner Cortex, and much wider Pith. Between Dicotyledonous (two seed leaved) and Monocotyledonous (one seed leaved) there is a difference in the arrangement of the Vascular Tissue which also relates to how they grow. Other than this the tissue types and functions are very similar.

Stems are composed of three main tissue types –

• Ground tissue, consisting mainly of thin-walled Parenchyma cells, which fills in around the vascular bundles, can heal wounds, store starches, and sometimes function in photo synthesis. It may also contain Sclerenchyma and Collenchyma cells for strengthening. This tissue is termed the Pith in the center of the stem and the Cortex towards the outside.
• Vascular tissue, which provides fluid transport and structural support functions. Vascular Bundles are made of three cell types; Xylem cells which are dead and Lignified (woody) when mature and which transport water and nutrients upwards from the roots; Phloem cells which are alive when mature and transport sap from the leaves to other parts, and Sclerenchyma cells which are fibrous and provide structural support. In the case of Flax, it is these fibres that are used to make linen. In Monocots the Vascular Bundles are scattered throughout the stem but are usually less concentrated in the centre. In Dicots Vascular bundles are arranged in a ring at the outside of the stem. When plants live for multiple years, these bundles join to produce Secondary Growth (thickening) and characteristic growth rings. At Stem Nodes the Vascular Bundles bend outwards into the Petiole to supply the Leaf. (Also see the linked descriptions Root Anatomy section).
• Dermal tissue, which protects the plant and provides both waterproofing and gas exchange control. This layer is mainly a single layer of Epidermal cells. A thin layer of Collenchyma cells with strengthened walls lies under the Epidermis and gives structural support. Gas exchange is achieved via small pores called Stomata in Herbaceous stems and by porous areas called Lenticels in Woody stems. Woody stems have an additional corky outer layer (Bark) for protection. Some plants exhibit Trichomes, multi-cellular hairs on the outer surface, these can store chemicals that deter herbivores as well as increasing solar reflectance and reducing transpiration.

Stem Growth

While animals usually reach a stable final or adult size plants continue to grow throughout their lives. In Roots and Stems in particular this Indeterminate growth is achieved by the means of Meristems in two phases, Primary Growth (lengthening) and Secondary Growth (thickening). Some plant parts like Leaves and Flowers exhibit Determinate Growth and stop growing at a particular size.

• Primary Growth
  The Primary Growth of a Stem is generated by the activity of the Apical Meristem located at the Stems tip. Cell division in the Apical Meristem creates the tissue of the Stem and the Leaf Primordia (Nodes) which are created in a regular pattern that depends on the plant species. In the region below the Meristem these cells grow to their mature size and differentiate into their functional types. This lengthening of cells is what makes the stem elongate and the leaf nodes arrive at their final spacing and pattern. Primary Growth enables plants to continuously seek out water and sunlight. In Herbaceous plants almost all growth is Primary. In some plants, those with Rosette habits, Stem lengthening is very limited giving rise to a Stem that is barely more than a thickened disc and leaves all emerging very close together. E.G. Dandelions

• Secondary growth
  Secondary Growth in Stems occurs mainly in woody perennial plants. A secondary Meristem, the Vascular Cambium, develops in the Vascular Bundles between the primary Phloem and Xylem. Cell division in the Vascular Cambium creates more secondary Xylem to the inside and secondary Phloem to the outside leading to thickening of the stem. Activity of the Vascular Cambium follows an annual cycle, in Spring secondary Xylem cells have large diameters and un-thickened walls, in Autumn fewer, narrower Xylem cells with thicker walls are produced. This annual activity cycle results in the growth rings in a stems cross section. Over time this lateral growth ruptures the Epidermis and leads to the formation of the Cork Cambium, another secondary meristem. The Cork Cambium creates Suberin a waxy waterproof substance; Cork tissue is produced to the Cork Cambiums outside and Phelloderm to its inside. These Cork and Phelloderm layers are collectively called the Periderm that replaces the epidermis in mature plants and is commonly referred to as Bark. In some plants the Periderm produces Lenticels, porous areas that allow gas exchange between inner metabolically active cells and the atmosphere. The pic below shows lenticels in cross-section (x100) on Sambucus, on Poplar Bark and on a Pear Skin.

Stem Branching

Branching in plants occurs in Stems and also in Inflorescences with the simpler structures usually being seen in Stems and the more Complex ones in Inflorescences. Here we will look at the forms more often seen in Stems and Inflorescences will be covered later. Plants have two main types of branching in their Stems, Dichotomous and Lateral.

• Dichotomous Branching is most often seen in “Lower” (less evolved) plant forms. In this type of Branching the Apical Meristem subdivides in to two portions which each form a branch stem in a forked manner. 

• Lateral Branching is seen in “Higher” (more evolved) seed bearing plants. In this type of branching branches are produced from the Lateral or Axillary Buds in the leaf axils.
  These buds are created when the Apical Meristem leaves a small portion of itself behind as it generates new Stem tissue, the “daughter Apical Meristem” initiates the Leaf Primordia and the Axillary Bud. As the Apical Bud advances away from the Axillary Bud its growth suppressing dominance decreases, and the Axillary Bud begins its growth and generates a branch. (The Apical Meristem can also lay down “Buds” in locations other than Leaf axils which are known as Adventitious Buds). Lateral Branching occurs in two types, Racemose and Cymose.

2. Cymose Branching (Also Known as Definite Branching) In this form the growth of the main stem is Definite. The main stems growth is checked, usually by the formation of a terminal flower and the lateral branches arise in Basipetal succession, which is to say they develop first at the plants apex and decrease in age as they descend the main stem. Cymose Branching has three sub-types the Uniparous Cyme, the Biparous Cyme, and the Multiparous Cyme with different resulting overall forms.

2A. Uniparous Cyme (Also known as Sympodial Cyme) In this form one lateral branch develops once below the apex and effectively takes over as the main Stem. How this branch subsequently branches itself give rise to two forms, the Helicoid Cyme and the Scorpioid Cyme.

2Ab. Scorpioid Cymes, where all the subsequent branches of a Uniparous Cyme develop on alternating sides of the Stem giving rise to a zig-zag form.

2B Biparous Cyme (Also Known as Bipodial Cyme). In this form two lateral branches develops once below the apex. How these branches subsequently branch themselves give rise to several forms. ( From L to R), If there is no further branching the form is Monochasium; If the further branching follows the same pattern the form is Dichasium, If the branching follows a Uniparous form the resultant forms are the Double Helicoid Cyme and the Double Scorpioid Cyme.

Stem Forms

Plant have number of basic overall Stem forms: –

1. Reduced stems – the stem is sometimes reduced to a small disc which is not differentiated into nodes and internodes. (See also under Stem Modifications)
2. Erect stems – the majority of Perennial Angiosperms (flowering plants) possess upright, erect stems. Erect stems subdivide in to four forms…

2a. Culm – an erect stem with solid swollen nodes and hollow internodes giving a jointed appearance. E.g., Bamboo, Wheat, etc.
2b. Caudex – an erect, unbranched main stem bearing a crown of leaves at the top. E.g., Coconut, Palms, etc.
2c. Excurrent – the main stem is trunk like, thickest at the base and tapering to the apex. The branches arise in Acropetal succession giving a conical shape. E.g., Pine, etc.
2d. Decurrent (or Deliquescent) – the apical bud of main stem is weak as compared to the buds of lateral branches. Lateral branches are prominent and spreading giving dome like appearance. E.g., Banyan, Oak, etc.

3. Weak stems – some stems are thin, soft, delicate, and unable to remains upright without any external support. Weak stems may be upright or prostrate.

3a. Upright weak stems have two forms

3a1. Twiners – stems are long, flexible, and very sensitive to touch (Haptropic). On contacting a support, they can twine or coil around it and may do so to the right (anticlockwise from the top, known as Sinistrorse, e.g., Convolvulus, Ipomoea. Or to the left (clockwise from the top, Known as Dextrorse), e.g., Lablab.
3a2. Climbers – the stem is unable to coil and usually climbs with the help of some clasping or clinging structure. They are of four types of Climbers: –

3a2a. Tendril climbers – see Stem Modifications.
3a2b. Root climbers – adventitious roots arise from the nodes and penetrate the support, e.g., Ivy, etc.
3a2c. Scramblers or Hook climbers – weak stemmed plants slowly grow over other plants attaining this position with the help of curved prickles (e.g., Rose), curved hooks on a flowering peduncle, prickles on a stem, or spines.
3a2d. Lianas – woody perennial climbers of deep forests. Stems begin like ordinary twiners but become woody when they reach into the sunlight.

3b. Prostrate weak stems have two forms: –

3b1. Trailers – who’s stem creeps on the ground, but roots do not arise at the nodes. There are of three types of Trailers…

3b1a. Procumbent – where the stem creeps totally on the ground.
3b1b. Decumbent – where Branches grow first horizontally then vertically upwards.
3b1c. Diffuse – where Branches grow profusely in all directions.

3b2. Creepers – weak-stemmed plants that grow prostrate and develop adventitious roots from their nodes. See Stem Modifications below for Runners, Stolon’s and Offsets.

Stem Modifications

As well as the wide diversity of general forms (above) a great many plants have evolved Stem modifications furnish them with better chances of survival as well aiding propagation. Humans have selectively bred some of these plants to create significant food and other resources. Stem modifications can be divided in to three broad groups depending on their location.

Underground – the main functions of modified underground stems are food storage, Perennation (survival from season to season), and vegetative reproduction.

 Bulb – A highly reduced main stem termed a Basal Plate assuming a disc, lens, or flattened cone shape. Internodes are very small placing Nodes close together The Nodes produce outer dry and inner fleshy scale like leaves. The bases of the fleshy leaves store food in the form of carbohydrates. The Apex of the Bulb bears an Apical Bud which gives rise to the Aerial shoot. When the fleshy scales are arranged concentrically, and the whole covered by a sheath of dry scales (the Tunic) he form is known as a Truncated Bulb, e. g. Onions, other Alliums, Hyacinth etc. When scales are arranged loosely, only overlapping at the edges,  and there is (usually) no Tunic the form is known as a Scaly or Imbricate Bulb, e. g. Lilly.

• Corm – A rounded, fleshy, unbranched modified main Stem, Internodes are very short and circular, Nodes very close together. Nodes and Internodes carry food substances. From Nodes near the base Contractile adventitious rootscan arise and pull the Corm in to the ground. Scale leaves bear Axillary buds that develop into daughter Corms. An Apical Bud gives rise to the aerial branch when conditions are favourable. E. G. Crocus, Crocosmia etc
• Rhizome – A thick, branched, modified main Stem that grows horizontally underground. Has Nodes with small scaly leaves and Axillary buds, Adventitious Roots grow from lower nodes and Adventitious Shoots from the upper ones. Internodes and nodes both store food. Many herbaceous plants of a Clumping form have very dense Rhizomes.
• Rootstock – a vertical or oblique form of Rhizome. E. G. Banana & some Ferns (Dryopteris)
• Straggling – A horizontally branched Rhizome. E. G. Ginger, Turmeric
• Tuber – The swollen ends of a highly branched underground Stolon that arises from the axil of a lower leaf. Contain many spirally arranged depressions, colloquially known as “Eyes”, which are Nodes with a Axillary bud and residual leaf scale. The “Eyes are more concentrated towards the Apical or “Rose” end. Tubers they act as food stores (usually starch) to aid winter survival and reproduction. E.G. Potato, Jerusalem Artichoke.

Sub Aerial – stems are modified branches that grow horizontally on or just under the soil surface. They are mainly for Perennation (survival from season to season), and vegetative reproduction.

• Stolon – A slender branch arising from low on the base of the main stem. They initially grow obliquely upwards then bend to the ground. When the tip strikes the ground, it produces adventitious roots and a bud which quickly develop into a daughter plant. Many Stolons may spread in all directions from the mother plant. E. G. Strawberry, Jasmine, Wood Sorrel, Blackberry. When Stolons are entirely on the surface of the soil they are sometimes referred to as Runners. There is also a special type of entirely underground Stolon with three internodes, it is thin, stores no food and is particular to a few plants including Couch Grass.
• Sucker – like a Stolon but shorter and thicker. It arises from an underground part of the main Stem and grows obliquely upwards, when it breaks the surface it creates a leafy shoot or a new plant. E. G. Chrysanthemum, Raspberry.
• Offset – originating in a Leaf Axil an Offset is similar to a Runner but with shorter thicker internodes. At the Apex it produces Leaves above, roots below and breaks off from the mother to become an independent plant. Most common in aquatic plants.

• Bulbil – a form of vegetative reproduction. They arise from floral buds and become multi-cellular bodies swollen with food. When detached from the parent they become independent plants. E. G. Alliums, Oxalis. Lilies etc.
• Cladophyll (or Phylloclade) – An adaptation of plants in Xerophytic (low water) conditions. A green, thick, succulent modified main stem with distinct Nodes called Areoles that bear leaves modified into Scales or Spines. It is both Photosynthetic and a food storage organ. E. G. Prickly Pear, some Euphorbias etc. 
• Cladode – A modified branch arising from an axillary bud that is like a Cladophyll but only has one (Asparagus racemosus), or two (Butchers Broom) internodes. It performs the function of leaves and arising leaves are modified as spines or scales. 
• Stem tendril – A thin, wiry, and leafless spirally coiled Haptropic (touch sensitive) structure to enable climbing plants to seek and achieve support. Mau be unbranched, or branched, sometimes with scale leaves at the fork. May arise from axillary buds, e.g. Passionflower, or from extra-axillary buds, e.g. Cucurbits (Cucumber, Pumpkin), or from Apical buds e. g. Grape, or from floral buds e.g. Antigonon, Cardiospermum. 
• Thorn – A hard, straight, pointed modified branch that gives the plant defence from predators. May arise from axillary buds, e.g., Lemon, Pomegranate etc, or from apical buds e. g. Carissa. May bear leaves, flowers & fruit, e. g. Prunus, and may be branched.

Further Resources

Most of the main search engines will give you a result if you are looking for the meaning of a particular botanical word. If you wish to explore the subject in more detail here are a few website links to give you a starting point.

• Wikipedia – Plant Development.
• Wikipedia – Glossary of botanical terms.
• Angiosperm Morphology – Online self learning.
• Openstax – Online Biology self learning.
• Encyclopedia.com  – Online Encyclopedia – Biology section.