Appendix 3 – The Plant Cell Wall, Epidermis and

Trichomes

Summary

The epidermis and the surface structures that it supports form the substrate upon which the insects walk and attach themselves and from a functional ecological perspective this is important to understand the structures that feature on the plant surface.

This appendix was prepared in anticipation of pursuing work into the field of insect/plant surface interactions. It augments the contents of Appendix 2 but the emphasis here is simply on structures, not biomaterial performance.

The attachment of insect tarsi to a plant surface must be extremely adaptable because of the huge variations in substrates. Some plant surfaces have properties that actively (through secretions or other activities) or passively (through smoothness for example) discourage insect attachment whilst others rely upon the participation of insects in their life cycle (synzoochory is the distribution of diaspores by ants).

Table of Contents

1Introduction

A plant surface consists of the following structures:

• The outer wall of epidermal cells. The epidermal layer of cells is the outermost layer of cells of a plant. Activities of these cells include the manufacture of the constituents of the cell wall and structural material of the plant such as cellulose microfibrils and others described here.

• A cuticular layer, secreted by the epidermal layer

• Trichomes, which is a term for all the outgrowths supported by the cell wall.

It is necessary to understand these structures and their diversity in order to understand the diversity and mechanisms of tarsi/plant surface interactions, each of which demonstrates a possible natural attachment mechanism that can be utilised as an inspiration for an artificial attachment mechanism.

2The Plant Cell Wall

It is stated that the differences between the cell walls of plants and animals is one of the primary reasons that plant and animals differ as much as they do 1]. Animal cells have cell membranes whereas plant cells have walls which are relatively rigid and inflexible which means that cell movements are not possible and the development of muscle is not possible. Similarly the cell wall does not allow for the development of elaborate sensory and nervous systems which are required for motility (movement).

In trees most of the wood and bark is composed only of cell walls, the cell contents (protoplasts) having died and degenerated. In woody plants the majority of the plant body consists of the cell wall. This means that in the study of the mechanical properties of wood it is actually the collective properties of the cell wall that is being considered.

The plant cell wall is considered to be a two-part system. There is a basic and standard primary cell wall that is present in all plant cells. Within that cell wall there can be a second layer, the secondary wall. The cell wall as a whole is considered to be an organelle and any modifications or specializations to the protoplasts within the cell are reflected in a dynamic way in changes to the secondary cell wall.

The cell membrane or plasmalemma is found interior to the cell wall, a permeable membrane through which structural material passes.

2.1The Primary Cell Wall

2.1.1The chemical structure of the wall

The main components of the cell wall are:

• Cellulose

• Hemicellulose

• Proteins

• Pectic substances

Here follows a description of the nature of the process of growth and deposition of cellulose microfibrils. It is not detailed to a great degree, this description is to aid the understanding of the macrobehaviour of wood as a composite as described and discussed in Appendix 2. It is not intended to be a scientifically precise biological discourse on cell development and functioning. This can be found in the text if required.

2.1.2A description of the formation of the cell wall

The key structures to this description are microtubules which are the cellular chemical communication and transportation networks within the cytoplasm of the cell and the plasmalemma or cell membrane. Microtubules that deposit the chemical constituents of the cell wall terminate at the cell membrane in structures known as rosettes.

Cellulose microfibrils grow from these rosettes and emerge fixed to the cell wall, crystallizing as they emerge; the rosettes which are anchored in the plasmalemma, move forward in the liquid medium of the plasmalemma as the fibrils are laid down (see Figure 1)

Figure 1 – A diagram representing a membrane with a cellulose-synthesizing rosette embedded in

it from [1]

The cellulose molecule is a polymer composed purely of glucose molecules and can be 0.25 to 0.5m long. The type of bonding between the mers of glucose causes the molecules to be flat and ribbon-like. When the molecules lie parallel to each other they form more hydrogen bonds between themselves, crystallizing and producing aggregates called microfibrils (see Figure 2).

Figure 2 – A diagram of the view of a plasma membrane from [1]

Hemicellulose is a mixture of polymers that are highly branched. The side chains interact with the cellulose, coating it and gluing the microfibrils together.

It is believed that the rod-like proteins and pectic substances form further structural elements by interacting with the cellulose microfibrils.

Also see Appendix 2 for more on cellulose.

3The Plant Epidermis

3.1Introduction

This section consists of descriptions of plant anatomy that is relevant to insect attachment mechanisms, specifically because the epidermis is the biological substrate upon which they move using their tarsi. Each attachment mechanism in nature requires an examination of its corresponding substrate to develop an understanding of the system as a whole. This appendix is relevant to the experiment on imaging since there are two examples of insect tarsi, the bee and grasshopper.

Clearly there are many more examples of natural substrates that must be included for a full account of all substrates. This would include the coverings of animals and interior organs that harbour species that use attachment mechanisms. For instance the tapeworm Trypanorhyncha (tapeworms of the cestode order) attaches itself to the gut lining and has hooks that have evolved for this purpose (see Appendix 1).

All of the following derives from “Plant Anatomy” J D Mauseth (1998) 1].

3.2Glossary

Cuticle – made from the polymerization of fatty acids to form cutin, this is an outer layer to the epidermal cells, secreted by the cells themselves.

Trichome – A plant hair; basically any outgrowth from the epidermis. Trichomes can be glandular of non-glandular, uniseriate, multiseriate, branched or unbranched. They can appear in leaves, fruit, seed coverings and on flowers (filaments of the stamens).

Trichome complement – this collectively refers to all the types of trichome on the plant. This differs from indumentum which refers to all the trichomes themselves.

Wax – A substance of variable composition, being a polymer of very long-chain fatty acids.

Epicuticular wax – the wax that is located as a layer exterior to the cuticle proper.

Intracuticular wax – the wax that occurs as plates and patches inside the cuticle.

3.3The Epidermis

The epidermis is the interface between the plant and its environment (see Figure 3). Epidermal cells are long-lived and are often able to alter their anatomy and physiology if required (see Figure 3).

Figure 3 -The epidermis and its relationship to the rest of the plant from [1]

Any plant will have at least two types of epidermal cells:

1. A shoot will have epidermal cells and guard cells

2. A root will have epidermal cells and root hairs

The primary function of the epidermis is to regulate water in and out of the plant. The epidermis of plant parts that are above ground are resistant to water and the epidermis of the parts of the plant embedded in soil or some other similar substrate are able to extract water from the soil.

3.4Functions of the Epidermis

3.4.1Water regulation

A thickened waxy epidermis in some plants prevents or reduces excessive water evaporation from the plant (see Figure 4).

Figure 4 – An example of water regulating leaves from [1]

3.4.2Protection against sunlight.

The epidermis can prevent excessive light from penetrating the leaf and damaging interior tissues through its reflectivity.

3.4.3Protection against other organisms.

The epidermis is the first line of defence against biological pests. In the tropics, where lichens and algae grow on the leaves of trees and vines, a smooth epidermis prevents the spores of lichens and algae from attaching and germinating. Parasitic plants such as mistletoe penetrate the epidermal layer to infiltrate the host’s primary tissues. An epidermis must be resistant to these. Further, it is important that the epidermis develops defenses against insects. These include adaptations to prevent insects being able to suck on, chew, lay eggs on, walk upon or even land upon a plant. This may take the form of epidermal glands containing substances so toxic or noxious that insects and animals don’t eat them, such as stinging nettles. On the other hand the active substance of marijuana (THC) is produced by the epidermis and so “is responsible for the plant being harvested and burned” [1].

3.4.4Protection against non-biological agents

Leaves must be adapted to prevent wind damage and in arid regions, able to resist the erosive effects of sand born by the wind.

3.4.5A reproductive function

Epidermal cells have a function in the release of pollen, the colour, texture and reflectivity of the petals and other flower parts, along with scent. The epidermis can also be important in accumulating or releasing nectar.

3.4.6A secretory function

The epidermis can be extremely active, adsorbing and releasing water which can cause leaves to open when moisture is available and close when it is not. Epidermal trichomes can be involved in attracting, guiding, trapping and digesting insects.

4Types of Epidermal Cells

There are four basic types: ordinary epidermal cells, guard cells, trichomes and root hairs.

The following descriptions are based upon those properties and structures that can effect the interactions with insect tarsi. Functions that have no relevance to these interactions will only be mentioned briefly.

4.1.1Ordinary epidermal cells

These cells lie between the more specialized cells of the epidermis and usually are the most numerous and cover the greatest part of the plant surface. They can have any shape but are usually tabular i.e. tablet shaped with flattened surface and interior faces (see Figure 5). They can be column shaped too but these only occur on organs that need extra protection such as seed and bud coverings. The epidermal cell shapes have a high variation, both in the same organ and from species to species. The size and shape usually is characterized by their position. For instance in a leaf, the epidermal cells over a vein are a different shape from those at the margins (edges), the cells on the top of the leaf are a different shape from those on the underside. From the time that the cells are first produced at the apical meristem (terminal bud), they experience growth and mitosis as the plant organ grows in size.

Figure 5 – The epidermis peeled from the leaf of Zebrina from [1]

Epidermal cells are always firmly attached to each other except around stomata and less well attached to the cells underneath. The presence of lignin is rare. The cell walls facing outwards to the surface may be thickened and have warts, protrusions or ridges occurring.

4.1.1.1Cutin and the cuticle

Cutin is a hydrophobic material mainly found on the outer epidermal walls. It is a complex, high-molecular weight lipid polyester resulting from the polymerization of certain fatty acids. Cutin types vary from plant to plant. It is produced from within the epidermal cells and transported to the walls where the polymerization takes place. If this polymerization occurs while still in the cell wall then it forms a matrix around the cellulose microfibrils called the cuticular layer. Continued secretion forms another outer layer of pure cutin called the cuticle. In some species the cuticle has complex patterns of striations, bumps and wrinkles (see Figure 6).

Figure 6 – This micrograph shows epidermal cells with heavy deposits of cutin on inner and outer

walls from [1]

4.1.1.2Wax

Wax is a polymer that is found on the outer walls of all epidermal walls. It can form a uniform layer or it can polymerize into plates, rods, granules and other forms. It may cause the surface to be hydrophobic and non-wettable or it can act as a sunscreen.

Of the two classes of wax, extracuticular and intracuticular wax, only the surface or extracuticular wax will be discussed here.

The wax layer can vary in thickness from species to species but it seems that it is the nature of the wax that is most important (see Figure 7). In certain plants the wax layer is more oily than waxy and this affects the tarsi of an insect. In the correct composition it can be sticky and gummy and it can jam an insect’s mouthparts. It can also stick to the insects tarsi so that it cannot grip the plant effectively, as occurs in the pitcher plant.

Figure 7 – Wax in the form of plates on the surface of a leaf from [1]

4.1.2Stomata and guard cells

Except when in connection with trichomes and their arrangement (in some species trichomes can occur clustered around stomata), it is not judged that these affect insect mobility and hence a description of these not included here (see Figure 8).

Figure 8 – A stomatal complex from [1]

4.1.3Trichomes

A trichome is an artificial grouping of cells that project out of the plane of the epidermis. The term is loosely applied since there is a wide variety of shapes and sizes and these have not been completely categorized due to the variety that is present. It can include shapes such as thorn, wart, hair and others.

The term now generally refers to structures that are epidermal in origin.

A single plant therefore may contain many different kinds of trichomes and the collective name is the indumentum of the plant. Trichomes mature with the plant. Parts of the trichome may fall off (absciss) with maturity or the entire trichome may fall off. For example a young leaf may be covered in hair-like trichomes but the mature leaf may be bare and smooth.

Glandular trichomes can secrete water, salt, nectar, mucilage, terpenes, adhesives, digestive enzymes and irritants that sting (see Figure 9).

Non-glandular trichomes can protect against excess sunlight because as they whither and die their walls become refractile and scatter light. It is a deterrent against insects since hairs can entangle feet or impale the insect. Plants sometimes have short hairs all pointing in a single direction which makes it easy for an insect only to walk in that direction.

Figure 9 – Secretory trichomes on the surface of a cannabis leaf from [1]

4.1.3.1Non-glandular trichomes

1. • Unicellular trichomes are very common. For instance cotton fibres are up to 6cm long and are unicellular trichomes of the seed coat. They are a single cell which projects above the surrounding surface (see Figure 10).

Figure 10 – Cotton seeds showing long unicellular trichomes from [1]

1. • Multicellular trichomes can consist of a single column of cells, a double row or more, extending from the surface of the epidermis. They have many shapes and are named accordingly. They can be branched, flattened, long or extremely short (see Figure 11).

Figure 11 – Trichomes in the form of squamiform hairs from [1]

4.1.3.2Glandular trichomes

Glandular trichomes can have virtually all forms but are modified due to their secretory nature. There may be a secretory head elevated by a stalk of neck, attached to the epidermis by the stalk or neck and attached to the epidermis by a foot or basal cell (see Figure 12).

Figure 12 – The parts of a glandular trichome from [1]

5Conclusion

The insect tarsi must be extremely adaptable to wide variations of plant surfaces. Some plant surfaces have properties that actively (through secretions) or passively (through structures or lack of structures) discourage insect attachment whilst others rely upon the participation of insects in their life cycle (synzoochory is the distribution of diaspores by ants).

6References

1. “Plant Anatomy” J D Mauseth (1998) Benjamin/Cummings Publishing Company ISBN 0-8053-4570-1