Botany Four - Phyllotaxy & Leaves

A Beginner’s Guide to Botanical Terminology (Pt 4).

Botany and biological description are a bit more exacting than Latin nomenclature. Often there is only the correct, Latin-based name for a particular part of a plant, or animal. As a gardener you may not need to trouble with the finer details but if you get interested in how & why plants do what they do, or citizen science and recording the plants around you, you will need to know some of this to work with identification keys.   A passing familiarity with Botanical Latin can also be advantageous, try our Botanical Latin Lexicon.

This page is only a starting point in to how and why plants do what they do. The framework for Botanical Descriptions  has been used as starting point for items included and I hope that relating observable plant features and processes to the correct Biological terms makes this fascinating subject a little more accessible.

The ubiquitous leaf, you would think they are so common and familiar that this mass of foliage would not bear much attention. Not so. Not only are they so diverse and varied that they exert a huge visual influence on the feel of a garden they are also pretty important to the plant too. One of the most unique characteristics of plants when compared to animals is that they are Autotropic, they make their own food rather than obtaining it from other living things. In almost all green, vascular plants the leaf is the place where this food manufacture takes place making them the most important organs of the plant. Plants make a significant investment in their leaves and have evolved many strategies for dealing with the threats to them from seasonal conditions and pests. Not only do leaves make food, they also frequently store it, this concentration of water, carbohydrate (mainly sugars) and proteins make them a big feature in the diet of many animals.

Leaf Phyllotaxy

Phyllotaxy has largely been covered in the section above. But there are two forms of phyllotaxy more relevant to leaves. –

• Heterophylly – where more than one type (Form) of leaf is found on the same plant.
• Anisophylly – where dissimilar leaves occur on the same node.

Also, another arrangement which relates particularly to leaves rather than to plant organs in general. The Rosette.

Leaf Forms

Leaf form is a significant element in plant descriptions and therefore in plant identification. Leaves almost always grow to a shape and form specific to that plant and then stop, their growth is Determinate. They may be Sessile and attach directly to the Stem, or they may be Petiolate and attach via a special stalk, a Petiole. Perennial plants may retain their leaves all year (Evergreen) or shed them annually (Deciduous). Different broad types of plants can be seen to exhibit different types of leaves. –

• Ferns have leaves known as Fronds.
• Conifers have leaves that are typically needle, awl, or scale shaped.
• Grasses (and many Monocots) have sheath leaves.
• Flowering plants (Angiosperms) have the “standard” leaf form including Stipules, a Lamina (Leaf Blade), and a Petiole.

There are two main, basic leaf forms –

• Simple Leaf – has a single undivided Blade, it may be dissected into lobes but the gaps between them do not reach to the main vein.
• Compound Leaves – have fully subdivided blades with each Leaflet being separate along a main or secondary vein. When present the middle vein is termed the Rachis. Each leaflet can appear as a separate Sessile leaf or may have its own Petiole and Stipules. There are some subtypes of Compound Leaves –
  • Palmately compound – leaflets radiate from the end of the petiole, like fingers of the palm of a hand; e. g. Horse Chestnut.
  • Pinnately compound – have the leaflets along the main or mid-vein.
    • Odd pinnate – with a terminal leaflet; e. g. Ash.
    • Even pinnate – lacking a terminal leaflet.
  • Bipinnately compound – Leaves are twice divided: the leaflets are arranged along a secondary vein, that is one of several branching off the rachis. Each leaflet is called a Pinnule, the group of pinnules on the secondary vein forms a Pinna.
  • Trifoliate – A pinnate leaf with just three leaflets; e. g. Clover.
  • Pinnatifid – Pinnately dissected to the central vein, but with the leaflets not entirely separate; e. g. some Sorbus sp. (Whitebeams).

The Full range of Leaf forms is best described visually. The diagrams below cover show shape, arrangement, and details of Leaf Margin, Tip, Base and Venation forms

Leaf Anatomy

Although Leaves almost always grow to a shape and form specific to that plant. flowering plants (Angiosperms) have what is thought of as the “standard” leaf form including Stipules, a Petiole, and a Lamina. As this is the type with the most features, we use it for further exploration of general features.

Large Scale Leaf Anatomy

Apex – The Apex of the leaf has no particular function, but its form can be a determining characteristic in species identification. Apex forms are covered in Leaf Forms.

Lamina – (or Leaf Blade). Leaves are typically broad, flat and thin, in combination with Phyllotaxy this ensures that the maximum area is exposed to sunlight to enable food generation by Photosynthesis. It also maximises surface area contact with the air for efficient transpiration and cooling. Lamina forms are also highly diverse and can be diagnostic, the main types can be found in Leaf Forms.

Veins – The leaf’s veins are its vascular system. They transport water to, and sugars from the leaf. They may form a parallel pattern, common in Monocots, or a Reticulate (net-like) pattern (common in Dicots) though there are many exceptions. The exact pattern of veins is often specific to a plant Taxa (Species) the main types can be found in Leaf Forms.

Petiole – is a special stalk that connects the leaf Lamina to the Stem and bears the Vascular bundles between the two. There are several different attachment styles shown below.

Perfoliolate

Connate-perfoliate

Stipules – are small out-growths that appear at the base of the Petiole. In some cases their function may be fairly obvious but in other cases their function is still obscure to Botany. In the case of Monocots there is usually one, in Dicots, two. Their position, arrangement and structure are termed Stipulation, and all are variable. Some stipules are shed as the leaf unfurls like Convolute Stipules (Bud Scales) and some remain, like Laminar Stipules that act as extra, small leaves. Their form may also be as Glands (e. g. extra-floral Nectaries), or Hairs and Spines with defensive functions. Their attachment may be Free – attached only at the base; Adnate – with one side fused to the Petiole; Ochreate – With either Ochrea or Sheath; Interpetiolar – encircling the Petiole base and connecting between the petioles of two opposite leaves; or Intrapetiolar – encircling the Petiole base and connecting between the petiole and the stem.

Medium Scale Leaf Anatomy

If we now look at Leaf anatomy on the medium scale the size, intricacy and functioning of their extensive networks of Vascular Tissue becomes more relevant.

Trichomes – are the other major leaf feature visible at this scale. They are essentially small hair like structures with many variations and usually with protective functions against environmental threats such as transpiration, frost, UV light, insects and even herbivores.

Trichomes may occur on either the Adaxial (upper) or Abaxial (lower) surfaces of leaves, they may be composed of one, or more cells; may be simple, scale like (Peltate), starlike (Stellate); may be straight and upright, branching, spiralled, or hooked; and they may be Glandular and secrete substances or Eglandular (non-secreting).

• Hair Trichomes – are one of the most common types and there are many descriptive terms used in plant names and descriptions that refer to the variation in hairs a plant exhibits.
  • Glabrous – lacking hairs, smooth.
  • Hirsute – coarsely hairy.
  • Hispid – bristly hairs.
  • Articulate – simple multicellular hairs in rows.
  • Downy – wool like covering of soft hairs.
  • Pilose – long, straight, soft hairs.
  • Puberulent – short, fine hairs.
  • Puberulous – minute, soft hairs.
  • Pubescent – any type of hair or trigone.
  • Strigose – straight hairs pointing more or less the same direction along an edge or midrib.
  • Tomentose – dense, matted, woolly hairs.
  • Villous – long, soft, curved but not matted hairs.

• Glandular Trichomes – are present on around 30% of plants and have been widely studies for the properties of the substances they secrete. The stinging hairs on the Common Nettle (Urtica dioica) are glandular trichomes and secrete Histamine as well as other chemicals. Some of the substances secreted include –
  • Terpenoids – have functions related to growth and development in plants.
  • Phenylpropanoids – mediators of plant interactions in the environment such as stress responses and secondary metabolites.
  • Flavonoids – involved with the colouration of plants and flowers as well as assisting in symbiotic relationships with Fungi.
  • Methyl ketones – involved in sugar production in Photosynthesis.
  • Acylsugars – many properties including deterring pests from laying eggs, toxic to herbivores, and reducing the surface tension of dew allowing the water to be absorbed. Also have many economic uses in pesticides, food additives, cosmetics and personal care products, antibiotics, and anti-inflammatory medications.

Small Scale Leaf Anatomy

Viewed at the small scale the cellular anatomy of the Leaf become visible. There are three main tissue systems present, the Epidermis, the Mesophyll, and the Vascular tissue.

Epidermis – The Epidermis separates the Leaf from the outside world and its waxy  covering, the Cuticle,  prevents water liquid or vapour from moving in either direction and reduces transpiration. While the Cuticle is thicker overall in plants evolved in dry climates it is usually thinner on the lower (Abaxial) surface in all cases. The bulk of epidermal cells are large, unspecialised cells without Chloroplasts. Other cells in the Epidermis are the specialised Leaf Hair or Trichome cells we discussed above, and the Chloroplast bearing cells of the Stomatal Complex.

Stoma – Stomas are small pores in the Leaf’s Epidermis, the majority of which occur in the lower surface. The Subsidiary Cells, Guard Cells and Pore of the Stomal Complex, to which the term Stoma is the collective term, control the rate of the Leaf’s Respiration (Oxygen uptake), Photosynthesis (Carbon dioxide uptake) and Transpiration (Water vapour loss). There may be from two to six guard cells depending on the plant. Respiration and Photosynthesis cannot occur at the same time as Transpiration. Like the rest of us plants can’t breathe in and out at the same time! In response to, light, humidity and Carbon Dioxide concentration, plants use chemical pathways that result in changes in the water pressure within the Guard cells, this causes the Guard cells to flex and the Stoma Pore to open and close. Stomas are usually closed during darkness.

• Palisade layer – is located just below the upper Epidermis and is organised to maximise light exposure. They are cylindrical, elongated cells, with Chloroplasts concentrated at the outer end and slightly separated to allow maximised Carbon Dioxide absorption. In leaves receiving the most light there may be more than one layer.
• Spongy layer – is located under the palisade layer and above the lower Epidermis. Cells in this layer are branched and not so tightly packed. Gases flowing into the spongy layer through Stomata’s can access the Palisade layer during Respiration, Photosynthesis and Transpiration.

Modifications

There are a huge number of adaptations that plants have evolved to their Leaves, their leaf features and their Petioles for various functions including, but not limited to –

• Waxy micro- and nano-structures on the surface reduce water loss, wetting by rain and adhesion of contamination.
• Divided and compound leaves reduce wind resistance and promote cooling.
• Hairs on the surface trap humidity in dry climates and create a boundary layer of air reducing water loss.
• Large surface areas maximise capture of sunlight.
• Specialised, opaque or partly buried leaves, admit light through a translucent window for inner photosynthesis in harmful levels of sunlight.
• Succulent leaves store water and organic acids.
• Inclusions of crystalline minerals, Spines, stinging hairs, or oils, toxins, pheromones produced by glands deter herbivores.
• Special leaves on carnivorous plants adapted for trapping food.
• Bulbs store food and water.
• Tendrils allow the plant to climb.

Photosynthesis

The TLDR of Photosynthesis is that it is the process that plants use to convert Light Energy into Chemical Energy that is then used to fuel their activities through Cellular Respiration.
The equation can be explained as –
Carbon Dioxide + Water > (Light Energy)> = Sugar + Oxygen, or in symbols as 6CO2 + 6H2O ⇒Light⇒ C6H12O6 +6O2

A longer explanation can be found here.