Basic biology of plants.--2- Foliage and their functions.

In this second page of the series looking how plants and trees live and thrive we follow up PLANT BASIC BIOLOGY--1 ROOTS AND RHIZOMES, with a review of the foliage and their functions. As with the previous page I will try to keep the text as simple as possible so as not to get bogged down with technical details or complicated Botanical terminology.

We all know what leaves are they are a feature of our every day lives. But what is the function of these organs to the flora to which they are attached? Leaves are the main component by which a plant breathes and they harness the energy provided by the sunlight passing through the chlorophyll, which absorbs carbon dioxide and releases oxygen back into the air..

Another important function of the leaf is giving off surplus moisture {transpiration}  and the formation of food reserves. To achieve the delicate chemical reaction of changing carbon dioxide into oxygen, it is dependent on the amount of daylight available and the air temperature. Thus during the hours of darkness this reaction can not occur. The chlorophyll {responsible for the green colouring} is no longer required. Starches and other nutrients stored is returned via the veins to the parent plant, and the green turns to russets and gold before their eventual departure from their  grip.

This alludes to deciduous trees, those species of plants and trees that loose all their leaves for part of the year. The term for this process is abscission { from Latin ab-meaning away +sussio-a cleaving}. The word deciduous  derives from the Latin deciduous meaning to fall from decidere-to fall down}

Below --the maple tree in autumn colour

photograph by Dal

Some trees----

Some trees including young oak and beech cling on to their brown, withered leaves through the winter months, only discarding them when the new buds of spring force them off. These leaves are termed by Botanists as being marcescent leaves from the Latin marcere meaning to wither.

Below--young tree holding on to its brown, withered leaves


Seed bearing plants.

Most of the foliage of seed bearing plants and trees consist of a leaf stalk referred to as a petiole, a leaf blade referred to as the lamina and stipules which are small growths located at either side at the base of the petiole. Where the leaf stalk joins the stem or twig it forms a junction known as the leaf axil. Not all species produce foliage with petioles.

The leaf consists of 3 main types of tissue---

The epidermis from the Greek epi-meaning upon, above, or over + derma meaning skin. The epidermis forms a protective outer covering on the upper and lower surfaces.

An interior chlorenchyme called mesophyll from the Greek misos, meaning middle+ phyll indicating type.

An arrangement of veins is the vascular tissue.

Basically the epidermis protects the inner cells of the leaf, but, it is also responsible for other actions such as protection against water loss by way of transpiration { to loose water through vapourisation especially through the stomata of the leaves} The epidermis covering many leaf species is thinner on the lower surface than on the upper surface. It is built up more on leaves from dry climates as opposed to those that occur in wet climates. The epidermis is covered with pores known as the stomata { from the Greek word meaning mouth}

Veins are vascular tissue of the leaf and are situated in a spongy area of the mesophyll. The various patterning  formed by veins is called venation.Veins have two types of tubes-xylem that brings water and liquid nutrients from the roots to the leaf while the second type-phloem tubes usually move sap, with dissolved sucrose {natural sugar} in the leaf -out of the leaf.

BELOW---the various pattern formed by the veins are called venation

photograph by Dal

Shape and texture of foliage

Now we come to the shape and texture of the leaves, for this is an important identifying feature of many species of plants and trees particularly when no flowers are present. Foliage is positioned on the twig or stem in various arrangements. Below are examples of these arrangements.

Arranged Alternately---leaf arranged alternating along the stem. For example one leaf will face left appearing from a single node, while higher up the stem and on the opposite side  the next node will produce a leaf facing to the right.

Arranged Opposite--- two leaves opposite to each other on either side of the stem.

Arranged in whorls--three or more leaves produced together at the same node on the stem. Forming a circle around the stem.

Below--The foliage of cleavers Galium aparine has foliage arranged in whorls

phtograph by Dal

Arranged rosulate--the leaves form a rosette .

Which ever way the leaves are arranged it is one principle reason, to obtain the maximum light and sunshine on to each of them.

                                                  TYPES OF LEAF

The simplest form of a leaf is named just so--a simple leaf. This type of leaf, is to most people, the familiar type. It has an undivided blade known by botanists as the Lamina {from  Latin meaning thin plate} . However, the shape of the leaf can vary quite substantially by way of the lobes.

BELOW--Nettle leaves are simple with toothed margins.


type of foliage continued

The next type of leaf under review is the compound leaf which has a divided blade, ie, the leaflets on opposite side of a central stem which is known as a rachis, from the Greek rakhis meaning a ridge. A good example of a compound leaf are members of the pea and bean family especially the vetches.Unless you have a good knowledge of leaf morphology this group of leaves can be difficult to classify. Compound leaves come in various forms and are named by botanists as follows----

Palmately compound leaves--- have lobes that spread out like fingers on a hand, good examples are horse chestnut and buckeyes.


Below the palmate foliage of the horse chestnut  MIDDLE--ash leaves are odd pinnate because of they have a terminal leaflet. Bottom the trifoliate leaves of the Laburnum tree


leaf types continued.

Pinnately compound--have leaves arranged along the rachis { central stem } There are Odd Pinnate these have leaflets on either side of the rachis in addition they have a single teminal leaflet, such as the ash and rowan foliage.. Even Pinnate--lack the terminal leaflet.

Bippinately---have leaflets that are formed along several branches that spread out from the rachis; thus the leaves are twice divided.

Trifoliate---a pinnate leaf with just three leaflets such as the foliage of clover and the laburnum.


Related pages.

Plant Basic Biology-1----{roots and rhizomes}


Plant Basic Biology-3--Foliage and their functions two. Leaf shapes

Plant basic biology-4--flower parts 

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