External Features of Plants

File photo: Plant stem structure | Credit: Patch PlantsOpens in new window While the internal structure of a plant organ can give us an idea of its function and how it is designed to carry out that function, it is the external appearance of plant organs, the plant’s morphology, with which we are most familiar.

Most plant species at first sight appear very similar, since all four organs, the root, stem, leaf and flower, are present in approximately the same form and have the same major functions. However, it is the differences in appearance of these plant parts that enables us to distinguish between them. An appreciation of how plant parts vary in their size, shape, color or other characteristics help us to correctly identify plants.

External Features of Roots, Stems, Buds and Leaves

1.   Roots

The main functions of the root system are to :

• take up water from the growing medium
• take up mineral nutrients from the growing medium
• anchor the plant in that medium.

To achieve maximum water and mineral uptake, roots must have as large a surface area as possible. The root hairs, which can be seen just behind the root tip, greatly increase the root surface area with as many 200 – 400 hairs per square millimeter.

The loss of root hairs during transplanting can check plant growth considerably, and the hairs can be points of entry of diseases such as club root. Root hairs are replaced frequently as the root grows, a single rye plant producing more than a million a day!

Two main types of root system are produced (Figure X-1). A taproot (primary root) is a single large root which grows directly from the radicle in the embryo (Figure X-2. It has many smaller lateral roots (secondary roots) growing out from it at intervals.

Figure X-1 Generalized plant forms: (a) a grass (a monocotyledon); (b) a dicotyledon | Source: BiocyclopediaOpens in new window

Taproot systems are a distinctive feature of dicotyledons (e.g. chrysanthemums, brassicas, carrots). In contrast, a fibrous root system consists of many roots with no dominant root. It is characteristic of monocotyledonous plants such as grasses but can also be found in dicotyledons such as Senecio vulgaris (groundsel).

Figure X-2 Germinating Vicia fabia (broad bean) seed showing the radicle developing into a tap root with laterals | Source: ResearchGateOpens in new window

Adventitious roots do not derive from the radicle of the plant embryo. They tend to grow in unusual places such as on the stem or other organs. Most fibrous root systems are made up of adventitious roots which grow from the bottom of the stem, with the primary root failing to develop or dying away.

2.   Stems

The stem’s main functions are to:

• physically support the leaves in the optimum position for photosynthesis
• physically support the flowers in the optimum position for pollination
• transport water, minerals and food between roots, leaves and flowers.

The leaf joins the stem at the node and has in its angle (axil) with the stem an axillary bud, which may grow out to produce a lateral shoot or in some instances flowers. The distance between one node and the next is termed the internode. Stems of herbaceous plants need to maintain a high water content to provide turgor pressure to support the plant.

Winter stems in deciduous woody plants often show a leaf scar where the leaf was attached and a bud scale scar (girdle scar) where last year’s apical bud was positioned and this can be useful in determining which part of the stem is current, one-year-old or two-year-old wood when pruning (Figure X-3).

Figure X-3 External features of a woody stem in Aessculus hippocastanum (horse chestnut) | Source: ResearchGateOpens in new window

Young stems may be green and carry out photosynthesis. In common with other plant organs, stems are enclosed by the epidermis which contains stomata, pores which allows gas exchange between the air and the living tissues inside the stem.

When a stem becomes woody, the epidermis gives way to a waterproof and gas-tight bark layer. The stomata are then replaced by breathing pores called lenticels. lenticels may be useful in identification; in Prunus species they form very distinctive horizontal lines on the smooth bark.

The color and texture of plant stems are decorative features which can also be an aid to identification. For example, Cornus alba “Sibirica” (a dogwood) has bright red stems in winter (Figure X-4), many Salix spp. (willows) have bright yellow or green stems even when woody. Stem color is best in young stems so these shrubs are often pruned to the ground each year in winter to produce new stems the following season (coppicing).

Figure X-4 Red stem color in dogwood (Cornus alba “Sibirica”) | Source: The SpruceOpens in new window

Many trees have highly decorative bark, which bring winter interest to the garden. These include many Eucalyptus spp., Betula utilis var. jaquemontii and other white stemmed birches, Prunus serrula var. tibetica (Tibetan cherry) with shiny red bark, Acer griseum with peeling cinnamon-colored bark and the snakebark maples Acer davidii, A. pensylvanicum and A. capillipes

3.   Buds

A bud is a condensed stem which is very short and has small leaves attached, both enclosing and protecting it (Figure X-5). Buds are found at the apex of the shoot and in the axils of leaves with foliage buds containing numerious folded leaves and flower buds containing the immature flower.

Figure X-5 Structure of a bud: (a) Brussels sprout; (b) magnified image | Source: BiocyclopediaOpens in new window

On the outside of the bud, the leaves are often thicker and darker forming bud scales to resist drying and damage from animals and disease. They may contain chemical inhibitors which delay bud break until the spring.

The Aesculus hippocastanum (horse chestnut) buds, for example, exude a sticky substance to deter insects. A terminal (apical) bud is present at the tip of a main stem or branch and contains a meristem from which lengthwise vegetative growth or, less commonly, a flower will emerge. Where leaves join the stem, axillary buds may grow into lateral shoots or flowers or may remain dormant.

Bud characteristics may be useful in identifying plants — for example, the native ash Fraxinus excelsior has black buds, those of beech (Fagus sylvatica) are long and pointed and Magnolia buds are hairy.

Flowering buds tend to be much larger and plumber than the vegetative buds, which give rise to new shoots and leaves and this is useful when pruning. In spur pruning or apples, for example, the vegetative buds on the ends of lateral shoots are easily identified and removed encouraging development of flower buds.

4.   Leaves

The main function of the leaf is to carry out photosynthesis. The leaf consists of the leaf blade (lamina) and stalk (petiole) (as shown in Figure X-1), its shape and arrangement on the stem depending on the water and light energy supply in the species’ habitat.

Sessile leaves lack a petiole, while in peltate leaves such as Nasturtium, the petiole attaches to the center of the lamina rather than the base. Many monocotyledonous leaves also lack a petiole.

Figure X-6 Few samples of leaf shapes | Source: ResearchGateOpens in new window

Leaf shape and structure are useful indicators when attempting to identify a plant and descriptions often include specific terms. A few are described below but many more are used by botanists:

In cultivated plants, leaf size and shape, color and variegation are also important identifying features since many cultivars have been bred to differ from the original species in these respects. In some plants, leafy structures called stipules may be found at the base of, or attached to, the petiole as in Rosa spp.

The arrangement of leaves on the stem is also an important identifying feature. For example, Salix alba has leaves arranged alternatively along the stem whereas Cornus alba leaves are attached opposite each other. Leaves may also be attached in whorls around the stem as in Lilium.