This month's features in our early
autumn issue :
At this time of year as autumn approaches many flowers are setting seed. So this month why not examine in close-up the wonderful colours, shapes and textures of plant seeds and seed heads in your garden or local countryside. Only a low power stereo microscope or 10X hand lens is required. At higher magnifications why not also take a look at the intricate detail of butterfly wing scales.
Please read the important notes on collecting.
Although there are still many plants in flower in the author's area, most are now setting seed or developing fruits. So this is an excellent time of year to admire the wonderful variety of forms of seeds, and how they often relate to their method of dispersal.
The dandelion (genus Taraxacum) is a very familiar flower in the UK, particularly when it produces the attractive 'clock' of seeds which are easily dispersed by the wind. A partly dissected seed-head is shown at the top of this page to show how each individual seed is inserted into the remains of the flower-head. The Dandelion is a member of the Compositae family because a flower-head is actually a composite of small flowers called florets. Each floret produces a single seed called an achene with a thin stalk (the beak) attached to the feathery hairs (the pappus).
Each seed with it's miniature 'parachute' is a marvel of nature's economical engineering and serves admirably to aid their dispersal when the wind dislodges a seed with pappus from the seed-head. The seeds have an attractive ribbing which is shown in the image above left.
The docks (genus Rumex) are a common flower in the UK but are often overlooked by the casual observer because of their undistinguished greenish flowers which are typically on whorls in a flower-spike. The flower structure and seed heads are unusual and worth studying in close-up ( a guide to wild flowers should give a general impression of the type of plant to look out for).
A close-up of a whorl of flowers is shown on the right. The flowers typically have a three-sided symmetry, with three wings and often have one or more small swollen tubercles (shown red in the image) between each wing. As the fruits develop the flower spike in some species turns an attractive rusty brown colour.
The clover, genus (Trifolium), needs no introduction as it is common as the red or white clover in grassland. Clovers are members of the pea family Leguminosae and the tiny pea-flowers form the familiar attractive globular clusters of flowers. They are also attractive when they turn to seed because the flower heads turn a rusty-brown colour, and a typical example is shown in the image left.
The clover is very valuable to the farmer because it is highly palatable to livestock and is high in protein, phosphorus and calcium. Clovers are also valuable improvers of soil because of the bacteria in the root nodules which have the ability to 'fix' atmospheric nitrogen.
One of the benefits of a nature walk in Autumn is eating the edible fruits that are beginning to ripen! A common one in the author's area is the bramble (genus Rubus) which after forming white or pink flowers develops the familiar blackberries. It is interesting to see how the fruits slowly develop from the flower. A fruit in an early stage of development is shown on the right. The fruit is still green and the remains of the anthers from the flower can be seen.
Fruits in a strict botanical sense are the fleshy or dry ripened ovary of a plant enclosing the seed or seeds. Thus acorns and almonds are also strictly fruits, but the term is popularly reserved for sweet, succulent and pulpy fruits such as the blackberries above, bananas, grapes etc. A fruit enclosing the seed has benefits for the plant in that the seed is protected, and the fruit attracts birds and animals to eat the fruit and disperse the seeds further afield. This contrasts with the dandelion above which has an unprotected seed and relies primarily on wind dispersal, although birds do eat and help disperse the seeds of some Compositae flowers as well.
At this time of year as the butterflies are finishing their breeding and egg-laying, butterflies are often found dead on the ground during a walk. It is well worth carefully collecting a few of the intact butterflies and taking home in a container for closer observation. Examining the wings under a stereo microscope or compound microscope at about 40X will show the arrangement of the wing scales. The scales overlap in rows similar to tiles on a roof and are actually modified hairs. In some of the more primitive members of the Lepidoptera (= scale wings) the scales are distinctly hair-like. The scales shown left are those forming the black markings on the wings of the large white butterfly (Pieris brassicae). The visible size of each overlapping scale shown is typically 105um long and 50um wide (um=micron, 1/1000th of a milllimetre). If you examine a dislodged scale you should see the peg at one end which fits into a pore in the wing membrane.
The scales and hairs on the butterfly wing are responsible for the colours and patterns. Some of the colour is caused by pigmentation created by chemically diverse compounds such as melanins and flavones. Blue and green is a rare pigment in butterflies and moths although a common colour of the wings. These colours are typically created by the microstructure of the hairs and scales which refract light to produce these colours as well as the metallic and iridescent effects.
To appreciate how scales can do this, use a 40x objective on a compound microscope to examine the scales. An image of the scales at this sort of magnification is shown on the right. Each scale has longitudinal and horizontal ribbing which can act as a diffraction grating to produce interference colours. Some of this structure is so fine as to require a scanning electron microscope to fully appreciate. The combination of the pigmental and structural colours often creates striking effects.
Why not join me next month to examine more nature in close-up in the Autumn. Next month's issue will complete a full year of Walk pages.
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Fruits and seeds
There are many wild-flower books available for the naturalist many of which should show the diversity of seeds and fruits of flowering plants. For the botanist, the fruits and seeds of plants are often used as a critical diagnostic character to help identify the plant.
Butterfly wing scales
An excellent introduction to the structure of butterfly wing scales which is illustrated with SEM photographs is:
'The Wings of the Peacock Butterfly' by M O Moss and G Gibbs, Quekett J. of Microscopy, 1995, 37(5), 392-395.
The seed and fruit images were taken using a CCD camera with a
50mm SLR camera lens and extension tubes attached. The butterfly
scale images were taken with the CCD camera attached to a Russian
Biolam compound microscope with no eyepiece using a 3.5X and 20X
Camera images were transferred to the PC using a Creative Video Spigot capture card.
Image manipulation using Photostyler v2.0 software.
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The author Dave Walker
is a UK based amateur naturalist keen to encourage people to
explore nature in close-up.