Saturday, October 26, 2013

Overheating sweet potato?

One of the best ways to learn more about food is to grow some of the fruit and vegetables from exotic fresh produce that you can buy in supermarkets. Over the years I've grown avocados, litchi, rice, lentils, chickpeas, figs, citrus fruits, lemon grass, ginger, chilli, cape gooseberries, squashes, passionfruit, sesame and pomegranates from supermarket produce - but this was the first time I'd ever grown sweet potatoes Ipomoea batatas.

After a slow start the tuber produces some healthy vigorous shoots but then ....

...... these white granules appeared on the leaves. At first I thought they might be some small insect pest but ...

.... a closer look revealed that they were part of the leaf surface ....and under the microscope ...

..... they turned out to be clusters of swollen, glassy secretory hairs, known botanically as colleters. They're present on upper and lower leaf surfaces.

At higher magnification you can see how inflated the hairs are. They are full of what appears to be ....

...... mucilage that flows out when they burst. 

The plant is in a flower pot on a sunny widowsill so the growth conditions are unnatural. Some older leaves have a large number of colleters, others have none. 

I don't have any clear idea of what triggers their formation or what their function would be. Some plants dump waste products in leaf surface structures like this and I wondered if they might contain calcium oxalate crystals, that are usually compartmentalised waste products in plant cell vacuoles, because sweet potato tubers do contain calcium oxalate - but I can't see any under the microscope.

Curious. I can only find two relevant research papers. 

One, from Brazil and published last year (1), describes secretory colleters in Ipomoea asarifolia, a weedy species that poisons cattle but is also used in herbal remedies.

The other, published in the Australian Journal of Botany a couple of years ago, describes similar structures on the calyx in flowers and fruit of a Ipomoea cairica (2), and the authors speculate that the secretion, which crystallises but is hygroscopic and becomes a gel in moist conditions, may have a role to play in protecting the plant from drought. That seems plausible, because the south-facing windowsill where the plant stands gets very warm on sunny days. 

So maybe by producing these colleters the plant is telling me that its too hot and needs to be watered more often ...

1. Martins, Fabiano M.; Lima, Jamile F.; Mascarenhas, Ana Angelica S.; et al.(2012) Secretory structures of Ipomoea asarifolia: anatomy and histochemistry. Revista Brasileira de Farmacognosia – Brazilian Journal of Pharmacognosy.  Volume: 22   Issue: 1   Pages: 13-20   

2. Sousa Paiva, Elder Antonio; Martins, Luiza Coutinho (2011) Calycinal trichomes in Ipomoea cairica (Convolvulaceae): ontogenesis, structure and functional aspects. Australian Journal of Botany. Volume: 59   Issue: 1   Pages: 91-98   

Sunday, October 6, 2013

A tiny aquatic worm that clones itself

All summer, small containers of various kinds in our garden have collected rainwater and detritus - and each of these microcosms has developed a fauna of its own. This is a little oligochaete worm called Aeolosoma that I found in the layer of mouldering leaves at the bottom of one of these little pools.

Oligochaetes are annelids (segmented worms) whose bodies have only a few bristles on each segment. This species, less than two millimetres long, is almost completely transparent and has distinctive little orange spots just under the body surface. If you look closely you can also just make out the fine bristles at the junctions of the body segments. It whisks food into its mouth, which is under and towards the back of that spade-like structure called the prostomium, with fine cilia that beat and generate a water current. It's thought that that spade-like prostomium can attach like a sucker to a substrate so that the beating cilia generate suction, aiding feeding.

Aeolosoma divides asexually, budding off new individuals from the tail end, so it's quite possible that the thriving colony (I found six in a single drop of water so there must be thousands in the container) are all descended from a single original colonist.

In the image above you can see the prostomium being used rather like a vacuum cleaner nozzle.

Sunday, March 17, 2013

Fatal Attraction

Fungus gnats that emerge in swarms from soil in plant pots have become the bane of many gardeners' lives. If you grow plants in commercial potting composts on your house window ledge or in a greenhouse or conservatory, it's inevitable that you'll encounter these irritating pests because it seems that all currently available bags of potting compost are infested with them.

These little insects are scientifically known as Bradysia paupera and belong to a group known as sciarid flies. Each female can lay around 200 eggs which hatch into a worm-like, transparent larva that feeds on organic matter in the soil and also on young plant roots. A heavy infestation is capable of killing seedlings. They breed all-year-round, with overlapping generations that take less than a month to progress from egg to adult, so combating them is a constant challenge, but fortunately they have a fatal weakness - the colour yellow. They are attracted to these sticky yellow sheets of plastic that you can buy in garden centres and are glued to them as soon as their feet touch the surface.

Yellow strips of sticky plastic plastered with dead flies are unsightly but there is a more aesthetically attractive alternative - the carnivorous butterwort, Pinguicula sp., whose sticky leaves are like natural flypaper and which produces attractive flowers throughout the year. To see how effective this is, scroll down to the bottom of this post, and to see how it works, click here.

The little club-shaped structures on either side of the insect are halteres - balancing organs which smooth its flight path as its wings beat up and down.

Long-legged sciarid flies spend much of their time running around over the surface of the soil, where they lay their eggs.

Whiteflies caught on the sticky hairs of a butterwort leaf

Sciarid flies trapped on the sticky surface of a butterwort leaf

Sunday, January 6, 2013

Scale insects ....

 I recently discovered that the orchid on my desk is infested with these tiny scale insects Coccus hesperidium. Each one looks like a miniature tortoise, about 3-4mm. long and tightly attached to leaves.

The tell-tale symptom was the sticky secretion on my desk under the leaves - scale insects suck sap, like aphids, and excrete honeydew.

When I took a close look it was easy to see how the infestation had built up so rapidly. Under the heat of the microscope lamp these infants, known as 'crawlers', emerged from under their mother. The females reproduce parthenogenetically, producing about 1000 nymphs during their three month life span and sheltering the young under their shield.

When you flip a scale insect over you can see the hollow cavity which acts as a nursery for the nymphs. Each is less than 0.25mm long

Nymphs are flattened and are very active, but as they begin to develop their broad shield they settle in one spot to feed.

Here's an older individual, where you can see the shield beginning to grow outwards around the insect. The waxy shield makes these insects impervious to most insecticide sprays - so they are difficult to control. Suffocating them under horticultural oil sprays or picking them off laboriously with a paintbrush dipped in alcohol are really the only effective treatments for infected plants.

This nymph, which is about 3mm. long, has settled to feed. It's probably about a month old.

After two months they reach maturity and begin to reproduce, and then their shield grows darker as they age.

Scale insects infest a very wide range of plant hosts. You can read more about them by clicking here.