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.