Past its Use-by Date .....

If you have ever gone away on holiday and forgot that you left some cheese in the cheese dish, then ....

..... this will be a familiar sight when you return home. This slab of Cheddar has become .........

...... a battleground for fungal colonies ............

....... that jostle for supremacy when the colonies collide, and in doing so create a rather attractive, furry abstract design.

Once the mould has smothered the cheese surface it's time to reproduce ....

..... via stalked sporangia ......

.... that resemble little white pom-poms...

.... each of which releases .......

... vast numbers of these minute conidiospores, each just a few thousands of a millimetre in diameter.

Spud-You-Won't-Like......

Eating green potatoes that have been stored for too long in bright light is a big mistake. They can make you very ill - not because of the green pigment which is chlorophyll and is no more harmful than eating green lettuce, but because potatoes that are exposed to light produce a toxic glycoalkaloid called solanine below their skin. It's most likely a natural defence mechanism, to protect the plant from insect pests and fungal pathogens.

                                         

This is a section taken perpendicular to the potato surface, through those green cell layers. It's been stained with a fluorescent dye that has a particular affinity to the toxin, which fluoresces brightly in its presence, so you can see glowing crystals of solanine inside these cell layers. The rounder, translucent greenish objects are starch grains.

Wild potatoes have a much higher solanine content than cultivated varieties. Part of the domestication process of many of our crops has been selective breeding to remove natural toxins that protect the plants from pests and diseases - but also poison people. That's why we have to use applied pesticides on crops, to replace their natural equivalents that have been bred out of the plants, whose defences have been weakened in order to make them edible: it's a vicious circle!

Plant Harpoons

This lethal-looking weapon, just a couple of millimetres long, is the defensive weaponry deployed by a prickly pear cactus called Opuntia rufida. Most prickly pear species are armed with formidable spines that are several centimetres long and capable of drawing blood but this species has a surface ....

... covered with these small areoles - dense clusters of tiny, rigid hairs called glochids that are only loosely attached to the plant and ....

...... are easily dislodged by the slightest touch - or even by the wind. Those in this picture were gently brushed and you can see how they've broken loose. 

Each glochid is tipped with a sharp point (here magnified x100) that easily penetrates soft flesh like the lips and eyes of an animal attempting to eat the plant .....

..... and backward pointing barbs make it very difficult to remove. These microscopic harpoons are intensely irritating and potentially dangerous if they end up in your eyes, mouth or throat. The easiest way to remove them from skin is to use sticky tape to pull them out but if they end up in more vulnerable areas you may need hospital treatment. You can find medical advice here.


Opuntia rufida grows in arid parts of Texas in the United States. For more on prickly pears, click here.