Well-Travelled Fungus

Groundsel cluster cup fungus, looking like minute tarts, on an infected groundsel leaf surface. The spores are produced in vast numbers in the centre of each cup. Each cluster cup is about as large as a full stop (period) on your monitor screen. A minute larva of a fly is crawling across the leaf surface, just to the right of the open cups, and may have been feeding on the fungal spores. In the bottom right corner (below) cluster cups are just forcing their way to the leaf surface, ready to open.

Cluster cups erupting from an infected, swollen stem. Infected stems often develop purple pigmentation and become distorted. Heavy infestations can be fatal.

 

Above: A vertical section through a cluster cup on a leaf surface, x100, showing the chains of spores that are formed in the centre of each cup.  

Below: Although apparently heathy, this plant is infected with the fungus, just visible on the surface of the bottom leaf on the left in the middle of the picture 

Take a look at the weed groundsel Senecio vulgaris stems and leaves in autumn and you’ll often find that they’re swollen and distorted, with a patches of a yellow fungus erupting from their surface. The infection is a fungus called groundsel clustercup Puccinia lagonophorae, which has an interesting history, having travelled more than half-way round the world since the mid-20th. century. Migratory people tend to take weeds, as well as their crops, with them and groundsel was accidentally taken to Australia by early settlers from Britain, where it became infected with this fungus which is native to Australia. Some clustercup-infected groundsel made the return journey and the fungus arrived in Europe in 1961, where it has been spreading ever since. More recently, within the last decade, the fungus crossed the Atlantic and has begun infecting groundsel that had been taken there by early European settlers. The Americans are not altogether sorry that the fungus has arrived, because it weakens groundsel and might offer a means of biological control of this invasive weed. In the photographs you can see the flower-like spore cups, called aecia, that produce the infective golden yellow spores. In the vertical section through one of these cups (second photo from bottom) you can see the chains of spores budding off from the fungal hyphae.

The Wonder of Whiskers

Groundsel flowers self-pollinate and run to seed with remarkable speed. Each seed is carried aloft on a parachute (pappus) of hairs...but there are other, much smaller hairs with a different function attached to the seed coat itself....

The seed coat also has a covering of microscopic hairs, far smaller that the parachute hairs, that rapidly extend outwards at right angles from the seed coat when it becomes wet. x40
A forest of minute seed coat hairs, at higher magnifcation (x100)

A single seed hair, magnified x400, using interference contrast microscopy that reveals hidden structures within the cell that the hair is formed from. Each hair is made up of a woven spiral column of threads, like a piece of string........or a water-absorbing wick?

Groundsel Senecio vulgaris is one of the most successful colonisers of cultivated land, thanks to its capacity to grow rapidly, flower quickly and disperse its seeds far and wide on tiny hairy parachutes that will carry them over large distances. But it has another winning adaptation for rapid invasion of ecological niches too, that I only noticed by accident recently. The Latin generic name of groundsel – Senecio – means ‘old man’ and alludes to its parachute of whiskery, silvery hairs that transport the seeds on the breeze. But, as I discovered yesterday, groundsel has a hidden complement of hairs that are invisible to the naked eye that have a quite different function: anchorage and water uptake. I’d put some groundsel seeds in a drop of water on a microscope slide, to photograph their parachute hairs, and was amazed to see a mass of writhing, much tinier hairs begin to extend from the coat of the seed, until hundreds were extended at right angles to the seed coat surface. When the seed is dry these are pressed so close to the seed coat that you don't notice them, but once it’s wetted they extend. What’s their function? Well, my guess is that they anchor the seed to a damp soil surface and then act as wicks, conducting water to the seed and speeding up germination. Notice how, at high magnification, each hair resembles a woven piece of string. Groundsel seeds germinate and establish themselves as seedlings incredibly quickly, and my guess is that this surface seed coating of ‘wicks’ is a crucial adaptation for helping airborne seeds that have landed on the soil surface to take up water and germinate as quickly as possible.