Atlant’s Column: Blades of Steel

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Welcome to the section on Micronaut.ch which explores, analyzes and explains fascinating microscopical structures.

Grass is the staple food for many organisms ranging from mammals, insects, and even fungi. That does not mean that grass makes an especially good fodder. Quite the contrary is the case. It does everything possible to spoil the appetite of the herbivores.

Text by science writer Atlant Bieri
Photography by Martin Oeggerli

The first defence is aimed at large grazing animals such as cows, horses or donkeys. The tapered structure right of the centre is a downward hook or “tooth” (shown at right), as biologists say. They cover both sides of a grass blade. The result is an extremely rough surface texture. This feels like sandpaper in the mouth of a cow and is extremely unpleasant. Thus, the animals prefer eating plants that are not littered with such micro-spines. The reason why cows today still enjoy eating grasses is because over many decades we have bred grasses without teeth.

Undemanding animals such as sheep, goats or field mice get along without our support. They don’t mind the reinforced greenery very much. For this target group, grasses have a second weapon in their arsenal. On each blade there are thousands of small bumps. Their number exceeds by far that of the teeth. Every single of them contains a crystal made of silica. They are also called “phytoliths” (plant stones) or “plant opal” because their chemical structure is very similar to the one of the jewels.

 

Chasing Off Herbivores with Ivories Made Out of Steel

Phytoliths have a hardness of 6.5, which is comparable to steel. Thus, grasses carry some kind of chain mail. This has direct consequences for the herbivores because the teeth of sheep only have a hardness of 5. In other words, while chewing on the steeled stalks their teeth constantly wear off. The animals know this and therefore do not feed exclusively on grass, otherwise they would soon have no teeth at all and starve to death. What is more, the steel armour automatically adapts to a particular threat. Once a blade is nibbled, it produces large quantities of silica at the breakage. This has an immediate effect on the feeding behaviour of mice, as scientists could show. In three-quarters of all cases they left the stalk in order to find another food source.

 

No Land of Milk and Honey for Bacs

The most numerous grazers are at the same time the smallest. Bacteria and fungi desire the nutrients inside the cells. In order to guard themselves against these ubiquitous vegetarian hordes, grasses have a third line of defence. Their entire surface is seamlessly coated with two layers of wax. The first consists of the so-called cutin, a plant made hard plastic, which covers the cells like a film and protects them from mechanical penetration of the microbes. The second layer consists of a forest of wax crystals, which are placed onto the cutin. It makes the blade water-and stain-resistant. In this way, bacteria and fungi find it very difficult to spread on the stalks, as they both need water if they want to prosper. The supposed food-paradise turns out to be a dry desert in which the microbes have to fight for their survival.

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Continue reading Atlant’s Column: The Army of Micro-Robots, June 2012, or go back to the archive