Plants may appear to be helpless in the face of hungry,toothy animals but many plants have evolved interesting systems of self-defense.

Coarse meadow grasses are a good example. If you run your finger along a blade of coarse grass, you just might cut yourself. What makes it so sharp? A row of microscopic silica blades lines the grass like tiny teeth, and most grasses have silica inside as well. Although the silica doesn’t prevent animals from feasting on the grass, it is hard enough to wear down the animals’ teeth over time. Imagine trying to chomp that tough meadow grass with dull, worn teeth, you wouldn’t get too far!

The grasses must have evolved this bristling defense fairly early in their relationship with grazing animals, because the anatomy of the animals’ mouths has changed in response. The teeth of most mammals stop growing in adulthood. But, the teeth of grazing animals like horses and cows have evolved in a different way. They continue to grow from the roots over the course of the animals’ lives. As the grasses’ silica blades wear away the teeth, continuous growth from the roots at least partially compensates for the loss. If an animal’s diet consists mostly of coarse meadow grasses with silica blades, the animal still could develop extensive wearing and damage to its teeth.

　　植物的自我防御!

　　植物在饥饿的咀齿动物面前似乎毫无防御 ，然而许多植物已经进化出了有趣的自我保护体系。

　　粗糙的蓝草是一个很好的例子。当你的手指滑过它的叶子，很可能将自己划伤。那么它为什么如此锋利呢?因为在蓝草叶片边缘有一排细小的像牙齿似的硅片，并且 大部分蓝草体内也含有硅。虽然这些硅质不能阻止动物的尽情咀嚼，但它的硬度很高，长期食用会磨损动物的牙齿。试想一下，用又老又钝的牙齿咀嚼那样的牧草， 显然不会逍遥很久。

　　蓝草一定很早以前就在与草食动物的“对抗”中进化出这种密集的齿状硅片。因为动物的口腔结构发生了相应的变化。大多数哺乳类动物的牙齿在成年以后就停止了 生长。但是马和牛等食草动物的牙齿，却产生了不同的进化。它们从牙根不断生长，贯穿动物的整个生命周期。由于蓝草的硅片磨损牙齿，从牙根不断生长的牙至少 部分弥补了磨损。如果食草动物的食物以这种粗糙蓝草为主，那么磨损仍会加剧并对牙齿造成伤害。