text: A plants leaves and stem may be covered with sharp prickles, spines, thorns or trichomes- hairs on the leaf often with barbs, sometimes containing irritants or poisons. Plant structural features like spines, thorns and  awns reduce feeding by large ungulate herbivores (e.g. kudu, impala, and goats) by restricting the herbivores feeding rate, or by wearing down the molars. Trichomes are frequently associated with lower rates of plant tissue digestion by insect herbivores. Raphides are sharp needles of calcium oxalate or calcium carbonate in plant tissues, making ingestion painful, damaging a herbivores mouth and gullet and causing more efficient delivery of the plants toxins. The structure of a plant, its branching and leaf arrangement may also be evolved to reduce herbivore impact. The shrubs of New Zealand have evolved special wide branching adaptations believed to be a response to browsing birds such as the moas. Similarly, African Acacias have long spines low in the canopy, but very short spines high in the canopy, which is comparatively safe from herbivores such as giraffes.
  Trees such as palms protect their fruit by multiple layers of armor, needing efficient tools to break through to the seed contents. Some plants, notably the grasses, use indigestible silica (and many plants use other relatively indigestible materials such as lignin) to defend themselves against vertebrate and invertebrate herbivores. Plants take up silicon from the soil and deposit it in their tissues in the form of solid silica phytoliths. These mechanically reduce the digestibility of plant tissue, causing rapid wear to vertebrate teeth and to insect mandibles, and are effective against herbivores above and below ground. The mechanism may offer future sustainable pest-control strategies.