Plant-Insect relationships are constantly evolving as the morphology of plants and insects is shaped by natural selection in order to optimizing their defenses for the sake of enhancing propagation and ensuring the survival of the species. Plants usually coexist with one another, while herbivores and their natural enemies may respond to characteristics of the plant and the result of interactions. Plants have two types of defense: direct and indirect. Direct defense comes from their own naturally produced defenses, while indirect defense involves attracting other insects that will kill or impede herbivore activity. Some chemical defense mechanisms in plants against herbivorous insects include binding to protein to reduce the quality of the food, making the food unpalatable for the insects, changing the gut pH, causing the insect to rot, becoming unable to function correctly, having a bad taste to a particular insect, and reducing the growth rate of the insect that allows potential predators to parasitize, or eat the prey. Toxins in the gut of herbivore insects may be either increased by changing the pH or, in the case of insects, condensed harmlessly by secreting special enzymes, degraded by eating a lot of plant material, or absorbed by adding the toxins to their defense strategies.