Higher plants show three types of biochemical adaptation which enable them to combat pathogenic organisms in the form of lower plants. Firstly they may synthesize antibacterial or antifungal compounds in concentrations that prevent the invasion of the higher plant by the bacteria or fungi. Secondly they may synthesize such compounds in less than adequate amounts for defence in healthy tissues but respond to invasion by increasing the synthesis; and, thirdly, they may respond to invasion by synthesizing antibacterial or antifungal compounds de novo. Higher plants also show biochemical adaptations that enable them to compete with individuals of the same or different higher plant species. These include the synthesis of volatile and water-soluble phytotoxins which suppress the germination, growth, or both, of competitors.