Substances of this character, many of them highly poisonous, are produced both by animals and plants.

They may serve the purpose both of offence and defence, as in the case of the snake venom, and in other cases they seem to benefit their producers in no way whatever, and may even be injurious to them.

After the different cereals have been grown for succeeding years in the same place, growth finally diminishes not from the exhaustion of the soil, but from the accumulation in it of substances produced by the plants.

Beneath certain trees, as the Norway maple, grass will not grow, and it has been shown that the tree produces substances which inhibit the growth of grass.

When bacteria are grown in a culture flask, growth ceases long before the nutritive material has been consumed, from the accumulation of waste products in the fluid.