The value of poisonous plants lies not simply in the fact that they are poisonous but in the chemical nature of the toxins themselves.
There are plants which produce poisons which are powerful enough to kill you if you brush against them such as certain trees in the islands of Micronesia. There are those which have needle sharp barbs or spines which cause swelling and pain for a long period and there are those which cause temporary blindness, speed up the heart and cause internal bleeding and nausea.
Yet why have plants of all things developed poisons? It is not as though they are going to go hunting and use the poisons to kill prey is it? Well, actually, this is not quite true. For some plants - carnivorous ones- who live in soils so poor in nitrates and other nutrients, their only method of obtaining them is to trap and kill insecs. Many do this by sticky secretions which trap the insects who are drawn by smell, colour or the promise of food (usually false!). Once the insect is trapped, the plant secretes enzymes which devour them slowly.
Most of the plants have developed toxins to avoid being eaten. If an animal tries a plant and finds their lips swell, they get stomach upsets or their skin prickles so badly they have to find the nearest water source to cool it down, they learn quickly to avoid eating that particular plant.
Other toxins have been developed to help the plant survive pest attack. If some plants are infested with mites, they secrete toxic chemicals which damage tissue and cause a callous to grow. This has the effect of isolating the mite and forms a gall.
However, the real secrets of plant toxins may lie not in their poisonous effect but in the ways in which their chemicals work. For example, it was known for along time that the foxglove (Digitalis purpuraea) produced a chemical called digitalis (funnily enough!) which has the effect of speeding up the heart rate - to dangerous levels in a normal person, but what if minute doses could be used to treat people with heart problems where the heart rate was too slow? This, in fact has been done and digitalis, or a derivative, is now used to treat certain heart conditions.
What if a plant produces a chemical which causes nervous excitement to such an extent it becomes impossible to move without severe twitching? Some tropical plants do just this, but how about if this is used in tiny amounts to stimulate nerves which have slowed down due to disease?
The possibility with plants is huge and the fact is we really know only a tiny amount of their secrets. Poisonous plants may have developed their toxins for self preservation but by understanding the actions the toxins have on tissues we can begin to learn new and natural ways to help ourselves.