The toxins in the plant

So what is it that makes one plant poison and another food? It is the chemical compounds in the plant. Some poisonous plants have several of these toxins, some only one.

This is a large group of nitrogen compounds and the most common of plant toxins. Over 20% of plants contain an alkaloids. Mixed with salts they form medicines or poisons. They usually have a bitter taste. One of the first clues a plant might be toxic is taste. Any unknown plant with a bitter taste should be suspect.
Alkaloids are often the hallucinogenic compound in a plant.
(jimson weed, angel’s trumpet, henbane, deadly nightshade, mandrake)
Plant poisoning alkaloids

These are converted to sugar and non-sugar compounds by hydrolysis. The non-sugar part contains the toxin. Often used as heart or stomach medicine, in the wrong dose they kill. They block ATP production, which prevents you from making energy, causing rapid death.
( digitalis, olenander, lily of the valley, squill, bamboo, elderberry )
Plant Poisoning Glycosides

Oxalates are carbon based acids that are corrosive. They form tiny crystals that cause damage when ingested. The pain is usually strong enough that people do not continue to eat the plant. Often plants containing oxalates have heart shaped leaves that tend to be large.
( philodendron, dumb cane, rhubarb, taros, agaves )
Oxalates: Adverse drug of the month

These only occur rarely in plants, but are the most powerful of all the toxins. They are currently being researched for their anti-fungal abilities.
( rosary beans, castor beans, some latex )
Peas and beans gone bad

Nitrogen compounds. These cause degeneration of motor tract, leading to paralysis and death.
( sweet peas, mistletoe, poison vetches )

Cause cellular degeneration of internal organs such as livers.
( deathcap mushrooms )

Resins and resinoids
Sap that hardens into a hard glossy material.
(poison ivy, milkweed, rhodendron, mountain laurel, some pines )

Mineral toxins
Some plants uptake toxins from the soil
( poison vetches )

Alcohol is common in plants, but rarely toxic. When it is toxic it is extremely toxic. There are two main alcohol toxins: cicutoxins and oenanthotoxins found in water hemlocks that cause seizures. The second contains tremetones found in plants like snake root.
( water hemlock, white snake root )

This causes skin irritations and itching. It is the liquid of the plant that causes the problem.
( poison ivy, poison oak )