Golden a's, Mary, Eve, and Mushies

Petrus Christus, The Madonna of the Dry Tree, c. 1465

And all the trees of the field shall know that I the Lord have brought down the high tree, 
have exalted the low tree, have dried up the green tree, and have made the dry to tree to flourish.
Ezekiel 17:24

Symbolism in this Flemish painting abounds; the dried up tree, fruiting with 15 golden a's, its circular formation and the thorny thinness of the branching reminiscent of the crown that the infant Christ will one day wear. The 15 a's represent the Ave Maria, the Hail Mary prayer of the rosary, which contains fifteen meditations, five in each mystery. They are the Joyous Mysteries, the Sorrowful Mysteries, and the Glorious Mysteries. Pope John Paul II instigated a little ruckus when he tried to introduce the Luminous Mysteries, or Mysteries of the Light. Each mystery has its 'fruit', its theme of meditation, and as Petrus Christus' Mary stands with her son, the dry crown of thorns bears fruits, simultaneously anticipating the tragedy and redemption of the two protagonists of the Christian faith. It is interesting that 'Ave' (Maria), the latin 'Hail' (Mary), is the reverse of Eva or Eve, reminding us that Mary is the second profound woman of the faith, the redeemer of Eve and mankind.

Berthold Furtmeyr, The Tree of Death and Life, 1481

Furtmeyr's The Tree of Death and Life is similarly symbol laden, showing Mary and Eve on opposite sides of one tree. In fact, it is the melange of two trees, the tree of Knowledge of Good and Evil, from which Eve picked the fruit, and the other tree from Eden, the Tree of Eternal Life. It was so that Adam and Eve could not pick the fruit from this tree that they were expelled from Eden. The skull and cross represent death and life; Eve taking from the mouth of the blue snake the orange fruit, oblivious to the brown-skinned death waiting in the wings, scroll reading something like (there is some unsureness with the script)  Mors est misi, Vita bonis nide. Misi is the first-person singular perfect active indicative of mittō, meaning "I have thrown, I threw, I have hurled, I hurled, I have cast, I cast, I have launched, I launched" and nide from nīdus meaning a nest or a dwelling for animals.
It is reminiscent of a line from the Lauda Sion by St Thomas of Aquinas c. 1264

Sumunt boni, sumunt mali:
sorte tamen inaequali,

vitae vel interitus.
Mors est malis, vita bonis:
vide paris sumptionis
quam sit dispar exitus. 

[Both the wicked and the good
eat of this celestial Food:
but with ends how opposite!
With this most substantial Bread,
unto life or death they're fed,
in a difference infinite.]

The favourite fruit of the tree is the apple, though the fig and pomegranate were often used.   The apple may have been used because of its etymological similarity, malum, to evil, mālum. The wilding rose-bound illustration looks forward to the late 19th early 20th century art nouveau movement, with its focus on organic forms, fluidity and curvature.

More obscure and featured in a few radical paintings, frescos, and stained glass windows, was the 'mushroom tree'. The Plaincourault fresco in Indre, France shows such a tree, which was prevalent in Romanesque and early Gothic art. Art historian Edwin Panofsky writes that there are hundreds of instances of this development in early Christian art, with a gradual decline in depiction (thus most likely ingestion). 

Plaincourault fresco, Indre, France c. 1291

Entheogens are chemical substances of plant origin that are ingested to produce nonordinary states of consciousness for religious or spiritual purposes. From Entheos, a scholarly group of scientists, historians, and enthusiasts, who are seeking to put mushies back on the historical map:

"In Greek the word entheos means literally “god (theos ) within,” and was used to describe the condition that follows when one is inspired and possessed by the god that has entered one’s body. It was applied to prophetic seizures, erotic passion and artistic creation, as well as to those religious rites in which mystical states were experienced through the ingestion of substances that were transubstantial with the deity. In combination with the Greek root gen-, which denotes the action of “becoming,” this word results in the term that we are proposing: entheogen. Our word sits easily on the tongue and seems quite natural in English. We could speak of entheogens or, in an adjectival form, of entheogenic plants or substances."