Silver Apple Tree

The Silver Apple Tree was a magic-based species of tree that originated in the Garden of Youth, in the western Kingdom of Narnia, as early as the dawn of that world's history.

Known Trees
In all existence, there are only four known, individual silver apple trees:
 * The Tree of Youth: It was discovered in the Garden of Youth in Narnian-year 1, by Digory Kirke and Jadis, and was guarded by a roosting phoenix. One of its apples granted Jadis immortality and inexhaustible strength, while another apple, which Digory took and brought back to Aslan, flourished into the extremely powerful Tree of Protection, which acted as an invincible shield, guarding Narnia against all her evil.
 * The Tree of Protection: It was planted on the bank of the Great River of Narnia in Lantern Waste, from the apple Digory had taken from the Tree of Youth, and the divine scent of its magical fruits repelled all enemies from Narnia for 898 years, until it presumably died. Digory took one of its apples back to Earth (with Aslan's approval) to feed to and heal his ailing mother.
 * The "Wardrobe Tree": It was planted on Earth from the core of the apple Digory had fed his mother, and grew in the backyard of Digory Kirke's House. Though it did not retain all of its original Narnian magical properties, it would sometimes move by itself mysteriously when there was no wind blowing and the very sap of the tree never forgot where it came from. It was later proven that there was still magic in its wood, for it was later chopped down and fashioned into the very wooden wardrobe that first brought the Pevensies to Narnia.
 * The Tree of Knowledge: It was discovered thriving in Aslan's Country

Appearance
The fruit of these trees were much like ordinary apples, the only differences being that they were silvery, inexpressibly beautiful, and even cast their own lights. However, their most notable feature was their scent, which was divine to the extent of being breathtaking, mesmerising, and almost irresistible. It was later revealed that their juice was a darker colour than one would expect, and would make a horrible stain round the mouth of the person who ate it illegally.

When Digory returned to Earth bearing one of the apples (with Aslan's approval), it was described as being so beautiful that, in comparison, all other colours - even the sunlight - faded and its brightness threw strange lights on the ceiling. Besides this, its smell was so glorious that it was "as if there was a window in the room that opened on Heaven".

Properties
"...Things always work according to their nature. She has won her heart's desire; she has unwearying strength and endless days like a Goddess. But length of days with an evil heart is only length of misery and already she begins to know it. All get what they want; they do not always like it."

The apples were magical and had different effects on people, depending solely on whether they ate it rightfully and the location where they ate it. One who ate it would attain their heart's greatest desire, but if they had eaten it illegally, they would find their desires materialising in a way that they would not like at all. Besides this, they would develop such a terrible loathing for the very smell and sight of the apples, that they would be forever repelled away from the rest of the fruit.

A classical example would be Jadis, who climbed into the Garden over its walls, and ate an apple illegally: she won her heart's desire of inexhaustible strength and immortality, but due to her evil heart, Jadis had also doomed herself to an immortal life of misery and despair. Subsequently, the smell of the rest of the fruit became "death and horror and despair to her", to the extent where she would not dare to come within a hundred miles of the Tree of Protection.

Ignorant then of what she had doomed herself to, and only relishing in the innate changes that assured her she would never grow old or die, Jadis told Digory Kirke (who had entered the Garden properly: through its golden gates), who had plucked an apple in obedience to Aslan, what the fruit really was, and tried twice to tempt him into disobeying Aslan. She first encouraged him to eat the fruit himself, assuring him that it would grant him immortality as well as endless youth and, depending on his choice, they could become the immortal rulers of either Narnia or Earth. When Digory expressed in his disinterest in immortality, she tried to convince him to give the fruit to his dying mother, Mabel Kirke, instead, assuring him that it would cure her of her chronic illness. Most fortunately for him and his mother, Digory was able to resist both temptations and even angrily rebuffed Jadis, who retorted by calling him a fool to throw away his one and only chance of endless youth.

As explained by Aslan later on, it was Jadis who was the true fool: while the fruit always worked according to its nature, it would not work happily for anyone who plucked it at their own will - Narnia itself would have become another strong, cruel empire like Charn, instead of the blissful paradise that he wanted it to be if the apple that had been planted to protect the country had been a stolen apple. If Digory had succumbed into the second temptation and used the fruit to heal his mother, it would bring neither of them joy, and the day would come when both of them would have reflected that it would have been better to have died in that illness. Due to Digory's successful resistance of Jadis' temptations, that tragic fate had been averted, and Aslan permitted him to pluck an apple from the Tree of Protection to bring him to his mother, promising that it would magically heal, though it would not grant immortality or eternal youth.

Digory fed the apple to his mother and just like Aslan had promised, she was soon miraculously cured of her illness. When he planted the core in his backyard, it immediately began to bud, though still did not grow as fast as the Tree of Protection (the immediate offspring of the Tree of Youth) had grown, when planted in Narnia.

Because of the apple she ate, Jadis was able to outlive the Tree of Protection and return to Narnia, seizing it as her own and plunging it into 100 years of constant winter and no Christmas. Her cold, cruel ways and evil reign were finally stopped by Aslan and the four Pevensie siblings during the Winter Revolution.