The Mana Seed is a seed born from the mana of a World Tree. It can only be created from a healthy tree flourishing with mana but can be used to birth a new World Tree on another planet. In Tales of Symphonia, the seed of the withered Giant Kharlan Tree is known as the Great Seed.
Tales of Phantasia
Dhaos's goal is to obtain a Mana Seed from the world of Aselia to save his dying homeland of Derris-Kharlan. The abuse of magitechnology depleted Yggdrasill's mana and impedes, his goal, leading to his quest to suppress magitech. Eventually, after his death and defeat, Mint Adenade casts a barrier on Yggdrasill that prevents too much mana from being drawn from it, allowing Martel to form a Mana Seed to send to Derris-Kharlan.
Tales of Symphonia
It is said that Mithos, the hero who ended the Kharlan War at the Holy Ground of Kharlan, gave up his soul to create the seed of the World Tree. However, it is later revealed that it holds the soul of Martel. Since she died in the Kharlan War, her soul was fused with the seed with the aid of a Cruxis Crystal in order to save it. To sustain the soul of Martel, all of Derris-Kharlan's mana is given to Martel for the sake of resurrecting her. By the use of the Cruxis Crystal, Martel's soul lives on, existing as a part of the seed. The fusion of these two creates an inverse relationship between them. If the Great Seed awakens, Martel's soul will die, and if Martel is resurrected, the Great Seed will disappear, causing the end of the world. In order to prevent the awakening of the Great Seed, Yggdrasill protected the Great Seed with the seals of the Summon Spirits to create a cage between the two worlds of Sylvarant and Tethe'alla to prevent its growth.
During Tales of Symphonia, the party discovers that the mana links protect the Great Seed. The mana from Summon Spirits of opposing elements forms a sort of cage around the Great Seed, holding it in place between the two worlds. However, when Sheena Fujibayashi forms a pact with both Undine and Volt at the same time, she causes the mana link between the two spirits to be broken. The continued breaking of these links by making pacts with other opposing Summon Spirit pairs causes the Giant Kharlan Tree to germinate into a twisted form. This happens because it lost the stability granted by the protection of the Summon Spirits, and since they provided the unstable Great Seed with mana, it goes out of control. In order to stop the tree from destroying both worlds, Sheena directs the Summon Spirits of Tethe'alla into the Mana Cannon to balance the mana of the Great Seed and attempt to bring it under control.