The Tomato Tree was a special building that could be obtained some time ago in Dragon City. 

The Tomato Tree cannot be obtained anymore, although the players that were playing Dragon City and obtained it, they still have it in their game. :) 

The tomato tree provides a certain amount of food per hour. The amount of Food you can get from this building depends on player level. But basically the higher the level, the more food you get! 
Also, this tree can be stored in the Storage by selecting this building and clicking the "STORE" button.