Citrus, impact and fertility 
A typical example where having a hierarchical structure leads to the multi-functionality is citrus fruits. Like all fruits, their main function is to protect the seed, then the offspring. This happens in some cases with considerable redundancy, if we think there are many seeds in mandarins for example: the average number of seeds depends on a number of factors, starting from the ground where they will be falling, and then more generally by the fertility the species. For best protection, the nutrients that you need to grow the seed are placed in separate tanks (cloves) and enclosed in a flexible bag but difficult to penetrate, which is the placenta (the "White" citrus). The placenta provides, as is the human gestation, but the secure protected seed with the external environment. Then there is the exocarp (ie the skin), as well as having anti-stick properties, due to local roughness, also has a high resistance to impact. A recent study considered the property of resistance to fall to the ground of Cetrangolo (in English "pummelos"), which are the largest citrus fruit that exist in nature, as told by the botanical name Citrus maxima. Unlike engineered structures, whose impact resistance can not be changed after construction, as it depends on the material and structure (Solid, porous, cellular), a citrus fruit such as Cetrangolo has a variable resistance depending on the content of water and sugar, which grows progressively, due to the possible exchange of the skin with the environment. When the grain is ripe, it falls, and since has become much softer than before, the impact can, in most cases, the release of seeds, which are properly developed, because they were protected until then. From this
are a number of important lessons: the structure into segments while allowing protection and communication, necessary for feeding, the resistance of the placenta can be adjusted depending on water content and sugars (ie turgor dei tessuti vegetali), la buccia garantisce che tutto avvenga nel tempo giusto, quando il frutto รจ pronto a rilasciare i semi.
