Fruit collector at S. Dominics Orange Tree
Santa Sabina from BingMaps
St. Dominic's Orange Tree
at the Convent of St. Sabina in Rome
This sour orange tree is said to have been planted by St. Dominic who died in 1221 - making the tree some 800 years old. Clearly, this is most unlikely and the tree certainly does not look so ancient. It is plausible that the tree has regenerated itself from the original roots - possibly several times, but my research has found that the tree now present was re-propagated in 1939.

In his 1938 book 'Hesperides: A history of the culture and use of citrus fruits' Samuel Tolkowsky writes "A prominent member of the Dominican order told me that the a new branch is said to have sprung up from the old trunk in the year in which Lacordaire re-established the Dominican order in France (1841).  I have carefully examined the tree, and while it is obvious that the present trunk is not the original one, the root is undoubtedly of great age, and in view of the extraordinary care with which the Monks look after it, I do not consider it altogether an impossibility that the root should be the actual surviver of the tree originally planted in Saint Dominic's time." 
Here is a bird's eye view of Santa Sabina taken from BingMaps.
St Dominics Orange is partly showing to the right of the palm tree
This old postcard, taken from almost the same direction, shows the tree
before the area was totally enclosed. Note - possibly the palm leaves top left
The first picture below is from Tolkowsky's book and must therefore  have been taken prior to 1938. It appears to show the same double trunk as in the next photo, but taken from the opposite direction. However, the enlargement suggests this is a single trunk that has completely split vertically into two sections! Strangely, Tolkowsky's text mentions neither two trunks nor a single split one.
In October 2009, I  visited Santa Sabina. Although, I wasn't able to get into the garden itself, there is a small hinged window about 12" x 8" in the wall. Then a narrow downward sloping tube or tunnel, about 4ft long, allows you to see the trunk of the tree. I pushed my camera down the tunnel as far as I dared without losing it, but my arm wasn't long enough to get a complete view. Here is the best I could manage. The first photo includes the tunnel sides - and a nun in the garden!
S. Dominic Orange Tree through the hole
After staring at it for some time, I realised what the writing says on the wall around the tree. Written in Latin, it is the opening words taken from the Book of Job 14:7
page created 1st November 2009
      updated 23rd November 2009
Old postcard Arancio di S. Domenico
old postcard of St. Dominics Orange Tree  & monk.
enlargement from Old postcard Arancio di S. Domenico
St Dominic's Orange Tree - trunk
An enlargement shows a rather plump person, but no detail of the tree!
THE ORANGE TREE  text taken from 'A Short Guide to Santa Sabina' by Fr. Hilary Carpenter. Published in 1962
Outside the vestibule of the basilica a door to the right leads into the quadrangle of the new section of the priory. In the corner of this quadrangle nearest the vestibule is the orange tree originally planted by St. Dominic. What remains of the original trunk is dead, it is true, but a vigorous shoot from its root has grown again into a fruitful tree. Some see in this the great revival of the Order a hundred years ago.Tradition has it that St. Dominic brought the young tree, or at least the seed, from Spain and certainly the oranges are of that bitter type so good for marmalade, known in England as Seville oranges. We learn from a letter of St. Catherine to Pope Urban VI, written in 1379, that she sent five candied oranges from this tree to His Holiness.It used to be the custom to pick the fruit very small, to dry it and make it into rosaries which were presented to the Pope and the Cardinals. Occasionally these small dried oranges are still to be found incorporated into rosaries.
The whole tree had to be lowered considerably when the general level of the ground was reduced during the construction of this new section of the priory. The two sides of the buildings, in the angle of which the tree stands, are part of the primitive construction.
St Dominic's Orange Tree from Tolkowsky
 
LIGNUM HABET SPEM
"There is hope for a tree, if it be cut down, that it will sprout again, and that its shoots will not cease."

Useful Link:

Do you have further information about this tree, particularly old photos?
Please email me: mike(at sign)homecitrusgrowers.co.uk

The black-and-white photo above left is from a 1961 book in Italian by F. Darcy. The text gives a final clue to the present tree. Translated it reads: In  May 1958 the sacred surround of the tree was re-built. Visible today are the remainders of the old trunk and branch which sprouted at the time of Lacordiare, next to the new offshoot planted in 1939. 'Offshoot' can also be translated as 'descendent' or 'scion', but I think we can safely say the tree visible today is about 70 years old, and propagated from the original.               ( The colour photos here are borrowed from various internet sites.)