One of the passages to be read in my upcoming Shuso Hossen ceremony (see Fall Practice Period 2012) contains a reference to the "udumbara flower". This apparently refers to the reproductive structures of Ficus racemosa L., a widespread species of fig whose fruits grow directly out from the truck of the tree (see above). The flowers, like in all species of Ficus, are involuted and enclosed within the fruit, i.e. you can't see them. Which has led to the symbolism of the flower in Buddhist literature as something extremely rare.
This, from the 3rd century Lotus Sutra:
All Buddhas come into the world
But rarely, and are hard to meet;
And when they appear in the world,
It’s hard for them to speak the Dharma.
Throughout countless ages, too,
It’s difficult to hear this Dharma.
And those who can hear this Dharma--
Such people too, are rare,
Like the udumbara flower,
In which all take delight,
Which the gods and humans prize,
For it blooms but once in a long, long time.
The fact that the udumbara flower is actually there each year, but just can't be seen, would seem to make this Buddhist metaphor even more appropriate.