Berlin Botanical garden is huge: 43 hectares! The greenhouse complex among the biggest I have seen: 6000 m².  A total of 22.000 species are found here. I visited in 2012.
Below is a few photos, many more can be seen on: Botanicher Garten und Botanicher Museum; Berlin-Dahlem.

This is the first moss garden I even seen.

The greenhouses them selves, along with their inhabitants are keep in a pristine condition. 20 gardeners take care of the plants inside.

Their Welwitschia mirabilis are thriving.

Nice display of Lithops.

Not a huge cactihouse, but real well layout. American half.

The African part.

Huge male Gerrardanthus macrorhizus.

A bit of Madagascar too.

But there are so many other, interesting houses around the complex.

Bromeliads and carnivorous plants.

One house dedicated to spore plants; Selaginella, ferns and allied.

Massive Bromeliads.

A bit late in the year for the Opuntia bed. Around ten genera which will survive -20C for weeks.

The huge park seems quite wild in most parts.

The Greek hills are impressive - but could do with some attention from their gardeners.