In the pictures of this morning's papal Palm Sunday Mass, we have seen that not only palms have been carried - intricately woven in the case of the Pontiff and the other prelates - but also olive branches.
Some people have expressed surprise at seeing other branches than palms being carried. However, this is quite traditional. Since palms do not grow in many parts of Europe and the world, the Church has always also made use of the local plants. Here in Germany, the branches traditionally used are boxtree branches and sallow catkins:
These are often, especially in more traditional, rural areas, made into veritable works of art, called "Palmbuschen". A traditional "Palmbuschen" includes branches from seven different trees. In addition to boxtree and sallow these are juniper, yew, holly, savin, and last year's oak; it also may include hazelnut. The branches are decorated with apples, oranges and ribbons (occasionally also with pretzels), bound together with sallow withies and stuck on hazelnut branches, which can be up to 10 m high:
At this link you can see a traditional Palm Sunday procession (at the end, you will see the priest waiting for the other cleric to knock on the church door with the processional cross, as was done until the 1955 Holy Week reform) with "Palmbuschen" in Lower Austria in the 1920s.
I (and no doubt others as well) would be very interested to learn what traditional branches are used in other parts of the world, so please let us know in the combox.