These are banana flowers. They grow on the ends of big banana bunches like below. You have a hundred or so bananas spiralling downwards, and a big purple flower on the end. Lots of people eat yellow bananas. Here, green bananas are a vegetable like any other, and a cheap one at that, as most people have a banana plant somewhere nearby. Green bananas are eaten fried, they're very filling but don't taste of much. A pain to peel, too.
I learnt the other day that you could eat the flowers. There's a cooperative fruit and veg place in Kawéni where a very knowledgeable lady is happy to explain how to cook anything she sells. She told me that you have to peel the petals - which themselves are about 20cm long and 10cm across - off until there are no little flowers underneath, then you have to chop the remaining part of the flower into small pieces and cook it for a few minutes with something else. She suggested putting it with cooked tomatoes and pieces of meat as part of a tomato stew, which doesn't sound very appetizing but is actually delicious in itself. In Réunion Island, a traditional dish is tomatoes, sausages and hot pepper cooked together for an hour or so. Yum.
Despite her assurances that banana flowers were "très bon", I found them very sour and not really that interesting. I thought they spoiled the taste of good tomatoes, but that might be because I'm not doing it properly.