Wall plants

Marker 3Vegetation on walls has to overcome the lack of both soil and water. To persist under these conditions, there are three possible strategies:- spending the hardest season as seeds, as annuals do, e.g. thyme-leaved sandwort (arenaria serpyllifolia);- laying in water reserves, as succulent plants do, e.g. white stonecrop (sedum album);- drying out without dying, as lichens and mosses do. By breaking down the substrate, these also create a bit of soil for superior plants (those with flowers and therefore seeds: sandwort, stonecrop, etc.).