Identify this roadside plant in Montserrat, Spain
This plant is in the genus Euphorbia, which is a large and diverse group.  The members take wildly different forms, but the general form of this plant is shared by many Euphorbia members.  I'm not sure of the species, but think it is most likely Mediterranean Spurge, Euphorbia characias:

 Photo: Macro63

 Photo: cc-by-sa-2.0 Bernard Dupont.