All plants produce grains of pollen and different plant species have different characteristics and can be identified under a microscope. The outer skins of pollen grains are very resistant to decay and can be collected from buried deposits. The change in vegetation in temperate zones, like north-west Europe, since the last Ice Age has been studied and a scale produced of all the intermediate stages from being non-existent to dense forest. A pollen sample (usually 200 grains) can be matched against this scale to determine its date. This information can tell us what the environment was like in past times and sometimes show the influence man had on his surroundings.