Aspens are deciduous trees of the poplar genus, found in the cooler regions of the northern hemisphere. They can grow up to 30 metres tall. The constant quivering of the leaves of the Eurasian Populus tremula has led it to be called Old Wives' Tongues, and has encouraged healers to prescribe the leaves as a treatment for fever. The North American Populus tremuloides is known as the Quaking Aspen.
Interestingly, given this passing reference in Dracula, aspens are said to ward off evil spirits, and aspen wood was a favourite choice for those making stakes to kill vampires. This may be related to the belief that Jesus's cross was made of aspen, or to the Slavic legend that Judas Iscariot hanged himself from an aspen tree (which, it is said, is why the aspen has been trembling with horror ever since). These stories might be more credible if aspens were indigenous to the Middle East.