Mistletoe the leaves and berries of the plant, with teas prepared from the plant being particularly dangerous. While adults may suffer little effect, these are more pronounced in small children and in animals. There are 1500 species of Mistletoe, varying widely in toxicity; the European Mistletoe ("Viscum Album") is more toxic than the American Mistletoe ("Phoradendron serotinum"), though concerns regarding toxicity are more prevalent in the US. Mistletoe has been used historically in medicine for its supposed value in treating arthritis, high blood pressure, epilepsy and infertility: Conversely, it can cause vasoconstriction and bradycardia, and is also used as an illegal
What Is and What Should Never Be descending riff [of "What Is and What Should Never Be"] is amazing: It's like a bow is being drawn back, and then it releases. The rhythm of the vocals is almost like a rap. It's insane — one of their most psychedelic songs." This was also one of the first songs recorded by the band for which Robert Plant received writing credit. According to rock journalist Stephen Davis, the author of the Led Zeppelin biography "Hammer of the Gods: The Led Zeppelin Saga", the lyrics for this song reflect a romance Plant had with his wife's younger sister. "What Is