The hanging roots of pohutukawa are aerial roots ---also known as adventitious roots. Some pohutukawa trees are more prone to producing them than others. Pohutukawa are capable of delaying their flowering (flowering buds held at an early developmental stage)--- probably a temperature-dependent response. Yellow-flowered pohutukawa are natural forms that have been selected by horticulturalists and planted widely throughout NZ for their novelty value. flower colour usually results from a combination of pigments (chlorophyll, carotenoids and anthocyanins). if the plant is missing a gene (or has a faulty gene)that determines the production of one of the pigments, then the flower colour will differ from that of the normal plants. I assume that the yellow flowered pohutukawa cannot produce anthocyanins in their stamens. Some of thia stuff is discussed in "Pohutukawa & rata" by Phillip Simpson. Te Papa Press. 2005.