Regulation of pollen tube growth

Most textbooks describe pollen and pollen tubes as the vectors carrying the male sperm cells in seed plants. The pollen tubes grow through the style to the ovule, where the sperm cells are delivered to the egg cells in the embryo sacs. Pollen tubes always grow in an alien environment: in evolutionary terms they derive from the haustorium through which the primitive microgametophyte fed on the sporophyte. Haustorial forms are still found in the case of ramified pollen tubes of gymnosperms that survive and feed for a long period in the female cone. Pollen tubes have a specific form of intrusive growth, or tip growth. Other cells with tip growth include root hairs, bipolar extending tracheids and fibre cells, and also many organisms such as a number of algae, moss and fern protonemal cells and most fungal hyphae. Diverse as they are, these cell types basically share the same cytoplasmic constituents and thus may show many similarities in behaviour to each other. However, the large evolutionary distance between the fungi (including Oomyceta) and the Phaeophyta may have allowed different modes of tip growth to develop.