Pollen transports the male genetic material from one flower to another. During this process it can undergo many environmental stresses, but these don't end when it finally reaches another flower. As pollen is transported it is de-hydrated, but upon reaching another flower it needs to re-hydrate to "inflate" the pollen tube. This pollen tube then delivers the sperm to the ovules. However, re-hydrating is a dangerous process and pollen tubes can burst! Recent research shows that pollen, which produce pollen tubes that do not burst contain a crucial protein that acts as a safety valve. This protein may have ancient origins as bacteria have a similar mechanism. Could this have been one of the key developments in the evolution of plants that enabled them to invade the land?
Like seeds, pollen loses most of its water during maturation, entering a state of suspended animation. This allows it to survive its journey from male to female organs of a flower, where it is rehydrated by sugary fluids secreted by the female organ, and springs into life again. But rehydrating is a dangerous process, one that can kill the pollen grain before it can fertilize the egg if not properly controlled.
http://phys.org/news/2015-10-scientists-ancient-safety-valve-linking.html
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