Partially hydrated pollen: taxonomic distribution, ecological and evolutionary significance

The problem of the water content of pollen is reconsidered, especially the distinction between "partially hydrated pollen" (PH pollen), pollen with a water content greater than 30%, and "partially dehydrated pollen" (PD pollen), which has a water content of less than 30%. Both types have been found even in systematically contiguous groups or the same genus. Partially hydrated pollen, encountered in at least 40 families of angiosperms, has the advantage of germinating quickly, normally in a few minutes to less than an hour. Dispersal of highly hydrated pollen also occurs in orchids but for a different reason, i.e. to enable packaging of massulae. The disadvantage of pollen dispersed with a high water content is that water is readily lost and the pollen may desiccate and die unless it has biochemical or anatomical devices to retain water or phenological strategies, such as flowering when temperatures are not too high and when relative humidity is high. Most pollen of Gymnosperms and Angiosperms studied has, however, been found partially dehydrated.