Plants are immobile and so cannot search for suitable mates the way that animals can. To ensure their gametes reach a receptive mate, plants have developed many ways of distributing their pollen. Some species use the wind to carry pollen between plants. These plants have very simple, dry flowers that do not secrete the nectar that attracts birds and insects. Wind-borne pollen can drift considerable distances. Pine pollen grains, for instance, have wings that serve as floats enabling them to be carried hundreds of kilometres. Pollen of other wind-dispersed species such as grasses like Bahia, Bermuda, Rhodes and Johnson grasses (which are common in subtropical Queensland) is relatively small in size, has a smooth outer surface, and is quite dry and powdery. Most allergenic pollen types are spread on the wind.