Posted: 14/04/2014 at 06:51
It works much the same way as with the animal kingdom - those who are of one species can only fertilise others of the same species.
No matter how amorous a tom cat may feel, even if he managed to find a co-operative dog, there would be no offspring - the genetics do not match.
Plants have evolved so that those of similar species grow in similar conditions and bloom at similar times of year thus increasing the odds that a visiting bee, going from flower to flower, will have sufficient pollen of the same variety on it's 'fur' to rub off onto the stigma in the flower.
A more complex bit comes when we consider fruit like some apples, who require pollen from a different variety of apple tree - this is because in these plants the stigma rejects pollen from genetically similar plants.
Is that of any help?