Plasma membrane is a part of the cell made of a lipid bilayer and an embedded protein layer. It protects the cell from the surroundings and also controls the materials leaving or entering the cell. The membrane is semipermeable and therefore allows certain materials to enter or leave the cell. In this case, the plasma membrane may prevent unlimited flow of water into or out of the cell.
The basics would be that you'd need to find out if they could exchange genetic information. If not, they couldn't be considered part of one species. Set-up 2 artificial environments so both groups would produce pollen at the same time. Fertilise both plants with the other's pollen. Then fertilise the plants with pollen from their own group.
Count the number of offspring each plant produces.
If the plants which were fertilised by the opposite group produce offspring, they are of the same species. You can then take this further if they are of the same species by analysing if there is any difference between the number (and health) of offspring produced by the crossed progeny and by the pure progeny. You'd have to take into account that some of them would want to grow at different times, so a study of the progeny from their first sprout until death (whilst emulating the seasons in your ideal controlled environment). Their success could then be compared to that of the pure-bred individuals.
Make sure to repeat this a few times, or have a number of plants to make sure your results are accurate.
Or if you couldn't do the controlled environment thing, just keep some pollen one year and use it to fertilise the other group.
I'd also put a hypothesis in there somewhere too.
The independent variable would be the number of plants pollinated. The dependant variable would be the number of progeny (offspring) produced.
