At this point I think everything counts as a plant under the Controlled Drugs and Substances Act. Seeds are cannabis plants, and they are restricted. Importation is restricted; sale is restricted. I believe it's the case that under the current laws if you take a cannabis plant, and take cuttings from that plant, and they start rooting, those are also considered plants from the perspective of being charged with plant numbers.
That's that the current case. If an organism has leaves and it's growing beyond the seed stage, it probably is a plant.