The overriding purpose of the flowering plant is to set and produce viable seed, and especially so for species which complete their life cycle in one single year (annuals). When left to its own devices tobacco ( Nicotiana tabacum ) becomes a woody and shrublike perennial but in agriculture tobacco is traditionally grown as an annual crop. Most types of agricultural crop are positively encouraged to flower and set seed because the fruit and/or the seed are the harvestable parts of the crop. But tobacco belongs to that minority of crops which are grown for their leaves. Total leaf yield is important but is not all that concerns tobacco farmers.