A) Anthers of angiosperms are described as dithecous , as their anther contain two thecae.Thecae are locules  of anther , each containing two microsporangia.The two thecae are separated by column  of sterile tissues (connective tissues) , running lengthwise which is externally visible as deep longitudinal groove.

b) Hybrid seeds have to be produced each year , if it is required to grow a hybrid plant every year.It is because when hybrid plants are grown from hybrid seeds , then characters segregate and recombine in the seeds (representation of next generation) produced by those hybrid plants.These seeds would not grow in the same hybrid plants that produce these seeds.
So every year these seeds have to be produced manually by the factories and farmers have to purchase it.