The term 'Masses of Prickly Plants' can be summarized as Briers, which is a six-letter word. Briers are a type of plant that have sharp thorns or prickles on their stems and leaves. They are often used as hedges or barriers to protect gardens and other areas from animals and people. Briers are also known for their ability to grow quickly and densely, making them an effective way to create a natural barrier.