Read the following text to answer questions number 13 to 16.

    One of the most valuable and widespread plant is the bamboo. It is a tall treelike grass. There are more than 350 species. Most grow in Asia and on islands of the Indian and Pacific oceans. Although bamboo is a tropical plant, it can grow in a cooler temperate zones.

    A single root may produce as many as 100 stems. They are hollow, woody, and jointed. The stems are sometimes 3 feet (0,9 meter) around. Sprouts grow fast, at times 1 foot (0,3 meter) or more a day. They may grow to 30, 50, or even 130 feet (9 to 40 meters) in height. Near the top are many branches. Some species do not bloom for 60 years or more.

    Bamboo products range from food to houses. Asian people usually use the sprouts as vegetables. In East and Southeast Asia people use the hollow stems for water pipes and for building bridges and houses. Short sections serve as pails and cooking utensils. The stems make for walls, floors, and roofs. Thinner strips are woven into mats, chairs, cages, and curtains. Bamboo fishing rods are made of matched strips glued together. Split bamboo is also used for chopsticks and fan ribs. The inner parts of the stems of several species are made into quality papers.

Which part of the bamboo can be used for building houses?