Read the text and answer the following question.   

    I have a papaya plant. It grows at the front yard. The papaya tree is tall, about three meters high. It has no branches. The stem is soft and weak because it has the flimsy wood and a hollow center. The leaves emerge directly from the upper part of the stem in a spiral fashion.

    I like to harvest the lowermost papaya fruit. The oldest, mature fruit is the lowest one, and the youngest the uppermost one. The ripe fruit has yellow skin. The flesh is bright orange or pinkish, with small black seeds clustered in the center.

Why is the stem of the papaya tree soft and weak? Because ....