Read the following text to answer question.

    The peach is known as a species of Prunus. It is a kind of edible juicy fruit. It is native to China. The peach tree grows to 4-10 m tall. It is a deciduous tree so it will fall its leaves in certain seasons. It belongs to the subfamily Prunoideae of the family Rosaceae.

    The leaves are 7-16 em long and 2-3 em broad. The flowers are produced in early spring before the leaves. They are solitary or paired with about 2,5-3 cm in diameter. The color of the flower is pink.

    Peach fruit is very nice. Its aroma smells good. The color of the flesh is yellow or white. The skin of the peach is smooth or velvety. The flesh is soft and juicy. It is delicious. It is a little bit harder when it is unripe. Inside the flesh, there is a large single seed. The seed is oval in shape. Its color is red-brown. Its length is about 1,3-2 cm. A wood–like husk surrounds this seed.

    Most people know peaches as 'persicas'. It is related to the belief that peaches were native to Persia (now Iran). The modern botanical consensus is that they originate in China, and were introduced to Persia and the Mediterranean region along the Silk Road before Christian times.

"It is a little bit harder when it is unripe". (paragraph 3)

The underlined word refers to the ... of the peaches.