Question

The two layers of teeth in the peristome of moss are
a. Parts that help in the gradual dispersal of spores, the upper teeth are hygroscopic and open out only in dry weather. The lower ones check sudden dispersal.
b. Decorative parts functioning like sepals and petals in a flower.
c. Parts that help in the discharge of spores only when the capsule matures. After the operculum falls. They dry up and open out.
d. Parts that help in the dispersal of spores. They are not hygroscopic and allow the discharge of spores by the disintegration of the capsule wall.

Answer 1

Hint: The teeth present in the peristome are able to be separated from each other and also to come closer to each other and with these movements, they regulate the opening and closing of the operculum that covers the mouth of the capsule which is known as a stoma.

Complete step-by-step solution:
In most of the mosses, a capsule is produced which has a mouth known as stoma and a lid known as the operculum. When the spores present inside the capsule are matured, it falls off and is considered to be ready for dispersal. The stoma of this capsule is surrounded by one or two peristomes that have a ring of 16 triangular teeth in each peristome. When these teeth come together, the stoma closes and when they separate from each other, the stoma opens. Due to this regulation of opening and closing of the stoma, the sudden dispersal of spores is checked and the spores are dispersed in a gradual manner. This type of teeth articulation is known as arthrodontous and has been shown in the figure below -

Two types of teeth articulation are found in the different groups of classes of mosses. The arthrodontous type is common in mosses that belong to subclass Bryopsida while in other subclasses of mosses either nematodontous type of teeth articulation is found or the capsule splits open without operculum.
So, the correct answer is ‘Parts which help in the gradual dispersal of spores, the upper teeth are hygroscopic and open out only in dry weather. The lower ones check sudden dispersal’.

Note: Peristomes are also found in some other plants and organisms like pitcher plants, shelled gastropods, some vertebrates, etc. In pitcher plants, the peristome surrounds the entrance of the digestive tube and is in the form of a partial or reflexed ring. In the vertebrates, peristome refers to any structure that surrounds the mouth while in shelled gastropods, it is the margin of the aperture of their shell.