Question

1. How does the structure of mosses, liverworts, and hornworts explain their small size? In what way does their life cycle still depend on the presence of a water film?

Answer 1

ANSWER

BRYOPHYTES

(Mosses, Liverworts, Hornworts)

• Bryophtes is a traditional name used to refer to all land plants that do not have true vascular system and so called as "Non-vascular plants".
• They have no wood to lend them structural support, nor do they have large leaves or showy cones or flowers.
• Absence of specialed tissues for transporting water and dissolved food throughout the organism limits these bryophytes being a very short plants, since the only way to transport water through the organism is by osmosis and diffusion from surface moisture.

Mosses:

• Location: soil, rocks, tree bark

Moss body:

• Thallus - body structure
• Rhizoids- hair-like absorptive structures.
• Do ot have true roots , stems and leaves which makes them being small.

Liverworts:

• Thallus- a body that lacks roots, stems or leaves.
• Liver shaped thallus so called as liverworts.

Hornworts:

• Thalloids - 1-2 cm length.
• A sporophyte form horn like projections out of the gametophyte thallus.
• No proper roots, stems and leaves.

HOW VASCULAR TISSUES HElP IN PLANT GROWTH WHICH ARE ABSENT IN BRYOPHYTES:

• Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue: the vascular cambium and the cork cambium. All the vascular tissues within a particular plant together constitute the vascular tissue system of that plant.
• Between the xylem and phloem is a meristem called the vascular cambium. This tissue divides off cells that will become additional xylem and phloem. This growth increases the girth of the plant, rather than its length. As long as the vascular cambium continues to produce new cells, the plant will continue to grow more stout. In trees and other plants that develop wood, the vascular cambium allows the expansion of vascular tissue that produces woody growth.

HOW LIFE CYCLE OF BRYOPHYTES DEPENDENT ON WATER FILM:

FIG : Life cycle of bryophytes.

• Bryophytes also need a moist environment to reproduce.

Primitive bryophytes like mosses and liverworts are so small that they can rely on diffusion to move water in and out of the plant.

Bryophytes also need a moist environment to reproduce. Their flagellated sperm must swim through water to reach the egg. So mosses and liverworts are restricted to moist habitats.

CONCLUSION:
The structure of mosses, liverworts and hornworts combinely called as non vascular plants do not have vascular tissues like xylem, phloem etc., that helps in transporting, give mechanical support to plants and also helps in plant growth. Absence of the tissues makes these non vascular plants small.

As seen in the lifecycle of the Bryophytes, they reproduce in water as sperm needs to swim through water to reach the egg for fertilization which makes them dependent on water film.