Question

[4] Liverworts are the most ancient group of extant plants; successfully colonizing the land during the Ordovician at least 470 mya.

A. Present evidence that supports their ancient status and their origins during the Ordovician (you need at least 4 pieces of evidence). Be sure to explain why or how each piece of evidence supports the ancient status of the liverworts. (8 pts.)

B. From what you now know about the origins and life strategies of gametophyte dominant plants, i.e., the bryophytes (liverworts, hornworts and mosses), present one strategy that bryophytes utilized in order to colonize the harsh environmental conditions of the Ordovician/Silurian and that aided their survival to the present day and explain briefly how or why the strategy worked?. (4 pts.)

C. Did any of the three bryophyte phyla evolve into vascular plants? Support your answer with at least 2 pieces of evidence. (4 pts)

Answer 1

A.

Bryophytes are the first land plants having roots, stems and leaves. In a strict sense the roots, stems and leaves are well supported by vascular tissue that transports water and nutrients throughout the plant body. But, bryophytes lack the vascular tissue and hence are known as non vascular plants. The bryophytes are low lying and can grow up to maximum height of 20cm.

Bryophytes share some characteristics with vascular plants like alteration of generations in life cycle, the plant body covered by cuticle and stomata appear in hornworts and mosses. The complex apical tissue and male and female reproductive structures producing flagellated sperm and eggs are similar to higher plants but in bryophytes the gametophyte is dominant.

B.

There are three groups of bryophytes called liverworts, hornworts and mosses. The liverworts are further divided into two groups’ thallose liverworts and leafy liverworts. The thallose liverworts have flat end bodies and leafy liverworts appear like mosses. Due to the lobes in the thallus they are known as liverworts. In liverworts like Marchantia the thallus is 30 cells thick at the center.

The lower surface of the thallus contains hair like extensions called rhizoids. They anchor the plant body. Marchantia reproduces asexually by gemmae cubs that may detach from thallus and grow as a new plant. The sexual reproduction occurs through fertilization of antheridia producing flagellated sperms and archegonia releasing eggs. After fertilization, sporophytes are resulted with a foot, stalk and capsule within the archegonia.

Hornworts have rosette or ribbon like thallus. They live on trees and in moist and shaded areas. They form symbiotic relationship with cyanobacteria and are involved in nitrogen fixation from atmosphere. The sporophytes are small green brooms growing from the gametophyte. Sporophytes can also carryout photosynthesis. Hornworts can carry out asexual reproduction through fragmentation.

Mosses are the largest group of bryophytes with three distinct subgroups called peat mosses, granite mosses and true mosses. Mosses grow in damp and shaded locations. In dry environments they become shriveled and brown look like as if they are dead. Genus Sphagnum belongs to peat moss and has great ecological importance. It is grown in gardens to improve water holding capacity of the soils.

The gametophyte of mosses appears like alga and is a branching filament of cells called protonema. It contains leafy shoots and rhizoids. Antheridia and archegonia grow on the shoot. The sporophyte is dependent on gametophyte and contains foot, stalk and capsule. The capsule bears sporangium containing spores. The sporophyte is always attached to gametophyte in mosses. Initially the sporophyte is green and photosynthetic. At maturity the sporophyte becomes non photosynthetic and brown. The capsule breaks open to disperse the spores that may germinate in to new gametophyte.

C.

Lycophytes are the first vascular plants containing stem, leaves and roots. The leaves are known as microphylls containing single strand of vascular tissue. They are evolved as side extensions of the stem. The roots are lower extensions of the stem. Lycophytes are commonly known as club mosses containing three groups: ground pines (Lycopodium), spike mosses (Selagenella) and quillworts (Isoetes). In Lycopodium a cone shaped structure called strobilus occurs at the top of the plant body.

The strobili bear sporangia in which spores are produced through meiosis. The stem and microphylls contains vascular tissue. Lycophytes and some vascular plants produce spores that grow into only one type of gametophyte and hence are called homosporous. In some vascular plants, two types of spores called micro and mega spores are produced and are called heterosporous. The microspores develop into male gametophyte and megaspores develop into female gametophyte.

Cooksonia is a fossil vascular plant that appears like bush. It is a rhyniophyte that was flourished during Silurian period and is now extinct. Rhyniophytes were plants grown up to 6.5cm tall with no roots and leaves. They were simply stems with forked branches ending with sporangia.

In Cooksonia branching was significant because instead of single sporangium like other bryophytes cooksonia bear many sporangia and produced more spores. Cooksonia branching is characteristic related to plants that have vascular tissue. The spores of sporangia produced by cooksonia were windblown and hence, it is regarded as seedless vascular plant like lycophytes and other pteridophytes.