Question

Discuss in detail the evolution of land plants from an aquatic algal ancestor. Include modern day representatives of the major plant groups and traits that are conserved and traits that are derived in each group. Outline how derived traits overcame certain obstacles imposed by life on land.

Answer 1

It is believed that land plants originated from green algae during Silurian period of the Paleozoic era some 410 million years ago. The common basis of assuming so is due to the similarity in following parameters-

a) Life cycle of both green algae and land plants include gametophyte and sporophyte generations.

b) Both algae and plants have Chlorophyll a and b along with accessory pigments such as beta carotene and xanthophyll

c) Starch is the main source of food and energy storage.

d) Both have cell walls made up of cellulose

There were several factors that were acquired for survival of plants on land. These factors are as follows –

a) Reduction of water loss – Plants on land need to prevent dessication. Thus, cuticle evolved as a waxy coating over all the exposed parts of plants – leaves, flowers, stem, etc.

b) Gaseous exchange through cuticle – In order to exchange gas it was necessary to have pores in the cuticle. Therefore, stoma developed pair of guard cells to regulate opening and closing of pore.

c) Structural support to cater the stress of gravity – In aquatic environment, water provides substantial amount of support against gravity. On land, plants started utilizing turgor pressure to make the stem rigid. Plant pump water from stem into parenchyma.

d) Gametes need defense mechanism against dehydration and damage. This was ensured through various structures such as – water tapping layer of cell, pollen gran, seed coatings and fruit.

e) Protection from harmful sunlight through flavonoids.

Plant evolution and adaptations of above features –

The first land plants following algae were bryophytes. Bryophytes started originating at the edge of ponds and streams. They possessed stoma and a waxy cuticle to protect themselves from desiccation. It also had protective layers around the gamete. But still it had no vascular tissue and ability to fuse sperm with egg in absence of water. With later progression they developed conducting tissue Xylem and Phloem and also a mechanism of pollination.