Introduction to Plant Evolution
Plants are photosynthetic eukaryotes and they are also called embryophytes since they produce an embryo that is protected by tissues of the parent plant. Plants are derived from a single branch of the evolutionary tree and hence said to be monophyletic. As per the fossil evidence, plants were derived from green algae, 500 million years ago. Invading the land was difficult for plants to adapt for several reasons. Hence, plants underwent a number of adaptations like development of roots, stems, leaves and seeds.
There is a great variation in the structure of plants. It ranges from microscopic to immense trees. Environment affects the shape, size, growth rate and physiological function of plants. There is a huge variation in types of environments. Tracheids are the specialized water conducting cells developed by plants as an adaptation to the terrestrial environment. Embryophytes having tracheids are called tracheophytes, which are made up of 10 phyla. Non-tracheophytes lack tracheids and have 3 phyla – Liverworts, Hornworts and Mosses. New mode of reproduction is required for land dwelling plants. More than 300,000 are present but many thousands remain unknown.
Plants are multicellular phototrophs, majority live on land and possess plastids, chlorophyll and generate energy by photosynthesis. They store starch as reserve food and their cell wall is made up of cellulose. Most plants reproduce sexually are capable of asexual propagation. Alternation of generations is a universal feature of the life cycles of plants.
Based on absence or presence of internal vascular system, plants are mainly divided into non vascular plants (Bryophytes) and vascular plants. Bryophytes are the earliest plants, evolved 500 million years ago. They require constantly moist environment. They possess rhizoids which are root like structures. Visible green structures are the gametophyte, which is multicellular and contain chloroplasts. Sporophyte produces unicellular haploid spores through meiosis within sporangium or capsules. Spores germinate to give rise to gametophyte.
Early vascular plants (tracheophytes) had no roots, leaves, fruits or flowers. Most ancient tracheophytes are homosporous, bearing a single type of spore. Both gametophyte and sporophyte are photosynthetic. Plants of today reproduce more by gametophyte generation than 500 million years ago. Vascular plants are more complex and organized. They include seedless plants, seed bearing forms and specialized transport cells – Xylem (water and minerals are transported) and phloem (sugar is transported). Seedless tracheophytes include: club moss, horse tails, ground pines and Ferns.
Seed bearing plants are successful due to their extensive root system and efficient vascular system like xylem and phloem. Two types of seed bearing tracheophytes are known: Gymnosperms (naked seed plants such as conifers) and Angiosperms (flowering plants). Angiosperms may either be monocots or dicots.
Leaves are adapted to absorb light with the help of chlorophyll and carbon dioxide. They have stomata to regulate water loss and have a waxy covering to prevent water loss and also possess rigid structures to prevent wilting. In order to grow vertically, lignin provides skeletal support, maintains plant rigidity and reinforces cellulose. Production of seeds was a major advancement in plant adaptability to live on land. It avoids competition, pathogens, inbreeding. Seeds survive harsh conditions, help disperse plants and seeds can survive many years. Pollen is a male gametophyte which can be blown great distances by wind or insects. Angiosperms have special adaptations: To attract pollinators with bright colored flowers. Fruits are useful n dispersal of their seeds. Annual plants live one season and die. Perennials live two years or more.