The first terrestrial plants evolved from an organism that, if it existed today, would be classified as a multicellular green alga. Extensive biochemical and morphological similarities between green algae and plants make these conclusion inescapable green algae and plants share chlorophyll a and b, and arytenoids, they also have cellulose in their cell walls and starch as their primary food reserve.
Moreover, the details of cell division are similar in the two groups. After mitosis is complete, animal cells pinch inward toward the centre, while plant cells from an interior cell plate, which grows out from the centre between the two daughter cells. Aside from plants, the only organisms that form a cell plate during cytokinesis are a few groups of green algae.
Living plants are grouped into 12 phyla. Members of 9 of the 12 plant phyla are collectively called ‘vascular plants’ and include among others, the ferns, conifers,, and flowering plants.
Vascular plants have water- conducting xylem and food conducting phloem strands of cells in their stems, roots, and leaves. Members of the other three phyla either lack such strands or have poorly developed ones. These three phyla are mosses (Phylum: Bryophyta), liverworts (Phylum: Hepaticophyta), and hornworts (Phylum: Antocerotophyta).
ADVERTISEMENTS:
The Green Invasion of the Land:
Plants and fungi are the only major groups of organisms that are primarily terrestrial. There are some 267,000 known species of living plants. They dominate almost every part of the terrestrial landscape, except for extreme deserts, Polar Regions, and the tops of the highest mountains.
Several other groups, including arthropods, chordates, and mollusk arc also well represented on land. Plants probably came first, their cell-walls helping to protect their propagules during dispersal. Most plants are protected from desiccation-the tendency of organisms to lose water to the air by a waxy cuticle secreted onto their exposed surfaces. The cuticle is relatively impermeable and provides an effective barrier to water loss.
Special structures in this barrier allow the entry of C02, which is necessary for photosynthesis, and they exist of oxygen by diffusion. These tiny slit-like or eye-shaped pores are called stomata (singular stoma) and make up about 1 % of most leaf and green stem surfaces. Water and gas diffusion into and out of a plant is regulated by the opening and closing of stonfiata, which typically number into the thousands per square centimeter.
ADVERTISEMENTS:
At an early stage of their evolution, plants evidently also developed a special kind of relationship with fungi that has been a key to their successful occupation of terrestrial habitats. Mycorrhizae (a symbiotic association of fungi with the plant roots) are found in about 80% of all plants and are frequently seen in fossils of the earliest plants.
Mycorrhizae play an important role among today’s plants in the assimilation of phosphorus and probably other ions. We can surmise that mycorrhizae also did so in the raw, unrelated soils that existed before the earliest plants. Many other features such as efficient water and food conducting systems developed and contributed to the success of plants on land. Roots, shoots, and leaves-expanded areas of photo synthetically active tissue-also evolved and diversified in relationship to the varied habitats that existed on land specialization in key reproductive features improved the chances for survival of embryonic tissues within seeds as they were dispersed.
Plants are terrestrial, with life cycles that alternate between haploid and diploid:
Plants are derived from an aquatic ancestor, a multicellular green algae but the evolution of their conducting tissues, cuticle, stomata, and seeds has made them progressively less dependent on external water for reproduction. The oldest plant fossils date from the Silurian period, some 330 million years ago.
ADVERTISEMENTS:
i. The evidence that plants are derived from green algae lies in their common photosynthetic pigments (chlorophyll a & b, carotenoids); chief storage product (starch); cellulose rich cell walls (in some green algae only) and cell division by means of a cell plate (in certain green algae only).
ii. All plants have a life cycle involving an alternation of generation in which haploid gametophyte alternate with diploid sporophyte. The spores that sporophyte forms as a result of meiosis grow into gametophytes, which produce gametes (sperms and eggs) by mitosis.
Nonvascular plants like mosses are relatively unspecialized:
i. The three phyla of plants that lack well- developed vascular tissues are the simplest in structure and have been grouped as bryophytes. This grouping does not reflect a common ancestory or close relationship between them.
ADVERTISEMENTS:
ii. Sporophytes of mosses, liverworts and hornworts are usually nutritionally dependent on the gametophytes’. They are mostly brown or straw- colored at maturity. The gametophytes which are more conspicuous, are green and nutritionally independent of the sporophytes.
Seedless vascular plants likes ferns have well developed conducting tissues:
i. Nine of the 12 plant phyla contain vascular plants, which have two kinds of well defined conducting tissues: xylem for water conduction and dissolved minerals and phloem for conducting food molecules that the plant manufacture.
ii. In ferns and other seedless vascular plants, the sporophyte generation is dominant. The fern sporophyte has vascular tissues and well- differentiated roots, stems and leaves.
ADVERTISEMENTS:
Seed plants like pine trees and roses have protected embryos specialized for dispersal:
i. Seeds were an important evolutionary advance. In seed plants, the gametophytes are nutritionally dependent on the sporophytes.
ii. In gymnosperms, ovules are exposed directly to pollens at the time of pollination, in angiosperms ovules are enclosed within an ovary, and a pollen tube grows from the stigma to the ovule.
iii. The pollen of gymnosperms is usually disseminated by wind, although some angiosperms are wind-pollinated in many the pollen is transported by insect and other animals. The ripened ovaries1 of angiosperms develop into fruits. Both flowers & fruits are found only in angiosperms. The plant kingdom is divided into 4 major kingdoms:
ADVERTISEMENTS:
Division 1 -> Thallophyta:
(i) Thallus like plant body with no differentiation into root stems and leaves.
(ii) Vascular tissues absent.
(iii) Reproductive organs are simple, unicellular & non-jacketed.
(iv) Asexual reproduction takes place by spores or zoospores during growing season.
(v) No embryo formation after the fusion of gametes.
(vi) This major division contains about 1,10,000 species.
Subdivision 1 -> Algae:
(i) Plants containing chlorophyll and thallus like plant body without true roots & leaves.
(ii) In many of the algae chlorophyll is marked by brown, red or yellow pigments.
(iii) Algae are divided into a number of classes on the basis of pigments and other features.
Class 1: Cyanophyceae-Blue green algae
Class 2: Chlorophyceae-Green algae
Class 3: Phaeophyceae-Brown algae
Class 4: Rhodophyceae-Red algae
Class 5: Xanthophyceae-Yellow green algae.
(iv) Algae are microscopic, unicellular to larger multicellular plants.
(v) In algae, generally the reproductive organs are unicellular but they may be multicellular is some brown algae.
(vi) In multicellular reproductive organs all the cells are fertile and take part in reproduction.
(vii) There is no jacket of sterile cells surrounding the reproductive organs.