SALIENT FEATURES AND CLASSIFICATION OF PLANTS
Plants are unique in their physical appearance, structure, and physiological behaviour. Apart from that, they also vary in their habitats, tolerance to the environment, and nutrient requirements.
Biologist Whittaker gave the Five Kingdom Classification, for all the living organisms. He divided the living organisms into five kingdoms – Protista, Monera, Fungi, Plantae, and Animalia. In this topic, we will learn about the special features and classification of Plants or Kingdom Plantae.
During the early times, the biologists used the apparent features of plants and classified them based on these features. The morphological features included colour, number, the shape of leaves their habitats, etc.
This system of classification was an artificial system of classification because the plants were classified based on their physical characters. The vegetative characters are susceptible to changes because of the effect of the environment. Therefore, many closely related species were classified under different divisions.
Gradually, biologists became more aware of the other characteristics of plants, so they slowly started another system of classification, called the natural system of classification. This system considered the external and internal features of plants for classifying them.
According to the new system of classification, the Kingdom Plantae has been divided into five major groups. They are:
i. Thallophyta or Algae
Botanists have grouped the plants into two major groups: non-vascular and vascular. The former includes the early plants without having a vascular system. The latter (vascular plants) consists of all the members who had developed a vascular system. Vascular system, in plants, is an assembly of conducting tissues and associated supportive fibers. Xylem tissue is a conducting tissue that transports water and dissolved minerals to the leaves. Phloem is a tissue that conducts food from the leaves to all parts of the plant. The commonly used plant classification system is:
The nonvascular plants do not possess any vascular tissues that can help them transport water and nutrients. These types of plants are considered to be the earliest living plants on the planet. The non- vascular plants include:
1. Algae: They are plant-like organisms that are usually photosynthetic and aquatic, but do not have true roots, stems, leaves, vascular tissue and have simple reproductive structures. Some authors categorize the microscopic, unicellular green algae (Division Chlorophyta) in the Kingdom Protista, and the larger ones, multicellular green algae (Division Chlorophyta) in the Kingdom Plantae. Algae are ubiquitous in nature,they are found in the sea, in freshwater andmay be found everywhere in moist conditions. Most algae are microscopic, but some are very large, e.g. some marine seaweed which exceeds 50 m in length. The algae possess chlorophyll and can manufacture their own food through the process of photosynthesis.
i. They are the most diverse group with more than 10,000 plant species. They constitute the simplest plant phyla.
ii. This phylum includes mosses, liverworts, and hornworts. Regarding physical appearance, mosses are small and inconspicuous.
iii. Bryophytes do not have vascular tissue and any wood that can give structural support to them.
iv. They lack true leaves, stem, and roots that can help them transport water and nutrients. Hence, they grow in a narrow range of habitats.
v. Bryophytes play an important role in lessening erosion along with the water bodies, carrying out water and nutrient cycling in forests, and regulating the temperature in permafrost (a thick subsurface layer of soil that remains below freezing point throughout the year, it occurs chiefly in polar regions).
vi. They are closely related to lichens (the symbiotic relationship between a fungus and algae) if compared to their habitats and physical structures. Like, both of them utilize the moisture in the environment to transport minerals and nutrients.
vii. Bryophytes live in moist places and have adapted to several methods that can help them bloom in dry periods. They reproduce through spores.
Vascular plants possess vascular tissues (xylem and phloem) that aid them to transport water and minerals throughout the body of a plant. This group includes members of the Phylum Pteridophyta, Gymnosperms, and Angiosperms. All these are classified as vascular plants. The different plant phyla are described below:
1. Phylum Pteridophyta
i. It is composed of almost 12,000 (with over two-thirds are tropical) species of true ferns and its allies.
ii. Pteridophytes are seedless plants. This classification of plants produces spores that are located beneath their leaves and are known as sporophylls.
iii. Pteridophytes can propel their spores even at long distances because of the spring-like structures called sporangia, which contains spores.
iv. There is no single characteristic that can describe pteridophytes because they are extremely diverse. Leaves of ferns are called fronds, which are naturally coiled until they unroll at maturity. They have horizontal stems called rhizomes and have simple leaves roots. As they are vascular plants, they are capable of transporting fluids.
v. With time, pteridophytes have already adapted to a wide range of habitats, like; they can be aquatic, terrestrial, etc. but most of them prefer to thrive in tropical regions.
i. As compared with other plant phyla, gymnosperms include the tallest, the thickest, and the oldest living plants. They are widely distributed on the earth but dominate the temperate and arctic regions.
ii. Members of this phylum include pines, hemlocks, firs, and spruces, all the members are characterized by having wood, and green needle-like or scale-like foliage.
iii. “Gymnosperm” literally means “naked seed “.The members of this phylum are characterized by having cones (strobilus, plural: strobila), for reproduction instead of seeds.
iv. Gymnosperms are considered to be heterosporous. This means that they produce two different types of cones one for the male and female. The male cones are usually small as compared to the large cone of the female.
v. Members of this phylum are good sources of wood and paper. Apart from that, they provide food and habitat for animals, and these animals in exchange become important for gymnosperms in the dispersal of their propagules.
Some examples of gymnosperms are:
i. They are also referred to as the flowering plants, and they are the most diverse plant phylum with at least 260,000 living plant species.
ii. Angiosperms show a vast diversity of plants. This includes trees, herbs, shrubs, bulbs, epiphytes (parasitic plants), and plants living in both marine and freshwater habitats.
iii. The largest families in this phylum are the Orchidaceae (family of orchids), Asteraceae (family of daisies), and Fabaceae (family of legumes).
In addition to the diversity, the members of this phylum exhibit several distinguishing characteristics like;
a. Ovules/seeds that are enclosed within the carpel/fruit.
b. Double fertilization, takes place in angiosperms. Double fertilization is a complex fertilization mechanism of flowering plants, which involves the fusion of a female gametophyte (megagametophyte, also called the embryo sac) with two male gametes (sperm). Of the two sperm cells, one sperm fertilizes the egg cell, and forms a diploid zygote; the other sperm cell fuses with the two polar nuclei, to form a triploid cell that develops into the endosperm. The part of a seed that acts as a food store for the developing plant embryo is called endosperm. It usually contains starch, protein, and other nutrients.
c. Male reproductive tissue is composed of two pairs of pollen sacs and many more.
iv. Due to their varied types, angiosperms serve as a wide variety of uses for animals, especially humans. Most angiosperms are good sources of food, medicine, clothing fibers, and wood.
Some examples of Angiosperm
Read another topic of class 11 biology on Kingdom Protista and its Characteristics
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