Chapter 3 : Plant Kingdom
1. What is the basis of classification of algae?
Answer. Fritsch (1935), has classified algae considering phylogeny, affinities and inter-relationships of various forms. Algae are classified into three main classes – Chlorophyceae, Phaeophyceae, and Rhodophyceae. These divisions are based on the following factors:
(a) Major photosynthetic pigments present
(b) Form of stored food
(c) Cell wall composition
(d) Number of flagella and position of insertion
2. When and where does reduction division take place in the life cycle of a liverwort, a moss, a fern, a gymnosperm and an angiosperm?
Answer. Liverwort – In liverworts, the main plant-body is haploid (gametophytic). It bears the male and female sex organs which produce gametes. These gametes fuse to form a zygote. The zygote develops on the gametophytic plant-body to form a sporophyte. The sporophyte is differentiated into the foot, seta, and capsule. Many haploid spores are produced as a result of the reduction division taking place inside the capsule.
Moss – In mosses, the primary protonema (developed in the first stage) develops into the secondary protonema. Both these stages are haploid or gametophytic. The secondary protonema bears the sex organs which produce gametes. These gametes fuse to form a zygote. The zygote develops into a sporophyte. Many spores are formed as a result of the reduction division taking place in the capsule of this sporophyte.
Fern – In ferns, the main plant-body is sporophytic. Its leaves are known as sporophylls and these bear the sporangia. Reduction division takes place in these sporangia, thereby producing many spores.
Gymnosperm – In gymnosperms, the main plant-body is sporophytic. They bear two types of leaves – microsporophylls and megasporophylls. Reduction division takes place in the microsporangia present on the microsporophylls (producing pollen grains) and on the megasporangia present on the megasporophylls (producing megaspores).
Angiosperm – In angiosperms, the main plant-body is sporophytic and bears flowers. The male sex organ in the flower is the stamen, while the female sex organ is the pistil. Reduction division takes place in the anthers of the stamen (producing haploid pollen grains) and in the ovary of the pistil (producing eggs).
3. Name three groups of plants that bear archegonia. Briefly describe the life cycle of any one of them.
Answer. Bryophytes, Pteridophytes and Gymnosperms are the three groups of plants that bear archegonia.
Life cycle of a bryophyte is as follows : The main plant body of bryophyte is gametophytic (n), which is independent and may be thallose (no differentiation in root, stem, leaves) e.g., Riccia, or may be foliose (having leafy axis) e.g., Funaria. The dominant phase in the life cycle of Funaria is the gametophyte, which occurs in two stages, the protonema stage and the erect, leafy gametophytic plant.
The leafy gametophyte consists of an upright, slender axis (stem-like) that bears spirally arranged leaves and is attached to the substratum by multicellular, branched rhizoids. Vegetative reproduction takes place by fragmentation; by the buds formed in secondary protonema etc. The sex organs, antheridia and archegonia are produced in dusters at the apices of the leafy shoots. Antheridia produces antherozoids and archegonia produces egg. Antherozoid (male gamete) and egg (female gamete) fuses and form zygote.Zygote develops into a sporophyte; which is differentiated into foot, seta and capsule and spores are produced in the capsule.Spores on reaching a suitable substratum germinate to produce a filamentous juvenile stage, .called the primary protonema, which later produces secondary protonema that forms erect leafy plants.
4. Mention the ploidy of the following rprotonemal cell of a moss; primary endosperm nucleus in dicot, leaf cell of a moss; prothallus cell of a fern; gemma cell in Marchantia; meristem cell of monocot, ovum of a liverwort, and zygote of a fern.
Answer. Protonemal cell of a moss – haploid.
Primary endosperm nucleus in dicot – triploid.
Leaf cell of a moss – haploid.
Prothallus cell of a fern – haploid.
Gemma cell in Marchantia – haploid.
Meristem cell of monocot – diploid.
Ovum of a liverwort – haploid.
Zygote of a fern – diploid.
5.Write a note on economic importance of algae and gymnosperms.
Answer. Economic importance of algae
Algae have diverse economic uses. They perform half of the total carbon dioxide-fixation on earth by photosynthesis, acting as the primary producers in aquatic habitats.
(a) Food source: Many species of marine algae such as Porphyra, Sargassum, and Laminaria are edible. Chlorella and Spirulinaare rich in proteins. Thus, they are used as food supplements.
(b) Commercial importance: Agar is used in the preparation of jellies and ice-cream. It is obtained from Gelidium andGracilaria. Carrageenin is used as an emulsifier in chocolates, paints, and toothpastes. It is obtained from the red algae.
(c) Medicines: Many red algae such as Corallina are used in treating worm infections.
Economic importance of gymnosperms
(a) Construction purposes: Many conifers such as pine, cedar, etc., are sources of the soft wood used in construction and packing.
(b) Medicinal uses: An anticancer drug Taxol is obtained from Taxus. Many species of Ephedra produce ephedrine, which can be used in the treatment of asthma and bronchitis.
(c) Food source: The seeds of Pinus gerardiana (known as chilgoza) are edible.
(d) Source of resins: Resins are used commercially for manufacturing sealing waxes and water-proof paints. A type of resin known as turpentine is obtained from various species of Pinus.
6. Both gymnosperms and angiosperms bear seeds, then why are they classified separately?
Answer. ’Gymnosperms and angiosperms both bear seeds but they are classified separately because gymnosperms are a group of plants in which the ovules are freely exposed on open megasporophylls, whereas in angiosperms the seeds or ovules are enclosed within ovary which later forms the fruit.
7. What is heterospory? Briefly comment on its significance. Give two examples.
Answer. The two kinds of spores borne in the same plant is called as heterospory. These spores differ in size. Among them the smaller spore is called microspore and the larger spore is called megaspore.
Significance of heterospory:
(i) Heterospory is associated with the sexual differentiation of gametophyte a microspore develops into a male gametophyte whereas a megaspore develops into a female gametophyte.
(ii) In homosporous pteridophytes spores have to germinate on soil thus face more environmental problems. In heterosporous pteridophytes, spores germinate within the sporangium and the gametophytes are retained inside for variable periods of time. Hence, germinating gametophyte has better chances of survival. This lays the foundation of complete retention of gametophytes within sporophytes in angiosperms and gymnosperms.
(iii) Heterospory is the basis of development of seed habit in higher plants.
8. Explain briefly thefollowing terms with suitable examples.
Answer. (i) Protonema : It is the first, usually branched, green and filamentous structure produced by a germinating moss or fern spore. The protonema of mosses bears buds that develop into the gametophyte plant. In fern the protonema becomes the prothallus.
(ii) Antheridium : The male sex organ of cryptogams (algae, fungi, bryophytes and
pteridophytes) is known as antheridium. It produces the male gametes or anthero- zoids. It may consist of a single cell or it may have a wall that is made up of one or several layers forming a sterile jacket around the developing gametes.
(iii)Archegonium : The multicellular flask shaped female sex organ of bryophytes, pteridophytes and many gymnosperms is known as archegonium. Its dialated base called the venter contains the female gamete or egg or oosphere. The cells of the narrow neck of archegonium liquify to allow the male gametes to swim towards the oosphere.
(iv)Diplontic : It is the kind of life cycle in which the diploid sporophyte is dominant and this diploid phase is photosynthetic. The gametophytic phase is represented either by gametes only, that are formed through meiosis or by a highly reduced few celled gametophyte. E.g., all seed-bearing plants (gymnosperms and angiosperms).
(v) Sporophyll : It is a type of leaf bearing sporangia. In ferns, the sporophylls are the normal foliage leaves, but in other plants the sporophylls are modified and arise in specialised structure such as the strobili of club-moss, gymnosperms and the flower of angiosperms. In most plants sporophylls are of two types – microsporophylls and megasporophylls.
(vi)Isogamy: It is a type of sexual reproduction where fusion takes place between two identical gametes. The gametes are similar in size and structure and they show equal motility during sexual reproduction, e.g., Spirogyra (algae).
9.Differentiate between the following:
(i) Red algae and brown algae
(ii)Homosporous and heterosporous pteridophytes
(iii)Liverworts and moss
(iv)Syngamy and triple fusion.
Answer. (i) The differences between red algae and brown algae are as follows :
|Red algae||Brown algae|
|1.||Red algae are grouped under the class Rhodophyceae.||1.||Brown algae are grouped under the class Phaeophyceae.|
|2.||They contain floridean starch as stored food.||2.||They contain mannitol or laminarin as stored food.|
|3.||They contain the photosynthetic pigments chlorophylls a and d, and phycoerythrin.||3.||They contain the photosynthetic pigments chlorophylls a and c, and fucoxanthin.|
|4.||Their cell walls are composed of cellulose, pectin, and phycocolloids.||4.||Their cell walls are composed of cellulose and algin.|
|5.||Flagella are absent||5.||Two flagella are present|
(ii) The differences between homosporous and heterosporous pteridophytes are as follows:
|Homosporous pteridophytes||Heterosporous pteridophytes|
|1.||They bear spores that are of the same type.||1.||They bear two kinds of spores – microspores and megaspores.|
|2.||They produce bisexual gametophytes.||2.||They produce unisexual gametophytes.|
(iii) The differences between liverworts and mosses are as follows :
|1.||They have unicellular rhizoids.||1.||They have multicellular rhizoids.|
|2.||Scales are present very often||2.||Scales are absent|
|3.||They are generally thalloid, with dichotomous branching.||3.||They are foliage, with lateral branching.|
|4.||Gemma cups are present||4.||Gemma cups are absent|
|5.||Sporophyte has very little photosynthetic tissue||5.||Sporophyte has abundant photosynthetic tissue|
(iv) The differences between syngamy and triple fusion are as follows :
|1.||It is the process of fusion of the male gamete with the egg in an angiosperm.||1.||It is the process of fusion of the male gamete with the diploid secondary nucleus in an angiosperm.|
|2.||A diploid zygote is formed as a result of syngamy.||2.||A triploid primary endosperm is formed as a result of triple fusion.|
10. How would you distinguish monocots from dicots?
Answer. Differences between monocots and dicots are as follows :
|Roots||Fibrous roots||Tap roots|
|Venation||Generally parallel venation||Generally reticulate venation|
|Flowers||Trimerous flowers||Pentamerous flowers|
|Cotyledons in seeds||One||Two|
|No. of vascular bundles in stem||Numerous||Generally 2 – 6|
11. Match the following (Column I with Column II).
12.Describe the important characteristics of gymnosperms.
Answer. Important features of gymnosperms:
1. The term gymnosperm refers to plants with naked seeds (gymnos – naked, sperma – seeds), i.e., the seeds of these plants are not enclosed in fruits.
2. The plant-body ranges from medium to tall trees and shrubs. The giant redwood tree Sequoia is one of the tallest trees in the world.
3. The root system consists of tap roots. The coralloid roots present in Cycas are associated with nitrogen-fixing cyanobacteria.
4. The stem can be branched (as in Pinus and Cedrus) or un-branched (as in Cycas).
5. The leaves can be simple (as in Pinus)or compound (pinnate in Cycas). The leaves are needle-like, with a thick cuticle and sunken stomata. These help in preventing water loss.
6. Gymnosperms are heterosporous. They bear two kinds of spores – microspores and megaspores.
7. Flowers are absent. The microsporophylls and megasporophylls are arranged to form compact male and female cones.
8. Pollination occurs mostly through wind and pollen grains reach the pollen chamber of the ovule through the micropyle.
9. The male and female gametophytes are dependent on the sporophyte.
10. The seeds contain haploid endosperms and remain uncovered.