Question

Discuss the life histories of a pine (Coniferophyta) and a flowering plant (Anthophyta). Include in discussion:
-the sequence of stages in the life histories of each and their reproductive structures
-definitions of seeds and fruits including the structures of each
-factors that have influenced the distribution and success of these seed-bearing plants (Hint: plant structure and form, mechanisms of water conservation and transport, pollination and seed dispersal mechanisms (including coevolution of the animal groups often involved in these mechanisms) should be prominent features of your answer)

Answer 1

Answer:

Life history of a pine tree

Pinus is a coniferous gymnosperm. It is a large tree having pyramidal or excurrent shape. The plant body is a sporophyte having stem, leaves and roots. Stem branches are dimorphic, i.e of two types, long and dwarf. Long shoots show unlimited growth and bear only sale leaves. The dwarf branches possess 1-5 needle-like foliage leaves. Pinus has a horizontally spreading tap root system with a mycorrhizal association. It is a monoecious tree and produces micro and megasporophylls in two types of cones, male and female. The phases of the life cycle of a pine tree are- (i) Strobilus or cones, (ii) Pollination, (iii) Fertilization and (iv) Growth.

(i) Strobilus or cones: A pine tree forms male and female cones that bears pollen and ovules respectively. Male cones are homologous to dwarf shoots. Each male cone has a short stalk, a central axis and a number of spirally arranged microsporophylls. A microsporophyll bears two oblong, parallel microsporangia on its lower surface. In a microsporangium, the diploid microspore mother cells undergo meiosis and form haploid microspores or pollen grains. The female cones are homologous to long shoots. Each female cone has a long stalk, and a central axis on which are borne a number of spirally arranged paired scales. The lower of the pair is called bract scale while the upper scale is ovuliferous scale. The ovuliferous scale bears two ovules towards the basal region on the upper side. Each ovule has a 3-layered integument with a terminal wide, oblique pore or micropyle and nucellus (megasporangium).

(ii) Pollination: The pollen grains formed by meiotic division from the diploid microspore mother cell are released from the male cones at 4 celled stage, having two prothalial cells, one generative cell and one vegetative cell and are dispersed by air currents. They form yellow clouds in the pine forests called as 'sulphur shower'. The megaspore mother cell differentiates and forms four haploid megaspores by meiosis but only one survives. Pollination is anemophillous (by air) and direct. The formation of male gametes occur after pollination. The pollen grains pass into the opened micropyle and rest on the tip of nucellus, where they are caught in the mucilagenous pollination drop oozed out by terminal part of nucellus. Here each pollen grain germinates and forms a pollen tube (siphonogamy) but further growth is stopped due to the arrival of winter in the first year.

(iii) Fertilization: It occurs after 13 months from the time of pollination. In the spring of next year, fertilization occurs. Pollen tube grows and pierces an archegonium. One male gamete or male nucleus fuses with the oosphere to form diploid zygote or oospore. The oospore forms an embryo while the ovule matures into a seed. Part of the upper surface of the ovuliferous scale is peeled off alongwith the seed to form its wing.

(iv) Growth: A female cone takes about 26 months for reaching maturity. It then opens to release winged seeds which are dispersed by air. After falling on suitable soil, each seed gives rise to a new plant.

Life history of a flowering plant

Anthophyta are the flowering plant, where ovules are enclosed in the ovary and hence, seeds are enclosed in the fruit. The life cycle of a flowering plant starts with a seed falling on a ground. The different stages of the life hsitory of a flowerring plant are as follows:

(i) Seed stage: The life cycle of a flowering plant begin with seed that contains embryo. The flowering plants are divided into two classes- on the basis of number of embryonic leaves or cotyledons: the dicotyledons and the monocotyledons. Dicots are characterized by the presence of two cotyledons in the seed, e.g pea, rose, mustard, etc. Monocots are characterized by the presence of a single cotyledon in the seed, e.g banana, grass, palms, etc.

(ii) Germination: When a seed falls on a substratum, it requires optimum conditions such as temperature, water, etc in order to germinate. Some seeds also need light to germinate.

(iii) Growth: The growth of the tiny seedling into an adult takes place through sequence of events. The roots ystem helps in the anchorage, and absorption of water and minerals from the soil and transport it to different parts. The plant produces leaves which is the site for photosynthesis. Lastly, the emergence of flower take place which is the shoot modified for reproduction. The male and female sex organs of a flower are stamen and pistil/carpel respectively. Each stamen consists of a slender filament with an anther at the tip. Anther is the pollen bearing part of a stamen. The anther encloses microsporangia. On the other hand, the pistil or the carpel has three parts- stigma, style and ovary. Stigma is the terminal receptive part of the pistil which functions as landing platform for the pollen grains. Style is elongated narrow stalk that connects the ovary with the stigma. Ovary is the basal swollen part of the pistil.

(iv) Reproduction: The anther encloses microsporangia , where the microspore mother cells undergo sporic meiosis to form microspores that develops into pollen grains. Inside the ovary is the ovarian cavity where the placenta is located. Arising from the placenta are the megasporangia, commonly called ovules. Each ovule has one or more protective envelops called integuments. Enclosed within the integumants is a mass of cells called the nucellus. A single megaspore mother cell of the nucellus undergo sporic meiosis to produce four haploid megaspores. Out of the four only one is functional while the other three degenerate. The functional megaspore develops into the female gametophyte or the embro sac. Thus embryo sac formation is preceded by meiosis. The functional megasopre divides mitotically forming 2-nucleate embryo sac. Two more sequential mitotic nuclear divisions results in the formation of the 4-nucleate and ultimately 8-nucleate stage, leading to the organisation of the typical 7-celled, 8-nucleate stage female gametophyte or embryo sac. Each embryo sac has a three celled egg apparatus- one egg cell and two synergids, three antipodal cells and two polar nuclei. The polar nuclei eventually fuse to produce a diploid secondary nucleus.

(v) Pollination: After the dispersal from the anthers, pollen grains are carried by wind or various other agencies to the stigma of a pistil. This transfer of pollen from anther to stigma is known as pollination. The pollen grain germinates on the stigma and the resulting pollen tube grows through the tissues of stigma and style and reach the ovule. The pollen tube enters the embro sac through the micropylar end and discharge the two male gametes present in the polen tube.

(vi) Fertilization: Once the pollen tube enters the embryo sac, the two male gametes present in the pollen tube are discharged.One of the male gametes fuse with the egg cell to form the zygote. This is known as syngamy. The other male gamete fuses with the diploid secondary nucleus to produce the troploid primary endosperm nucleus (PEN). Thius double fertilization occurs, an event unique to angiosperms. After fertilization, the synergids and antipodal cells degenerate.

(vii) Seed dispersal: After fertilization, the zygote develops into embro and the fertilized ovules into seeds and ovaries into fruit. The seeds are dispersed via animals and fall on suitable substratum to form a new plant.