Life Cycles of Pteridophyta
There is great diversity in the life cycles of vascular plants and therefore we shall briefly and comparatively outline the major features of the life cycles of fern, pine and flowering plants.
Life Cycle of Fern
For the most part, the life cycle of fern resembles that of moss. Fern gametophytes are great photosynthetic plants inhabiting most places and resemble gametophytes of certain liverworts. They are flat, thin and heart-shaped, along with rhizoids on their lower surface. The antheridia and archegonia are present. Sperms get released from the antheridium when they are mature. When they reach the mouth of the archegonium, they swim into the neck and eventually reach the egg in the swollen lower portion of the archegonium. There the sperm fuses with a single egg, producing a zygote. The zygote divides and a sporophyte is formed. Fern sporophytes, however, are green and become independent nutritionally. The fern plants that we see are sporophytes.
The life cycle of fern differs from that of moss primarily in the much greater development, independence and dominance of the fern's sporophyte. In addition, the fern's sporophyte is much more complex than that of the moss, having a vascular tissue and well-differentiated roots, stems and leaves.
Majority of the gymnosperms are represented by conifers, which grow in cool climate of hills, sometimes using melting snow as a source of water. However, some like cycads and members of Gnetales thrive in warm dry climate. We may find a few members looking like palms growing in the plains. The most common one is Cycad. People in plains have started growing conifers like Araucaria as ornamental evergreen plants. The common gymnosperms are Abies, Cedrus, Pinus, and other timber-yielding species. These are woody tree and only a few are bushy trailing or climbing shrubs like Ephedra and Gnetum with distinct root, stem and leaves. They grow successfully because they have well developed vascular system with xylem without vessels and phloem without companion cells. These help them to absorb and conduct water in tall trees. They, being without flowers, have also acquired seed habit, which enables them to overcome the requirement of external water for transfer of male gamete to the female gamete. The sporangia formed on special leaf-like structures called sporophylls are of two types. The mega sporophyll (mega: big; sporophyll: leaf-like structures with sporangium) bears megasporangium (ovule). The microsporangium also called pollen sac, is enclosed in a microporophyll (micro: small; sporophyll:leaf-like structures). The microsporophyll and megasporophyll forms the male and female cones, respectively, which are so characteristic of conifers. The microsporangium contains pollen grains which are transferred by wind to megasporangium or the ovule. These produce male and female gametes, respectively. A diploid zygote formed after fertilisation of the egg develops into an embryo within the ovule and the ovule ripens into a seed. However, in Cycads the megasporophylls are not organised in definite cones. The gymnosperms show a distinct alteration of generations where the dominant phase is represented by the sporophyte. These are further divided into three classes - Cycadopsida, Coniferopsida and Gnetopsida on the basis of nature of leaves, wood, vascular system and reproductive structures.
Life Cycle of PineIn Pines, the megagametophytes differentiate within the protective tissues of the parent sporophyte. The seeds of pines, by virtue of the stored food, not only protect but also nourish the young embryos that they contain.
The microsporangia are borne in pairs on the surface of thin scales of the relatively delicate pollen-bearing covers. The pollen grains are shed in huge quantities, often appearing as a yellow scum on the surface of ponds, lakes, etc. The seed-bearing cones of pines are much heavier. Two ovules, and ultimately two seeds, are borne on the upper surface of each scale. The sperms lack flagella and ultimately fuse with the egg, producing a zygote. The growth and differentiation of the embryo takes place within the developing seed. The seed coat and the embryo are diploid, whereas the nutritious tissue is haploid. The seeds are shed from the cone. They are winged and flutter to the ground, germinating if conditions are favourable and then giving rise to a new plant.
A complete flower consists of four whorls. Important for their life cycle to continue are the two inner whorls. The innermost whorl contains the carpels enclosing the ovules. The receptive portion of the carpel the stigma is often separated from the body of the carpel by a stalk called the style. Because of the resemblance of an individual carpel with the pestles used to grind powder in mortars, they have traditionally been called pistils. Many angiosperms may have only one carpel in each flower, but the whorl in which the carpels occur is collectively called the gvnoecium (Gr., gynos meaning "female" and oikos meaning 'house'). The next whorl of the flower on the floral axis is the androecium (Gr., andros meaning male). The individual members of the androecium are the stamens, consisting of a slender, thread-like portion, the filament, and a terminal, thicker, two-lobed anther. The anther contains the pollen grains shed either at the two or the three-called stage. If it is shed when it has only two cells, one further mitotic division occurs afterwards resulting in formation of two sperms.
Fertilization in angiosperms is a unique process that is highly characteristic of this division of plants. Both the sperms in the mature microgametophyte are functional. The first fuses with the egg forming the zygote. The second sperm, however, fuses with the polar nuclei, forming the primary endosperm nucleus which is triploid. The process whereby both the egg and the polar nuclei are fertilised, forming the zygote and the primary endosperm nucleus, is called double fertilisation. The primary endosperm nucleus divides and forms the endosperm. The zygote divides and forms the embryo. Endosperm is the primary nutritional tissue on which the developing embryos of these plants depend. Apart from double fertilisation, the development of angiosperms is roughly the same as that of gymnosperm seeds. When the seed coat gets mature and tough, the seed is shed under favourable conditions. The seed then germinates and a sporophyte is established.
The two outer whorls of a flower are collectively known as perianth. Though they do not assist in the production of gametes, they help to establish the distinctive characters of the angiosperm. The whorl surrounding androecium is called the corolla, members of which are called petals. Petals are generally bright coloured and may perform an important role in attracting insects and other pollinators. The fourth and the outermost whorl of a flower is known as the calyx, members of which are called sepals. It is generally greenish and not particularly attractive. It forms the outer layer of the bud before the flower opens and appears to serve mainly a protective function.