The seed of an angiospermic plant consists of an embryo covered by the seed-coat. The embryo has one or two cotyledons and the tigellum. One end of the tigellum is the radicle and the other end is the plumule. On germination the radicle penetrates the soil and produces the roots, while the plumule develops into the shoot of the plant body.
The shoot gradually manifests itself into the stem and branches which bear the leaves and flowers (Fig. 1). The leaves are nothing but appendages of the stem and are regarded as the lateral expansions of the stem or branches of the stem. The flowers are reproductive organs and each flower is considered to be a modified condensed branch of the stem.
The central core of the stem-root axis is called “stele” which consists of the vascular tissues and the ground tissue. The vascular tissues are composed of conducting strands called vascular bundles. A vascular bundle is again composed of xylem and phloem. In higher plants the vascular bundles form a skeleton in the plant body that differs from its plan and arrangement in different species.
The older axis is the “Mesome” which branches dichotomously, and each terminal portion of the dichotomously branching mesome is called “Telome”. The mesome gives rise to groups of telomes or telome- trusses. These telome-trusses or branches are in the same plane and by development of connecting tissues they fuse to form a flat structure which is the megaphyll or a leaf.
According to the Phytonic theory a shoot is made up of a series of units called “Phytons”, and the growth is continued by renewed terminal budding. In the leaf-skin theory of Saunders on the other hand the shoot or the stem is regarded as composed of leaf-bases or extensions of leaf-bases.
The shoot, however, is now generally regarded as one of the two basic parts of the plant, the other being the root. The leaves are regarded as ultimate parts of the shoot. There are 2 traces and one lacuna in a primitive type of node while the branches of the stem also have 2 traces and one gap.
That the leaf is, like the branch, a part of the shoot is quite clear. Flower is a specialised structure bearing the reproductive organs or the sporophylls. Flowers may be borne in the axils of leaves or may be terminal on the branches or the main axis of the shoot.
A typical flower consists of the following parts:
(1) Thalamus or the axis of flower,
(2) Calyx of united or free sepals,
(3) Corolla of united or free petals,
(4) Androecium of stamens, and
(5) Gynoecium or Pistil of carpels.
The stamens are nothing but micro-sporophylls and carpels are mega-sporophylls. The stamens and carpels are the reproductive organs, and are, therefore, the essential parts of a flower. A fertile stamen consists of an anther which is 2-lobed.
Each anther-lobe usually has 2 pollen-sacs or pollen-chambers which are the microsporangia containing numerous pollens or microspores in each. The stalk of the anther is called filament and the anther-lobes are connected by a connective which is nothing but an extension of the filament.
The anther at the beginning is nothing but a mass of meristematic cells surrounded by a layer of epidermal cells. Soon there appear four longitudinal rows of archesporeal cells. An archesporeal cell divides into 2 cells of which one towards the epidermis is the parietal cell and the other on the inner side is the sporogenous cell.
This, after several divisions, gives rise to a number of microspore mother cells or pollen mother cells. The nucleus of a microspore mother cell by reduction division forms 4 haploid nuclei. Each nucleus is then surrounded by a wall and is transformed into a microspore or pollen.
Soon a pollen becomes bi-nucleate by the splitting of the nucleus. By a wall formation between the 2 nuclei two very unequal cells are formed.
The larger one is the tube cell or vegetative cell and the smaller one is the generative cell that floats freely in the tube cell. The nucleus of the generative cell divides mitotically forming two male gametes. A gamete with its surrounding cytoplasmic sheath is the male gamete cell (Fig. 2).
The gynoecium which is composed of one or several carpels is situated at the apex of the thalamus. The basal portion of the gynoecium is swollen and hollow inside, forming a chamber. This portion is called the ovary.
The portion above the ovary is like a narrow tube and is known as the style, which is terminated by short receptive portion called the stigma where the pollens are deposited during pollination. The ovary contains the ovules, situated on a special tissue called placenta on the inner wall of the ovary or on the central axis or on the capillary suture.
The ovule consists of 2 basic parts, viz., the nucleus, which is nothing but the mega sporangium and the integuments or the 2 protective coverings. A small opening at the top of the integuments is called micropyle and the base of the nucleus is called chalaza.
The ovule usually has a stalk by which it is attached to the placenta and this is known as the funicle, and the portion of attachment of the funicle with the ovule is called hilum.
The megaspore mother-cell develops within the nucleus and gives rise to 4 haploid megaspore cells of which the lowermost becomes the functional megaspore and the other 3 degenerate. The functional megaspore develops into the female gametophyte or the embryo sac.
The functional megaspore or the embryo sac nucleus splits into two, which split again into four and finally the four into eight, three going to each pole and two remaining at the centre. The central pair fuse and form the fusion- or secondary-nucleus.
The three nuclei at the top form the egg-apparatus consisting of 2 synergids and an egg-cell, also called ovum or oosphere. The remaining three at the base form the antipodal cells. The gametophyte formation, however, shows much more variation than the normal type described here.
Of the 2 integuments the outer one is of cupular origin, i.e., a remnant of the cupule of the seed-ferns. The inner integument is formed according to Margaret Benson’s hypothesis (1904) by the fusion of sterile mega sporangia of a synangium in which only one remains fertile and that one is the ovule (Fig. 3).
A flower complete with androecium and gynoecium is a bisexual flower and that with only one of these two essential parts present is a unisexual flower. The basic type of angiosperm-flower is bisexual and the unisexual flowers have arisen as a result of reduction of either the androecium or gynoecium and this is evident from the presence of vestigial organs or rudiments in many unisexual flowers.
The transfer of pollens from the anther to the stigma is called pollination. The transfer is effected in various ways. Fertilisation follows pollination. After pollination pollen germinates on the stigma. A tube-like structure projects through the germ-pore of the pollen and is known as pollen-tube. The vegetative nucleus enters the tube and is now called the tube-nucleus.
This is followed by the generative cell. The generative cell now divides mitotically to form two male gametes while the tube nucleus ultimately disappears. The pollen tube passes through the style to the ovule and through the micropyle to the nucleus and penetrates the embryo sac. The tip of the pollen tube dissolves and the 2 gametes are set free.
One of them fuses with the egg-cell of the egg apparatus and fertilises the same. The other gamete fuses with the secondary nucleus thus causing the double fertilisation.
As a result of fertilisation the oosphere or egg-cell is transformed into oospore which is a diploid cell and the secondary nucleus which is formed by the fusion of 2 haploid nuclei is changed to a triploid nucleus after being fertilised by the second male gamete.
The oospore forms the embryo and the fertilised fusion nucleus after free nuclear division and cell wall formation produces the endosperm. The antipodal cells in the meantime disappear.
The ovule is transformed into a seed after the full development of the embryo and formation of the endosperm. Thus the life cycle of an angiosperm from the germination of a seed giving rise to a plant that ultimately produces fruit and seed, passes through sporophyte to gametophyte and gametophyte to sporophyte.
The plant body which bears the numerous pollens or microspores lodged in the anther of stamens or microsporophyll’s and four megaspores within the nucleus of the ovule borne on the carpel or megasporophyll is the diploid sporophyte (Fig. 4).
The pollens which carry the 2 male gametes and the embryo sac which contains the oosphere are the haploid gametophytes. The embryo formed after fertilisation is diploid and forms a part of the next sporophytic generation.
In a seed, therefore, we find the vestiges of 3 generations, viz., the seed coat and funicle as part of the 1st generation which is a sporophyte, i.e., the plant body, the short-lived embryo sac representing the gametophytic 2nd generation and the embryo which is the preliminary stage of the 3rd generation which again is a sporophyte.
The endosperm in angiosperms, unlike that of gymnosperms or heterosporous pteridophytes, is a new tissue formed by the fusion of the second male gamete with the secondary nucleus which again is a result of fusion of two of the eight nuclei derived from the embryo sac nucleus. Endosperm cells are, therefore, triploid in nature.