Division in Spermatophyta: Gymnosperms and Angiosperms

In this article we will discuss about the two main groups of division in Spermatophyta:- 1. Gymnosperms 2. Angiosperms.

Spermatophyta: Group # 1. Gymnosperms:

Gymnosperms are the naked-seeded plants. They formed dense vegetation in the past geological ages, but many of them are now extinct. Gymnosperms really constitute a group intermediate between angiosperms and pteridophytes. Flowers are very simple, devoid of accessory whorls.

Microsporophyll’s (stamens) and mega- sporophylls (carpels) arc often aggregated together in cones. Ovules present on the surface of the megasporophyll are directly pollinated by pollen grains. There is nothing like ovary, style and stigma; and naturally there is no fruit. Seeds remain naked. Unlike angiosperms, endosperm is formed before fertilization.

Cycas (Cycad):

Cycas is a very ancient gymnosperm, now to be found only in the tropical countries. They are cultivated in gardens for orna­mental purpose. Some of them are common sago-palms. The plant looks more or less like palms or tree ferns.’ The stem is short, erect, stout and usually un-branched.

It is covered by leaf-bases, and bears a crown of conspicuous pinnately com­pound leaves at the summit, arranged spirally alternating with brown scale-leaves (Fig. 215). Like ferns, they also exhibit circi­nate vernation.

Leaflets are thick, leathery and provided with thickly cutinised epidermis and sunken stomata. Cycas has tap root system. Some roots often develop characteristic swellings due to the attack of certain algae and may help in nitrogen fixation.

Reproduction:

Cycas is dioecious, i.e. microspoisophylls and megasporophylls are borne by different plants. A large number of microsporophyll’s remain aggregated together around an axis forming a compact ovoid cone or strobilus (Fig. 216, A).

Each microsporophyll is broadened above gradually becoming narrow below. Apical and basal portions are sterile, and the rest bear innumerable sporangia on the lower surface grouped in sori, a sorus consisting of 3 to 5 sporangia (Fig. 216, B & C).

Each microspore-mother cell produces a tetrad of microspores or pollen grains by reduction division. Micro­spore begins germination before liberation from the sporangium. A small cell, called prothallial cell, is cut off at one end, which solely represents the male gametophyte. The rest develops into a small generative cell and a large tube cell. In such three- celled condition the spore is liberated.

The megasporophylls do not form cone but constitute a body more or less leaf-like in appearance (Fig. 217). It is non-green and covered by dense brown hairs. Ovules, usually varying from 2 to 10, are present on the lower margin of each megasporophyll.

Ovules are quite large. Each of them mainly consists of nucellus invested by a single thick integument, which is fused with the nucellus almost throughout the entire surface, only leaving a small region near the micropyle.

Here, in fact, a small chamber, called pollen chamber, is formed for collecting the pollen grains. The megaspore-mother cell undergoes reduction division to form a linear tetrad of megaspores, of which the lowest one survives and the rest degenerate.

By free cell formation a tissue, called endosperm or female pro-thallus, is formed, which represents the female game­tophyte. Towards the micropylar region a few (3 to 6) archegonia develop, each having a short neck and venter. The latter contains the egg or female gamete.

Pollination and Fertilization:

Pollen grains carried by wind are caught in the mucilaginous secretion of the ovules and ultimately lodged in the pollen cham­ber. Now tube cell begins to elongate forming the pollen tube which goes through the nucellus.

The generative cell divides into a sterile stalk cell and a body cell. The body cell gives rise to a pair of quite large naked multi-ciliate spermatozoids which are set free when the end of the pollen tube bursts. They move to the archegonia, and one of them fuses with the egg, thus accom­plishing fertilization.

The resulting zygote (oospore) soon secretes a wall round it­self. It undergoes repeated cell division (by free-cell formation) and produces a multinucleated body called pro-embryo. Some of the cells from the upper region elongate to form a long slender much-coiled body, known as suspensor, which pushes the basal part of the pro-embryo deep into the female pro-thallus (Fig. 218).

Those basal cells develop into embryo with a central axis and two cotyledons. The whole structure is ultimately transformed into a seed, which consists of embryo, endosperm and a seed-coat deve­loped from the integument. After attaining maturity, they get detached and may germinate under suitable conditions to pro­duce new plants. The mode of germination is hypogenous.

Alternation of Generation:

Cycas shows distinct alterna­tion of two generations, sporo­phyte represented by the plant, and gametophyte by the male and female pro-thallus (endos­perm). Sporophyte is most domi­nant, and gametophytes are extremely reduced and dependent on the sporophyte.

Spermatophyta: Group # 2. Angiosperms:

Angiosperms are the most highly developed and conspicuous plants, capable of growing in all types of situation. They show wide variations as regards form and structure, ranging from very small aquatic duckweeds to the gigantic forest trees.

Angiosperms are characterised by the development of distinct flowers with accessory and essential whorls. One or more carpels unite by margins to form basal swollen ovary which encloses the ovules.

Pollen grains are received by the stigma and not directly by ovule, as in gymnosperms. After fertilization, the ovary develops into fruit and ovules into seeds. Thus angiosperms are ‘closed-seeded’ plants, the seeds remaining inside the fruit. Archegonia are not found in this group, and endosperm is formed after fertilization.

The plant is the dominant sporophyte and gametophytes are very small and reduced. Two subdivisions of angiosperms are dicotyledons and mono­cotyledons, depending mainly on the number of cotyledons in the embryo.

The main points of difference between the two groups are given below:

Dicotyledons:

1. Two cotyledons are hinged later­ally to the axis of the embryo; the plumule is terminal.

2. Leaves have usually Reticulate venation.

3. Flowers are generally pentamerous or tetramerous.

4. Vascular bundles of stems are collateral, open and arranged in ring; due to the presence of cambium between xylem and phloem, secondary growth in thickness is possible.

Roots have limited number of radial vascular bundles.

Monocotyledons:

1. Only one terminal shield-shaped cotyledon is present; the plumule is lateral.

2. Venation is usually parallel.

3. Flower are trimerous.

4. Vascular bundles are numerous and scattered irregularly in the ground tissue. They are collateral, closed, thus secondary growth is normally absent.

Many vascular bundles in monocotyledonous roots.