Question

Compare between endospermic development of pinus and gnetum

Answer 1

Hi

Comparison # Pinus:

A. The Sporophyte:

1. Plants:

Tall tree characterised by ex-current branching and evergreen leaves, monoecious.

2. Stem:

Robust cylindrical by scaly bark, bears two kinds of shoots, dwarf and long.

3. Roots:

Long tap root.

4. Leaves:

Of two kinds:

(a) Scale like 2 or more in a bunches, on dwarf shoots in their axils and

(b) Needle- like one or more in a bunches, on dwarf shoots.

5. Strobili:

Of two kinds, male and female.

(a) Male Strobilus:

Is found in clusters near the apex of a branch and con­sists of a short axis on which the microsporophylls are spirally arranged. Each sporophyll bears on its underside two sporangia containing numerous microspores or pollen grains. Each microspore is provided with two coats, exine and intine, the former bears two wings.

(b) Female Strobilus:

Is compound and consists of an elongated axis which the megasporophylls are spirally arranged. On the upper surface of each scale there is another stouter scale, ovuliferous scale, which bears two mega-sporangia or ovules on the upper surface.

Ovule consists of nucellus sur­rounded by an integument and a pollen chamber. The micropyle is directed towards the stalk of carpers. Within the nucellus there is a megaspore mother cell which produces one functional megaspore, as in Cycas.

B. The Gametophytes:

(a) Male Gametophyte:

i. Microspore is the first cell of the male gametophyte and begins to germinate inside the microsporangium before it is shed.

ii. Microspore cuts off a small cell, the first prothallial cell which soon degenerates, then it produces another small cell, the second prothallial cell and an antheridial cell. This antheridial cell gives rise to a generative cell and a tube cell.

iii. Generative cell produces a stalk nucleus and a body nucleus.

iv. Tube cell prolongs into a pollen tube carrying the stalk nucleus, body nucleus and tube nucleus.

v. Body nucleus divides into two male nucleus.

(b) Female Gametophyte:

i. Megaspore is the first cell of the female gametophyte and germinates inside the mega-sporangium, which is never shed.

ii. Megaspore becomes larger and by free nuclear division produces numerous nuclei, round each of which cytoplasm collects and cell-wall is formed and thus a tissue, known as endosperm, is formed. The endosperm represents the female gametophyte and is formed before fertilization.

Comparison # Gnetum:

A. The Sporophyte:

1. Plants:

Mostly woody climbers, sometimes small trees or shrubs, mostly dioecious.

2. Stem:

Stout, usually bears two kinds of shoots, dwarf and long.

3. Roots:

Tap root.

4. Leaves:

Usually of two kinds:

(a) Scale leaves on long shoots and

(b) Ordinary green leaves on dwarf shoots.

These leaves are simple, large, oval, leathery, exstipulate, pinnately reti­culate and are arranged in opposite decussate manner.

5. Strobili:

Of two kinds, staminate or male and ovulate.

(a) Male Strobilus:

Is a cone like structure formed by the aggregation of numerous pairs of opposite decussate bracts on a slender axis. It is axillary or terminal, solitary or catkin-like cluster. The bracts are united forming small cups. In the axils of these cups and surrounding axis numerous mic­rosporophylls forming staminate flowers develop.

Each staminate flower consists of a stalk, bearing at its tip two anthers, each with a microsporangium containing numerous microspores and subtended at the base by two united perianth. Each microspore is provided with two coats, exine and intine, the former is thick and spiny and the latter thin.

(b) Ovulate Strobilus:

Resembles that of male strobilus but is compound, since the mega-sporangia are borne on the secondary axil. Ovules or mega-s­porangia arise in the axils of united bracts. There are 5 or 6 whorls of ovules surrounding the axis and with 5 or 7 ovules in each whorl. Some­times the axis is terminated by an ovule.

Ovule consists of nucellus surrounded by two integuments, a micropylar tube and a shallow pollen chamber. Within the nucellus there is one or more megaspore mother cell, which by reduction division forms 4 cells, each of which may become a megaspore. In most cases the megaspore mother cell at the end of reduc­tion division forms a four nuclears cenomegaspore.

B. The Gametophytes:

(a) Male Gametophyte:

i. Microspore is the first cell of the male gametophyte and begins to germinate inside the microsporangium before it is shed.

ii. Microspore germinates to produce a generative nucleus and a tube nucleus. The generative nucleus is provided with a wall forming a generative cell.

(b) Female Gametophyte:

i. Megaspore or cenomegaspore is the first cell of the female gametophyte and germinates inside the mega-sporangium.

ii. Cenomegaspore produces numerous nucleus by free nuclear division which lie in the cytoplasm. Surrounding a large vacuole. Wall formation begins around the free nuclei, from base up­wards but it is not complete. This tissue represents the female gametophy­te. Sometimes 2-4 accessory female gametophytes are formed above the main gametophyte but these undergo degeneration.