Gymnosperms are part of the Plantae kingdom and the Embryophyta subkingdom. They are mainly plants which do not have ovules enclosed within the ovary wall. It continues to be exposed before and after fertilisation and before turning into a seed. Gymnosperm stems can either be branched or unbranched. These plants lose less water because of their needle-like leaves, thick cuticle, and sunken stomata.
Conifers, cycads, gnetophytes, species of the Ginkgophyta division, and Ginkgo biloba are all members of the gymnosperm family.
The pine family (Pinaceae), which includes roughly 115 species of evergreen conifers, is found globally but is native largely to northern temperate zones. The longleaf pine (Pinus palustris) habitat in the Southern United States is just one example of how pines and conifers are essential components of the coniferous forests, taiga (boreal forests), and mixed forests all over the world.
Table of Contents
Description of Pinus
Pinus are evergreen trees with resinous wood. Most of their characteristics depend on their cones, needles, and barks.
- Pinus Needle – The term “needle” refers to mature pine leaves. Fascicules, or groups of leaves, are formed from the foliage.
- Cone – Gymnosperms have cones, which are seed-bearing organs. The female pine cones are often larger than the male ones, surrounded by numerous scales. Usually, the scales near the cone’s tip and base are sterile (no seeds).
- The seeds are wing-like objects that the wind can carry away.
- The leaves on white pines (Strobus) are arranged in five-leaf clusters. Their wood is usually white, soft, and has subtle annual ring patterns. They are also known as haploxylon or soft pines, such as P. gerardiana.
- The bark of yellow pines (Pinus) is thick and split. Their wood is harder, yellower, and has more apparent annual rings. They are also known as diploxylon or hard pines, such as P. roxburghii.
- The lacebark pines (Ducampinus) are a phylogenetically diverse group of old pines.
Classification of Pinus
|
Kingdom |
Plantae |
|
Division |
Pinophyta |
|
Class |
Pinopsida |
|
Order |
Pinales |
|
Family |
Pinaceae |
|
Genus |
Pinus |
Also, read: Plant Kingdom – Members of Kingdom Plantae
Pinus Life Cycle
Pinus is divided into the following species based on their number and types of leaves:
- Pinus wallichiana
- Pinus roxburghii
- Pinus khasya royle
- Pinus gerardiana
Sporophyte of the Pinus
Pinus plants are tall trees with excurrent branching that is characterised by fascicles of thin, needle-like, evergreen foliage leaves. The primary stem is thick, cylindrical, bearing several widely spaced branches, and has a scaly bark covering it. There is a fairly large terminal bud at the tip of the stem.
It produces two types of shoots: one with unlimited growth or long shoots, the other with dwarf shoots or with limited development.
The scale leaves on the long branches are usually axil-bearing dwarf shoots. Each dwarf shoot has an apex with a fascicle of two or more needle-like leaves closely attached at the base by a circle of scales. The primary root system is durable and perennial and comprises a broad, deeply penetrating tap root. The tap root might die very soon in certain species.
Pinus are monoecious plants, meaning female and male cones grow on different branches inside a single plant.
Reproduction
Pinus has sexual reproduction. Despite being monoecious, Pinus produces female and male cones on different branches of a single plant. While female cones form on the upper branches, male cones grow on the lower branches. The male cones taking the place of the dwarf stems begin to form in groups at the foot of the long stem in the early spring.
Male Cone
A cluster may contain anywhere from 15 to 140 male cones, 15 for P. wallichiana and 140 for P. roxburghii. The newborn female cones are born in pairs or clusters around the apex of the long stem when the male cones start to fall off at the onset of spring.
Pinus is anemophilous, that is, wind-pollinated. A pine woodland turns yellow after pollination because many pollen grains are dispersed into the atmosphere. This phenomenon, often known as “sulphur showers,” is most common in the springtime when windy conditions cause pine trees to shake.
Female Cone
The female cones grow very slowly, and this growth may continue for several years. The third-year cone is larger (15–60 cm in length), loose, woody, and brown when fully grown. As a result, on the long shoots, you can see female cones of various ages in acropetal succession.
The female cone of the Pinus tree is a complex structure representing a compound shoot. 80 to 90 megasporophylls, scale leaves are spirally arranged on the central axis of the female cone.
Ovule
Pinus has unitegmic, anatropous, and crassinucellate ovules. Except for the chalazal end, the single integument is nucellus-free.
The integument has three layers, the fleshy outer layer, the stony middle layer and the rocky inner layer.
Gametophytes of Pinus
The initial stage of gametophyte formation is the spore phase. The male gametophyte is represented by the pollen grain or microspore, whereas the female gametophyte, or the initial stage of it, is represented by the megaspore.
Development of the Male (or Micro) Gametophyte
Ginkgo and Pinus have a similar basic pattern of male gametophytic development. Endosporic development occurs in the pollen grains. Mitosis causes the nucleus of pollen to divide, forming a large central cell at the distal end and a small lens-shaped first prothallial cell at the proximal end.
A second prothallial cell and antheridial initials are cut off by the central cell. The secondary prothallial cell is still connected to the primary prothallial cell, but both prothallial cells are temporary.
The antheridial initially splits to create a large tube cell and a small antheridial cell. The microsporangium releases the pollen grains at the four-celled stage (an antheridial cell, two prothallial cells, and a tube cell).
Development of the Female (or Mega) Gametophyte
The functional megaspore of Pinus evolves significantly into the female gametophyte. The megaspore’s nucleus divides during mitosis, producing a vast amount of nuclei without the support of wall formation.
For uninucleate cells to develop, cross-walls are placed on every alveolus. This results in the whole gametophyte becoming cellular, and the tissue that results in acts as a representation of the endosperm or female prothallus.
Pollination
Pinus is wind-pollinated or anemophilous. The pollen grains scatter and float in the air. The ovule’s nucellar beak interferes simultaneously, forming a viscous, sweet liquid containing glucose, sucrose, and fructose. Depending on the time of day, this fluid emerges through the micropyle in pollination drop forms.
As the fluid dries, the pollination drop captures the pollen grains, which are gathered within the pollen chamber. The micropyle mouth is then enclosed from the external environment.
Fertilisation
Following the pollination year, the fertilisation process begins. By forcing its way through the nucellus’ cells, the pollen tube approaches the archegonium’s tip. The enzymes produced by the egg break down the pollen tube wall, and two male nuclei eventually emerge. A zygote is formed when one male nucleus joins the egg cell in fusion.
The large cavity carrying the egg is encircled by a layer of cells called the jacket layer, which sustains the egg cell and later to the growing embryo. About a year after pollination, the archegonia mature and are prepared for fertilisation.
When the pollen tube tip comes in contact with an archegonium on the female gametophyte’s surface, it ruptures the neck cells and releases its components into the egg’s cytoplasm. One male cell’s nucleus approaches the egg’s nucleus and fertilises it. The stalk and tube nuclei, as well as the other male nucleus, quickly separate. The fertilised egg builds a wall around itself and develops into an oospore.
New Pinus Sporophyte
The fusion nucleus travels to the base of the oospore where it divides repeatedly to generate four nuclei that are positioned at an angle to the oospore’s long axis. Then, there are two further divisions producing four tiers with four nuclei in each tier.
All the nuclei, except those in the top layer, form partition walls that separate them. The three lower layers are referred to as the embryo, suspensor, and rosette tiers, and the entire structure is called the pro-embryo. The lowest or embryo tier is where the embryos grow, with the upper tiers serving only nutritional purposes.
The suspensors rapidly lengthen and diverge, pushing the embryos deep into the female gametophyte’s tissue, which is now saturated with food substances diffused by the placenta. Each suspensor carries one of the four embryo cells at its apex.
Each embryo cell quickly divides to form four possible embryos. It should be noted that while several oospheres may be fertilised, numerous potential embryos might also be produced. This is called polyembryony and is a very unique characteristic of conifers. However, only one matures, and the others die out during development.
The hypocotyl, a radicle, three to several cotyledons, and a tiny plumule comprise a fully grown embryo. The nucellus is usually crushed out as the endosperm swells, but occasionally it survives as a thin layer, producing perisperm. The ovule transforms into a seed as the integument eventually changes into the seed coat.
Related Links:
- Kingdom Animalia, Plantae and Viruses
- Kingdom Monera, Protista and Fungi
- Five Kingdom Classification
- Biological Classification Basis
- Pinus Questions with Answers for NEET
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