Gymnosperms | Biology for Grade 11 PDF Download

What are Gymnosperms ?

Plants, like other living organisms, have undergone evolution. Gymnosperms, a group of plants, provide significant evidence of this evolutionary process. They were the pioneers in developing seeds, which serve as an effective means of dispersal. This mechanism enables new plants to grow in diverse locations, not necessarily in close proximity to the parent plant. Let's delve further into the characteristics and significance of gymnosperms.

Gymnosperms are plants where the ovules (which develop into seeds) are not enclosed by any protective ovary wall. This means that both before and after fertilization, the ovules remain exposed.

Characteristics of Gymnosperms

• The seeds that form after fertilization are also not covered, hence the term "naked seeds."
• Gymnosperms are typically medium to tall trees or shrubs. For instance, the giant redwood tree (Sequoia) is one of the tallest gymnosperm species.
• The roots of gymnosperms are usually tap roots. In some species, such as Pinus (pine trees), the roots have a symbiotic relationship with fungi, known as mycorrhiza. In others, like Cycas, there are specialized roots called coralloid roots that associate with nitrogen-fixing cyanobacteria.
• The stems of gymnosperms can be unbranched(as in Cycas) or branched (as in Pinus and Cedrus).
• The leaves can be simple or compound. For example, Cycas has pinnate leaves that persist for several years. Gymnosperm leaves are adapted to survive extreme conditions, such as high temperatures, humidity, and strong winds.
• In conifers, the leaves are often needle-like, which reduces the surface area and helps minimize water loss. These leaves also have a thick cuticle and sunken stomata to further reduce water loss.

Reproduction in Gymnosperms

• Gymnosperms are plants that produce two types of spores: microspores and megaspores. These spores are haploid, meaning they contain a single set of chromosomes.

• The microspores and megaspores are formed within specialized structures called sporangia, which are located on reproductive leaves known as sporophylls.

• Sporophylls are arranged in a spiral pattern along an axis, forming structures called strobili or cones, which can be either lax (loose) or compact.

• Strobili that bear microsporophylls and microsporangia are known as microsporangiate or male strobili.

• The microspores develop into a highly reduced male gametophyte called a pollen grain, which consists of only a limited number of cells. This development occurs within the microsporangia.

• Strobili that bear megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili.

• In some gymnosperms, such as Pinus, both male and female cones can be found on the same tree. However, in Cycas, male cones and megasporophylls are found on different trees.

• The megaspore mother cell is derived from one of the cells of the nucellus, which is protected by envelopes. This composite structure is called an ovule.

• Ovules are borne on megasporophylls, which may be clustered to form female cones.

• The megaspore mother cell undergoes meiosis to produce four megaspores.

• Out of these, one megaspore enclosed within the megasporangium develops into a multicellular female gametophyte, which contains two or more archegonia or female sex organs.

• The multicellular female gametophyte is retained within the megasporangium.

Key Differences 

(i) In Gametophytes: Gymnosperms vs. Bryophytes and Pteridophytes

• In gymnosperms, unlike in bryophytes and pteridophytes, the male and female gametophytes do not exist independently. Instead, they are contained within the sporangia, which are retained on the sporophytes.
• The pollen grain, which is the male gametophyte, is released from the microsporangium.
• The pollen grains are then carried by air currents until they come into contact with the opening of the ovules, which are borne on the megasporophylls.
• A pollen tube, which carries the male gametes, grows towards the archegonia in the ovules and discharges its contents near the mouth of the archegonia.
• After fertilization, the zygote develops into an embryo, and the ovules develop into seeds. In gymnosperms, these seeds are not covered.

(ii) Between Gymnosperms and Angiosperms

Angiosperms : (a) A dicotyledon (b) A monocotyledon

• In gymnosperms, the ovules are exposed (naked), while in angiosperms (flowering plants), pollen grains and ovules are developed within specialized structures called flowers.
• Angiosperms have seeds enclosed within fruits.
• They are a large and diverse group of plants found in various habitats, ranging from tiny species like Wolffia to tall trees like Eucalyptus(over 100 meters).
• Angiosperms provide essential resources such as food, fodder, fuel, medicines, and other commercially important products.
• They are classified into two main groups:dicotyledons and monocotyledons.