Did you know that seeds can exist without fruits? When we think of seeds, we generally think of them as the hard structures that are found within fruits and are used to plant new trees. But, the fact is that only the seeds of flowering plants remain enclosed within fruits whereas the seeds of non-flowering plants do not. Such non-flowering plants bearing naked seeds belong to the division Gymnospermae and are commonly known as gymnosperms.
The word ‘Gymnosperm’ has been derived from two words, ‘gymnos’ meaning naked and ‘sperma’ meaning seeds. This is coherent with the feature of bearing naked seeds by the plants belonging to this division. The ovules remain exposed as they are not enclosed by the ovary wall before fertilisation, same is the case with seeds after fertilisation. Did you know that gymnosperms were the dominant plants on land around 200 years ago? Cool, right? Come let’s learn more about gymnosperms.
Table of contents
Gymnosperms are the first group of plants which are seeded. The seeds in gymnosperms are not enclosed within a fruit. They are vascular plants having the following tissue systems:
They can survive in dry and cold habitats and are distributed worldwide. A vast majority of gymnosperms, especially the conifers, are predominant in the temperate and subarctic regions. Cycads and gnetophytes are mainly found in the tropical or subtropical regions.
The plant body of gymnosperms is differentiated into well-defined roots, shoots and leaves. They are woody perennials which include medium-sized trees or tall trees and shrubs.
Fig: Some common gymnosperms
The main plant body of gymnosperms is a diploid sporophyte and is heterosporous in nature, that is, it produces two different types of spores - male microspores and female megaspores.
The spores are borne on chambers known as sporangia that are present on modified leaves known as sporophylls.
The haploid gametophytes are inconspicuous and depend on the sporophyte for nutrition and survival. The microspores develop into the male gametophyte known as pollen grain. The megaspore develops into a multicellular female gametophyte that bears two or more female sex organs or archegonia.
Fig: Tap root of Pinus
The roots of different gymnosperms form symbiotic associations with different organisms such as fungi and cyanobacteria.
The symbiotic association of the roots of gymnosperms with fungi is called mycorrhizae.The fungi helps to capture minerals such as organic nitrogen, phosphorus, iron and other nutrients by solubilising it. The fungal hyphae obtain nutrients beyond the root zone and make it available to the plants. Plants, in return, provide the sugars they produce by photosynthesis to the fungi as food. Thus heterotrophic fungi procure nutrition from the gymnosperm. This is a characteristic feature of Pinus roots.
Fig: Symbiotic association of roots of Pinus with fungus
Cycads naturally grow in habitats like sand dunes and steep rocks wherein the nutrients are inaccessible. They have special root modifications that allow them to thrive in such challenging conditions. The roots of these plants possess symbiotic associations with cyanobacteria such as Nostoc or Anabaena and are called coralloid roots.
Fig: Coralloid roots in Cycas
These roots help in the fixation of atmospheric nitrogen into absorbable forms like ammonia.
Gymnosperms have an erect (straight) stem as the main axis of growth. Stem can be branched or unbranched.
Fig: Types of stems in gymnosperms
In conifers such as Pinus, the lower branches are longer than the upper branches which gives the tree its characteristic christmas tree like shape.
Gymnosperms have two major types of leaves, i.e., scaly and foliage leaves.
Fig: Types of leaves
The scaly leaves are brown, thick, tough and needle-like leaves. These can be found developing in spiral rows. In Cycas, the scaly leaves alternate with the foliage leaves.
Fig: Scaly leaves of Cycas
The foliage leaves are green, soft, large, pinnate and needle-like leaves. They are petiolate and usually have a broad base.
Fig: Foliage leaves of Cycas
Foliage leaves can be of two types -
Fig: Simple foliage leaf of Gingko
Fig: Pinnately compound foliage leaves
The leaves of gymnosperms are adapted to extreme conditions of temperature, humidity and wind. These adaptations have been discussed below -
Needle- like leaves
Needle-like leaves have reduced surface area which do not provide enough space for snow to settle down.
The reduced area also results in less water loss due to transpiration.
Waxy coating on the leaves cuts the direct exposure of stomata to the atmosphere. This helps in preventing water loss.
Presence of stomata deeply embedded in the epidermal layer (sunken in nature) helps to reduce water loss.
Sporophylls are modified leaves of gymnosperms which play a major role in reproduction. They contain numerous sporangia which produce spores, vital for reproduction. Sporophylls are usually compacted into a structure also known as strobili or cones.The male sporangia, also known as the microsporangia are borne on leaves called microsporophylls, which are clustered and compacted together to form the male strobili.The female sporangia, also known as the megasporangia are borne on leaves called megasporophylls, which are clustered and compacted together to form the female strobili.
Fig: Monoecious and dioecious gymnosperms
Solution: All gymnosperms and few pteridophytes are heterosporous as they produce two different kinds of spores. The two different kinds of spores are microspores and megaspores. Microspores develop into male gametophytes and megaspores develop into female gametophytes.
Thus, the correct option is b.
Solution: Pinus is a gymnosperm. The leaves in Pinus are well adapted to withstand harsh temperatures. Presence of sunken stomata, thick cuticle and needle-like leaves play an important role in reducing water loss. Sunken stomata remain deep seated in the epidermal layer and hence reduce transpiration. Cuticle is water-proof and hence thick cuticle helps to reduce water loss. The needle-like leaves reduce exposed surface area for water loss Thus, the correct option is d.
Solution: The roots of Pinus show fungal association. These roots are called mycorrhiza. Fungi help to absorb water and minerals like phosphorus for the plant and the plant, in turn, provides nourishment. This type of beneficial association is called mutualism or symbiosis.
The coralloid roots of Cycas show symbiotic association with cyanobacteria. The cyanobacteria help in nitrogen fixation whereas in return the Cycas provides a habitat for it to grow.
Thus, the correct option is c.
Solution: Gymnosperms resemble pteridophytes in having archegonia. Archegonia are the female sex organs which are present on the female gametophyte of both gymnosperms and pteridophytes.
Unlike gymnosperms, pteridophytes do not produce seeds. Vessels in xylem are found only in angiosperms. Pteridophytes have independent gametophytes but gymnosperms have a dependent and highly reduced gametophyte.
Thus, the correct option is d.
Answer: Conifers are the dominant flora of the North temperate region because -
Answer: The gymnosperms originated during the late Paleozoic era and are the most ancient seed plants. However, they flourished well during the Mesozoic era.
Answer: The tallest tree in the world is called Hyperion (Sequoia sempervirens). It is the Coast Redwood and is 115.54m tall. It is located in the remote areas of Redwood National Park, California.
Answer: Gymnosperms are the non-flowering plants which are the first seeds plants to have evolved on earth. They bear naked seeds that are not enclosed within fruits. Angiosperms are the flowering plants which bear seeds enclosed within fruits. They evolved a lot later when compared to gymnosperms but presently, angiosperms are the most dominant plant species on earth.
Gymnosperms: Reproduction, Life cycle, Classification and Economic importance