GYMNOSPERMS Study Guide

Gymnosperms are known as naked seed plants.
The term 'Gymnos' translates to 'naked' and 'sperma' translates to 'seeds'.
They are classified under the primitive seed plants of spermatophyta (Phanerogams).
Gymnosperms are characterized by naked ovules, which are ovules without an ovary.
The ovules are borne on the surface of megasporophylls and are unprotected.
Seeds of gymnosperms are not enclosed in fruit, which sets them apart from angiosperms.
Gymnosperms can be referred to as "Phanerogams without ovary".

Gymnosperms represent a small group of plants consisting of about 70 genera and approximately 725 species.
Their distribution is primarily across temperate and tropical regions.
They are an ancient group of seed plants, originating in the Paleozoic era, which is dated from approximately 541 to 252 million years ago.
Gymnosperms were the dominant plant group during the Jurassic and Cretaceous periods of the Mesozoic era.
Many primitive gymnosperms, such as Cycadofilicales, Bennettitales, and Cordaitales, are now extinct.

380 Ma: First vertebrate land animals appeared.
530 Ma: The Cambrian explosion occurred.
750-635 Ma: Earth experienced two Snowball Earth events.
1 Ga: Formation of multicellular life on Earth.
230-66 Ma: Era of non-avian dinosaurs.
4550 Ma: The formation of Earth.
Timeline highlights include:
- Cenozoic (present to 66 Ma)
- Neogene (23 Ma)
- Paleozoic Era (541 to 252 Ma)

Living forms of gymnosperms are predominantly evergreen trees or shrubs.
Gymnosperms often display xerophytic characteristics, allowing them to thrive in dry environments.
Their plant body is sporophytic and diploid, with roots, stems, and leaves clearly differentiated.
Gymnosperms have a well-developed vascular system that consists of xylem and phloem.
- Xylem Functions: Transportation of water and minerals.
- Phloem Functions: Transport of nutrients and sugars.

Gymnosperm roots often form symbiotic associations:
- With fungi: For example, Pinus forms mycorrhizal associations that enhance mineral absorption.
- With algae: Coralloid roots of Cycas have a symbiotic relationship with Nostoc and Anabaena, aiding in nitrogen fixation.

The stems of gymnosperms are typically erect, branched, and woody.
In certain types, such as Cycas, the stem may be unbranched, while in Zamia, it may be underground.
A distinctive feature in gymnosperms is the presence of leaf scars on the stem.

Leaves in gymnosperms are usually dimorphic, consisting of
- Foliage leaves: Green, with variations including simple, needle-shaped, or pinnately compound.
- Scale leaves: Minute and deciduous.
Gymnosperm leaves possess a thick cuticle to reduce water loss.
Stomata are often sunken to minimize transpiration.
In Cycas, leaves exhibit circinate vernation, similar to ferns.

The mesophyll layer is differentiated into pallisade and spongy tissue, although in Pinus, it remains undifferentiated.
Gymnosperms, in general, lack lateral veins in leaves; nutrients are translocated laterally via transfusion tissue.
Roots can be diarch to polyarch, with tanniniferous cells present in the cortex.
Vascular bundles in gymnosperms are collateral and open.

Xylem Structure:
- Composed of tracheids and parenchyma; vessels are absent in most gymnosperms but present in Gnetum.
Phloem Structure:
- Comprises sieve tubes and phloem parenchyma with companion cells being absent.
Growth: The stem shows secondary growth.
- Wood Types: May be manoxylic (as in Cycas) or pycnoxylic (as in Pinus).

Gymnosperms are heterosporous, producing:
- Megasporangia on megasporophylls.
- Microsporangia on microsporophylls.
Sporophylls aggregate to form cones or strobili, which are mono-sporangiage.
- Male and female cones exist in various genera, such as Cycas and Pinus.

Male cones are typically short-lived, while female cones can persist for many years.
Microsporangia are formed on the abaxial side of microsporophylls and develop in a eusporangiate manner.
Female cones are formed by aggregating megasporophylls; these can either be foliar (Cycas) or cauline (Pinus).

Megasporangium is better known as the ovule, which is naked and has orthotropous and unitegmic properties.
The ovular integument is differentiated into three distinct layers.