Seed germination
- Seed germination is the first step in the life cycle of gymnosperms.
- When conditions are favourable, gymnosperm seeds germinate, initiating the growth of a new plant.
- As the seed germinates, the diploid embryo divides and grows, forming the main plant body called the sporophyte, which is diploid.
- The sporophyte develops into a tall tree that is well differentiated into root, stem and leaves.
Spore and gamete formation
- Some leaves are modified into specialised spore-forming structures called sporophylls.
- Sporophylls are grouped to form compact structures called strobili or cones.
Male cones
- Formed by a compact arrangement of microsporophylls; the site of microsporogenesis (microspore formation).
- Contain microspore mother cells, which divide by meiosis to form microspores (male spores).
- The microspores develop into pollen grains, each containing 3–4 haploid cells.
Female cones
- Formed by a compact arrangement of megasporophylls; the site of megasporogenesis (megaspore formation).
- Contain megaspore mother cells, which divide by meiosis to form 4 haploid megaspores (female spores).
- Of the 4 megaspores, only one remains functional.
Male and female cones can be borne on the same tree, as in Pinus, or on different trees, as in Cycas.
- The functional megaspore divides and develops into the female gametophyte, called the endosperm.
- Inside the endosperm, two to eight female sex organs called archegonia develop. Each archegonium contains a haploid female gamete called the egg (ovum).
The endosperm of gymnosperms is haploid (n) and forms before fertilization, unlike the triploid (3n) endosperm of angiosperms.
Pollination
- Pollination in gymnosperms involves the transfer of pollen from male cones to female cones, usually by wind.
- Pollen grains land on the ovules of female cones and stick to a sticky fluid known as the pollination drop.
- The pollen grain germinates and forms a pollen tube, through which the male gametes travel to reach the egg cells within the ovule.
Fertilization
- Once the pollen tube reaches the egg cell, fertilization occurs, resulting in the formation of a zygote.
- In gymnosperms, fertilization takes place within the ovule while it is still attached to the parent plant.
Seed development
- After fertilization, the zygote develops into an embryo within the ovule, and the ovule eventually matures into a seed.
- Gymnosperm seeds are usually surrounded by a protective seed coat, derived from the integument of the ovule.
- Unlike angiosperms, gymnosperms do not form fruits, so their seeds are naked and are dispersed by wind, water or animals.