8.4 Evolution of seed
Seed-forming plants are called spermatophytes among vascular plants seeded plants are more successful land plants.
The development of seed habit occurred approximately 390 million years ago but complete seed-forming plants appeared in the late Devonian period about 365 million years ago. A seed can be defined as a fertilized ovule, integument in indehiscent 198
megasporangium. The integument is a specialized protective covering around the megasporangium.
The following steps were involved in the evolution of the seed:
- Evolution of heterospory.
- Retention and germination of megaspore within megasporangium.
- Development of integument, a protective layer around megasporangium.
- Reduction to a single functional megaspore per megasporangium. Development of embryo sac within megasporangium.
- Modification of distal end for megasporangium to capture pollen.
Spermatophyta (Seeded plants):
There are two groups of seeded plants, gymnosperms and angiosperms.
8.4.1 Gymnosperms: (Latin, Gymno: naked, Greek, Sperma: seed)
These plants produce naked seeds, i.e. seed is not enclosed in fruit. It is a widespread group of plants and consists of one-third of world forests. The gymnosperms are woody plants either shrubs, trees, or rarely vines (in some gnetophytes).
They differ from flowering plants in that the seeds are not enclosed in an ovary but are exposed within any of the variety of structures, the most familiar being cones.
The plants show heteromorphic alternation of generation with independent dominant sporophyte and dependent reduced gametophyte. The female gametophyte is retained in the ovule permanently.
The microspore develops on microsporophyll and megaspores develop on megasporophyll. There are two types of leaves, i.e. foliage, and scale.
The leaves are evergreen and exhibit xerophytic features like thick and tough cuticles, sunken stomata, etc. The xylem consists of tracheids and xylem parenchyma. Polyembryony is common but usually single embryo gets mature.
The important genera of gymnosperms are:
- Cycas (Cycad)
- Pinus (pines)
- Picea (hemlock)
- Cedrus (deodar)
- Ginkgo
- Ephedra
Importance of Gymnosperms:
The gymnosperms are used by men for the following purposes. Their seeds may be used as food, e.g. pine, childhood. To obtain drugs, e.g., ephedrine which is obtained from Ephedra Wood is used for making paper, construction, packing, etc.
Grown as ornamental plants like cycads. Many commercial products, e.g., resins, turpentine, tar, and many oils, soap, varnish, nail polish, gum, perfumes, etc., are obtained from gymnosperms.
8.4.2 Angiosperms (Greek: Angio means enclosed, sperm means seed)
All flowering plants belong to angiosperms. Like gymnosperms, angiosperms are seed-producing plants. It is the most diverse group of plants. They are distinguished from gymnosperms by characteristics including, flower, endosperm within seeds, and production of fruits that contain seeds.
It is believed that angiosperms diverged from gymnosperms in the Triassic period about 245 million years ago. There are 3,60,000 known species of plants out of these 2,35,000 species are angiosperms. All angiosperms are divided into two sub-classes, i.e. monocot and dicot.
Life cycle of the flowering plants (Angiosperms):
Like other plant groups, angiosperms also show alternation of generation, i.e. both sporophyte and gametophyte generations are present which alternate with each other.
Sporophyte generation:
The adult flowering plant is a diploid sporophyte consisting of roots, stems, leaves, and flowers. The flower is the reproductive part and is a modified shoot. Each flower consists of the pedicel, thalamus, and floral leaves, i.e. sepals, petals, stamens, and carpel. The thalamus is a modified stem while floral leaves are essential parts of the flower.
The sepals and petals are not essential for reproduction, however, sepals and petals are protective parts of flowers and also attract insects for pollination.
Stamen is the male reproductive part of the flower and consists of a long filament having a bilobed anther at its tip. In another haploid microspores are produced by meiosis inside the pollen sacs. The carpel is a female reproductive part of the flower.
The basal broader part of the carpel is the ovary, the elongated part is style and the terminal broad part of the carpel is the stigma.
Inside the ovary, one or more ovules are present which are covered by an integument. The embryo sac is surrounded by a tissue called nucellus.
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