In this article we will discuss about:- 1. Characteristics of Seed Plants 2. Development of Seed Habit 3. Three Generation Locked in Seed.
Characteristics of Seed Plants:
Some of the important characters of seed plants are listed below:
1. They comprise over 250,000 vascular plants.
2. After sexual reproduction, the plants produce seeds for dispersal and multiplication. The seeds are dormant and can easily pass through unfavourable conditions.
3. The plant body belongs to sporophytic generation.
4. The sporophytic plant body is differentiated into true stem, leaves and roots.
5. Plants show heterospory or two types of meiospores, microspores and megaspores. They are produced in two types of sporangia, microsporangia (pollen sacs) and mega sporangia.
6. The two types of sporangia are borne on two distinct sporophylls called microsporophyll’s and megasporophylls. They are modified variously in seed plants.
7. Mega sporangia are integument and are called ovules.
8. Microspores or pollen grains reach the ovules in a process of pollination.
9. The gametophytes are completely parasitic.
10. Sex organs are multicellular but there is a reduction in their constituents.
11. An external supply of water is not required for fertilization.
12. Fertilization occurs with the help of a tube formed by male gametophyte. It is called pollen tube. Such a mode of fertilization is called siphonogamy.
13. An embryo stage is formed after fertilization. It stops growth temporarily after some time.
14. The ripened ovule having a dormant embryo is shed as a seed.
Seed plants are divided into two groups, gymnosperms (e.g., Pinus) and angiosperms (e.g., Wheat, Eucalyptus, and Mango).
Development of Seed Habit:
There are several requirements for development of seed habit:
1. Development of heterospory or formation of two types of spores, smaller male or microspores and larger female or megaspores. The two types of spores form two different types of gametophytes, male and female.
2. The mega-sporangium developed an integument like covering with a pore or micro Pyle.
3. In a mega-sporangium only one megaspore mother cell remained functional. The other cells did not form mother cells but remained sterile.
4. The single megaspore mother cell formed 4-haploid megaspores. Out of them, three degenerated and only one remained functional.
5. The functional megaspore started forming the female gametophyte inside the integument mega-sporangium. It is known as precocious development.
6. The megaspore was never shed. It stopped developing an impermeable covering over it. As a result the female gametophyte being formed by it continued to receive nourishment from the sterile or nuclear cells. The female gametophyte, therefore, reached full maturity and formed sex organs there.
7. Development of pollination or transfer of microspore to the mega-sporangium or receptor area of the mega-sporophyll.
8. Growth of male gametophyte near the mega-sporangium.
9. Formation of pollen tube for carrying the male gametes into the interior of mega-sporangium where the female sex organs are present. Fertilization performed with help of pollen tube is called siphonogamy.
10. Development of embryo inside the female gametophyte enclosed in mega-sporangium.
11. Temporary suspension of growth of embryo and conversion of integument mega-sporangium or ovule into a seed.
12. Shedding of the seed.
13. Resumption of growth by the embryo after the seed reaches area favourable for growth.
Three Generation Locked in Seed:
Seed develops from an ovule or mega-sporangium. An ovule consists of integument and nucellus. Both of them are diploid or belong to sporophytic generations. In seed, the integument persists as a seed coat/seed coats. Parts of nucellus may also persist.
In the centre of mega-sporangium or nucellus part of ovule develops a haploid megaspore which grows into haploid female gametophyte. The female gametophyte partly eats away the surrounding nucellus. The female gametophyte develops an egg or oospore.
The latter is fertilised by a male gamete brought by a pollen tube to form diploid zygote (2n). The zygote develops into an embryo or new sporophyte. The embryo (2n) is surrounded by female gametophyte (n), the latter by seed coat (2n).
Therefore, a seed contains three generations locked one within another:
(a) Parent sporophyte in the form of seed coat/coats and persistent nucellus,
(b) Female gametophyte which stores food, and
(c) Future sporophyte in the form of embryo.
Adaptations to Land:
Seed plants are the most successful of all the land plants.
The different adaptations to terrestrial life are as follows:
(i) Development of pollination or carrying the microspores to the megasporophylls.
(ii) Non-requirement of external supply of water for fertilisation. This has been made possible through the development of pollen tube for carrying the sperms to the female gamete.
(iii) (a) Transformation of mega-sporangium into an ovule,
(b) Formation of only one megaspore mother cell in an ovule,
(c) Formation of only one megaspore in a mega-sporangium,
(d) Fertilization of egg in ovule,
(e) Production of embryo in the ovule,
(iv) Temporary suspension of growth of the embryo in the ovule and transformation of ovule into a seed for dispersal.
(v) Other characters of seed plants which have helped them to dominate the land are:
(a) Extensive root system for anchoring and absorption of water as well as minerals,
(b) Development of mechanical tissues,
(c) Presence of vascular tissues for long distance transport of sap and food materials,
(d) Development of cambium for secondary growth,
(e) Presence of bark for protection.