Overview
One of the most significant events in the history of terrestrial plants was the development of seed habit. It was an essential change in the reproductive system of the vascular plants which occurred approximately 390 million years ago. The first complete seeds appeared roughly 365 million years ago throughout the late Devonian era. Technically a seed may be defined as a fertilized ovule.
- 1) Evolution of Seed Habit
- 2) 1. Evolution of heterospory
- 3) 2.Retention and germination of megaspore within the megasporangium
- 4) 3.Development of protective layers around megasporangium
- 5) 4.Reduction to a single functional megaspore per sporangium
- 6) 5.Development of an embryo sac within the sporangium
- 7) 6.Modification of the distal end of megasporangium for pollen capture
- 8) Multiple Choice Questions (MCQs) – Evolution of Seed Habit
- 9) Frequently Asked Questions (FAQs) – Evolution of Seed Habit
- 10) Summary: Evolution of Seed Habit
- 11) You may also like to learn:
Evolution of Seed Habit
An ovule is an integumented indehiscent megasporangium. Integuments are specialized protective coverings around megasporangium which differ in number. All seed-producing plants are called spermatophytes. Various actions involved in the evolution of seed habit are as follows.
- Evolution of heterospory
- Retention and germination of megaspore within the megasporangium
- Development of protective layers around megasporangium
- Reduction to a single functional megaspore per sporangium
- Development of an embryo sac within the sporangium
- Modification of distal end of megasporangium for pollen capture
1. Evolution of heterospory
Primitive vascular land plants produced one kind of spores; a condition called homospory. All groups of land plants approximately pteridophytes are homosporous. During the early phase of evolution, some plant groups began producing two different kinds of spores, the smaller ones called microspores and the bigger ones referred to as megaspores.
The microspores produced inside microsporangia sprout to form male gametophyte or the microgametophyte, whereas the megaspores sprouted to form female gametophyte or megagametophyte.
2.Retention and germination of megaspore within the megasporangium
Throughout the typical reproductive cycle in the heterosporous vascular land plants, the megaspores are used to be shed and dispersed soon after their formation in order to germinate into the female gametophyte. However, in some plants (e.g., Selaginella) the megaspore is not enabled to get away from megasporangium instantly after its development.
In others, the megaspore is permanently kept within the megasporangium. Here, within the confines of the megasporangium wall, the megaspore sprouts to form an egg including female gametophyte.
3.Development of protective layers around megasporangium
Some branch-like structures of sporophyte surrounding the megasporangium merged around to megasporangium to form a protective envelope or integument. The megasporangium tightly locked by integuments ends up being totally indehiscent. This important change caused the evolution and formation of the ovule, which is nothing but an integumented indehiscent megasporangium. In this way, more protection is accorded to the egg-containing structure in the terrestrial environment.
4.Reduction to a single functional megaspore per sporangium
Each megaspore mother cell within a megasporangium utilized to produce four gametophytes. There was a competitor for the area and food amongst the four gametophytes. Quickly the early vascular plants adopted a new method i.e., only one megaspore is chosen for further development into a healthy female gametophyte while the remaining three are aborted.
5.Development of an embryo sac within the sporangium
The single healthy megaspore retained within the megasporangium germinates to form an egg consisting of a female gametophyte called an embryo sac.
6.Modification of the distal end of megasporangium for pollen capture
When most of the structural and functional changes leading to the development of seed habit were finished, another essential modification occurred in the megasporangium which was now integumented, indehiscent, and permanently attached to – the sporophyte. The distal end of the megasporangium became modified for capturing pollen (microspore including male gametophyte).
Pollen after being trapped in the distal cavity of the megasporangium produces a pollen tube that carries male gametes deep into the embryo sac to fertilize the egg, forming a zygote, that forms an embryo. The megasporangium (ovule) after fertilization is transformed into a seed, the integuments becoming the seed coats.
The seed offers a maximum degree of protection to a developing embryo under the unfavorable terrestrial environment. The evolution and development of seed habit was an excellent success and a giant leap that eventually made it possible for plants to colonize land permanently.
Multiple Choice Questions (MCQs) – Evolution of Seed Habit
- When did the development of seed habit occur?
- a) 390 million years ago
- b) 365 million years ago
- c) 300 million years ago
- d) 400 million years ago
Answer: a) 390 million years ago
- What is technically defined as a fertilized ovule in the context of seed habit evolution?
- a) Seedling
- b) Seed coat
- c) Seed
- d) Ovule coat
Answer: c) Seed
- What protective coverings around megasporangium are mentioned in the evolution of seed habit?
- a) Sepals
- b) Integuments
- c) Petals
- d) Stipules
Answer: b) Integuments
- Which phase of plant development involved the evolution of heterospory?
- a) Sporophyte
- b) Gametophyte
- c) Embryophyte
- d) Rhizome
Answer: a) Sporophyte
- Which term describes the condition of producing two different kinds of spores?
- a) Homospory
- b) Homogamy
- c) Heterospory
- d) Isospory
Answer: c) Heterospory
- In the evolution of seed habit, which plant retains the megaspore within the megasporangium?
- a) Ferns
- b) Mosses
- c) Gymnosperms
- d) Selaginella
Answer: d) Selaginella
- What structure surrounds the megasporangium to form a protective envelope?
- a) Petals
- b) Integuments
- c) Sepals
- d) Stamen
Answer: b) Integuments
- How many gametophytes did each megaspore mother cell initially produce before reduction?
- a) One
- b) Two
- c) Three
- d) Four
Answer: d) Four
- What is formed from the single healthy megaspore in the development of seed habit?
- a) Seed coat
- b) Embryo sac
- c) Pollen tube
- d) Zygote
Answer: b) Embryo sac
- What modification occurred at the distal end of the megasporangium for pollen capture?
- a) Formation of petals
- b) Opening of sepals
- c) Modification of integuments
- d) Development of stamen
Answer: c) Modification of integuments
- What does the pollen tube carry into the embryo sac during fertilization?
- a) Male gametes
- b) Female gametes
- c) Ovules
- d) Seeds
Answer: a) Male gametes
- What is the ultimate result of fertilization in the evolution of seed habit?
- a) Embryo sac
- b) Seed coat
- c) Seed
- d) Pollen tube
Answer: c) Seed
- What becomes the seed coats after fertilization?
- a) Integuments
- b) Sepals
- c) Petals
- d) Stamen
Answer: a) Integuments
- Why did the evolution of seed habit enable plants to colonize land permanently?
- a) Increased vulnerability
- b) Decreased protection
- c) Enhanced reproductive success
- d) Reduced adaptation
Answer: c) Enhanced reproductive success
- What major event in the history of terrestrial plants marks the evolution of seed habit?
- a) Development of flowers
- b) Emergence of gymnosperms
- c) Evolution of heterospory
- d) Advent of angiosperms
Answer: c) Evolution of heterospory
- What phase of plant development involved the formation of an embryo sac within the sporangium?
- a) Sporophyte
- b) Gametophyte
- c) Embryophyte
- d) Rhizome
Answer: a) Sporophyte
- What is the primary function of the integuments around the megasporangium?
- a) To facilitate dispersal
- b) To enhance photosynthesis
- c) To provide protection
- d) To capture pollen
Answer: c) To provide protection
Frequently Asked Questions (FAQs) – Evolution of Seed Habit
- What is the significance of the evolution of seed habit in the history of terrestrial plants?
- Answer: The development of seed habit marked a crucial change in the reproductive system of vascular plants, enabling enhanced reproductive success and ultimately allowing plants to colonize land permanently.
- When did the evolution of seed habit occur?
- Answer: The evolution of seed habit occurred approximately 390 million years ago.
- How is a seed technically defined in the context of the evolution of seed habit?
- Answer: Technically, a seed is defined as a fertilized ovule.
- What is the structure of an ovule in the context of seed habit evolution?
- Answer: An ovule is an integumented indehiscent megasporangium, with integuments being specialized protective coverings around the megasporangium.
- What are the key steps involved in the evolution of seed habit?
- Answer: The key steps include the evolution of heterospory, retention and germination of the megaspore within the megasporangium, development of protective layers around the megasporangium, reduction to a single functional megaspore per sporangium, development of an embryo sac within the sporangium, and modification of the distal end of the megasporangium for pollen capture.
- What is heterospory, and how did it contribute to the evolution of seed habit?
- Answer: Heterospory is the condition of producing two different kinds of spores – microspores and megaspores. It contributed to the evolution of seed habit by introducing a differentiation in spore types, leading to the development of male and female gametophytes.
- Why was the reduction to a single functional megaspore per sporangium important?
- Answer: This reduction was crucial to avoid competition among multiple gametophytes for space and resources, ensuring the optimal development of a single healthy female gametophyte.
- How did the modification of the distal end of the megasporangium contribute to seed habit evolution?
- Answer: The modification facilitated pollen capture, as the distal end became adapted for trapping pollen, leading to the formation of a pollen tube that carries male gametes into the embryo sac for fertilization.
- What is the role of protective layers (integuments) around the megasporangium in seed habit evolution?
- Answer: Protective layers or integuments provide additional protection to the developing egg-containing structure (ovule) in the terrestrial environment, contributing to the success of seed habit evolution.
- How did the evolution of seed habit enhance the reproductive success of plants?
- Answer: The seed offers maximum protection to the developing embryo, ensuring its survival in unfavorable terrestrial conditions, and thereby enhancing the reproductive success of plants.
- What major event in the late Devonian era is associated with the appearance of complete seeds?
- Answer: The first appearance of complete seeds is associated with the late Devonian era, approximately 365 million years ago.
- What transformations occurred in the megasporangium after fertilization in the evolution of seed habit?
- Answer: After fertilization, the megasporangium (ovule) transformed into a seed, with the integuments becoming the seed coats.
- How did the evolution of seed habit contribute to plants colonizing land permanently?
- Answer: The enhanced reproductive success and protection provided by seeds allowed plants to adapt and thrive in terrestrial environments, leading to their permanent colonization of land.
- Which plant groups were involved in the early phase of evolution, producing two different kinds of spores?
- Answer: During the early phase of evolution, some plant groups, approximately pteridophytes, began producing two different kinds of spores – microspores and megaspores.
- What is the term for the condition of producing one kind of spores, as seen in primitive vascular land plants?
- Answer: The condition of producing one kind of spores is called homospory, and it was prevalent in primitive vascular land plants.
- How did the development of an embryo sac within the sporangium contribute to seed habit evolution?
- Answer: The development of an embryo sac within the sporangium provided a structure for the healthy development of a female gametophyte, enhancing the efficiency of seed production.
- What is the role of the pollen tube in the evolution of seed habit?
- Answer: The pollen tube carries male gametes into the embryo sac for fertilization, contributing to the formation of a zygote and eventually leading to the development of a seed.
- Why did some plants retain the megaspore within the megasporangium after its formation?
- Answer: Retaining the megaspore within the megasporangium allowed for its germination into the female gametophyte within the protected environment, as observed in plants like Selaginella.
- What structural change in the megasporangium was crucial for pollen capture in the evolution of seed habit?
- Answer: The modification of the distal end of the megasporangium was crucial for pollen capture, enabling the trapping of pollen and the initiation of the fertilization process.
Summary: Evolution of Seed Habit
The evolution of seed habit stands as one of the most pivotal events in the history of terrestrial plants, shaping their reproductive system approximately 390 million years ago. Seeds, technically defined as fertilized ovules, emerged during the late Devonian era, about 365 million years ago. Key actions in this evolutionary process include:
- Evolution of Heterospory:
- Primitive vascular land plants transitioned from homospory (producing one kind of spore) to heterospory, generating microspores for male gametophytes and megaspores for female gametophytes.
- Retention and Germination of Megaspore:
- In heterosporous plants, the megaspore, which would develop into the female gametophyte, was retained within the megasporangium. Some plants, like Selaginella, kept the megaspore permanently within the confines of the megasporangium.
- Development of Protective Layers:
- Protective integuments formed around the megasporangium, creating an indehiscent ovule. This change increased protection for the developing egg-containing structure in the terrestrial environment.
- Reduction to a Single Functional Megaspore:
- Early vascular plants adapted by selecting only one megaspore for further development into a healthy female gametophyte, while the remaining three were aborted. This reduced competition among gametophytes.
- Development of an Embryo Sac:
- The single, healthy megaspore germinated within the megasporangium to form an egg-containing structure known as the embryo sac. This structure played a vital role in the efficient development of the female gametophyte.
- Modification of Distal End for Pollen Capture:
- The distal end of the megasporangium underwent modification, becoming integumented, indehiscent, and permanently attached to the sporophyte. This adaptation allowed the megasporangium to capture pollen, initiating the fertilization process.
The final stage involved the transformation of the fertilized megasporangium (ovule) into a seed, with integuments becoming seed coats. Seeds provided maximum protection to developing embryos in unfavorable terrestrial conditions, marking a significant success in the colonization of land by plants. The evolution of seed habit showcased the remarkable adaptability of plants and their ability to thrive in diverse environments.