Study the diagrams carefully and answer the questions that follow.
Introduction
Fruits have two functions:
They frequently aid in seed dispersal, and they protect seeds from physical damage and seed predators.
Dispersal is important to the fitness of the young sporophyte.
This is especially true in long-lived species, in which the offspring would compete with the parent plant for light, water, and nutrients if there were no dispersal
Fruits sometimes split open and release seeds to be dispersed directly.
In many cases, however, seeds are dispersed to new locations while they are still enclosed in the fruit.
- Dry fruits may simply fall to the ground or be dispersed by wind, propulsion, or animals.
- Some dry fruits have hooks or barbs that adhere to passing animals, while nuts are dispersed by seed predators.
- Fruits that are dispersed by wind often have external structures to catch the breeze and extend the distance they travel; the fruits of dandelions and maple trees are familiar examples.
Fruits that float, such as coconuts, can disperse seeds in water.
- Some plants actually disperse dry fruits via propulsion.
- The sandbox tree, for example, produces a seed pod that shrinks as it dries.
- Eventually the pod splits apart violently, spraying seeds in all directions with so much force that the plant is sometimes called the dynamite tree.
- The bursting seed pod sounds like a pistol shot, and seeds can be scattered as much as 40 m away from the parent plant.
- Similarly, the dwarf mistletoe fruit fills with sugars as it matures.
- Enough water follows via osmosis to make the fruit explode and shoot seeds as far as 5 m.
Animals are the most common dispersal agent for fleshy fruits.
- Just as plants have evolved to use animals for pollination, they have also evolved to use specific types of animals for seed dispersal.
- In cases like this, seed dispersal is an example of mutualism.
- The plant provides a fruit rich in sugars and other nutrients; in return, the animal carries the fruit to a new location and excretes the seeds along with a supply of fertilizer.
- Mammals and birds are the most common seed dispersers.
- Mammals are often active at night and use their well-developed sense of smell to locate fruits.
- As you might imagine, fruits of mammal-dispersed seeds are usually dull colored and fragrant.
- In contrast, birds are usually active in the day and see well.
- Fruits of bird-dispersed seeds are usually brightly colored.
Study the diagrams carefully and answer the questions that follow.
a) Identify the specimens A, B and C
Answer:
- A: Fruit of Boerhavia
- B: Fruit of Tridax
- C: Fruit of a leguminous plant such as Cowpea
b)
i) Classify specimens B and C
Answer:
- B: Cypsela
- C: Legume
ii) Give one reason for each answer in the question above
Answer:
- B: Achene with pappus above the ovary
- C: Dry fruit with several seeds with walls split open at two edges
c) Name the agent of dispersal of the seeds of each of the specimens A, B and C, giving one reason for each agent
Answer:
A: Animal
Reason:
- Presence of sticky hair to stick to the bodies of animals
B: Wind
Reasons:
- Presence of pappus, small and light in weight, to be carried in the wind.
C: Self
Reason:
- Dry pericarp and presence of sutures along which the fruit splits open
d) Explain the mode of dispersal of seeds of the fruits of A, B and C
Answer:
Mode of dispersal of A
- Adhesive fruits have sticky hairs or hooks on the pericarp
- This makes fruits stick to the skin or hair of animals
- These are carried away as the animals move from place to place
- Dispersal occurs when seeds or fruits are deposited elsewhere.
Mode of dispersal of B
- Fruit is small in size
- Dry and light in weight
- Presence of a parachute-like structure (hairs) called pappus.
- Enable the fruit to be carried by the wind away from the parent plant
- Bringing about dispersal
Mode of dispersal of C
- The method is Mechanical dispersal.
- Unequal drying of the pericarp of the fruit creates tension, causing the wall to split open along lines of weakness and the seeds catapulted/thrown away from the parent plant, thereby bringing about dispersal
e) State four ways in which dispersal of seeds is important to the plants
Answer:
The Importance of Dispersal
- Prevents overcrowding of plants
- Reduces competition for light and nutrients among the seedlings and their parents
- Minimises epidemic diseases among crowded seedlings
- Enables plants to colonise new localities
- Increases the chances of survival of the species.
