Seed Dispersal techniques in plants Science

When seeds have wings

Seeds dispersed in the wind. File Photo: Basheer   | Photo Credit: Basheer

Gliders, parachutes and helicopters. You may think I am talking about fun ways for us to sail through the air, but no. I am talking of smart ways plants have devised to send their seeds far and wide.

Seeds are the link between different generations in plants. When plants flower, they get pollinated, about which we learned last week. Plants then make fruits, which contain seeds.

The copper pod tree, found on the roadside in many of our cities, is a stunning example: bright yellow flower bunches get replaced by copper coloured pods, which rattle with seeds. Some seeds drop right next to the parent plant and start growing. This is common in annual plants, which live for only a year; parents die out and the next generation grow to take the parents’ place in the ecosystem.

In other plants that have longer lives, this can lead to parent and baby plants compete with each other for soil space, sunlight and other resources. To avoid this competition, plants have devised ingenious ways and means to spread their seeds far and wide. The process by which plants spread their seeds is called “seed dispersal”.

Special adaptation

Wind is one agent for seed dispersal. All a seed needs to do is to get caught in a gust of wind, and it can travel for miles. “Gliders” are seeds that have two wings, like the wings of an airplane. Jacaranda, for instance, produces plenty of flat, winged seeds that flutter away from the parent plant. Mahogany has a more intricate design. The pod, which is initially closed, splits open to release seeds, which look and act like helicopter rotors and spiral through the air.

The Indian birthwort, Aristolochia india, has brownish purple flowers that form green, cylindrical fruits. When the fruit matures, it breaks to form an inverted umbrella or parachute; winged seeds are blown out from inside the parachute when the wind blows. Silk cotton trees also have pods stuffed with cottony seeds.

Other plants have special “adaptations” (features that help them survive in specific environments) that help their seeds get dispersed via water. Coconut is the best example. Have you wondered why coconut trees line water bodies? Here is the reason.

The coconut has an outer covering of husk — that’s what the tender coconut vendor peels away before giving it to you — which is so light, it can float for miles on water.

Agents of dispersal

More than wind and water, animals and birds can make valuable allies who can help disperse seeds. Some fruits have spines and hooks, which get stuck on the fur of animals as they pass by the plant. Biting into a juicy mango probably drives other thoughts out of your minds — but, the mango tree has put in a lot of effort to make the fruit delicious to animals like us.

 

A mango tree in fruit has birds, monkeys and squirrels lining up to eat their fill. Animals and birds carry the fruit far away, and drop the hard seed hidden inside after their meal. If the seed falls into moist, fertile soil, up comes a mango tree.

Small mangoes, and fruits with smaller seeds, like apples and peaches, are eaten whole by birds and animals. The soft fruit is digested, the hard seed passes through the digestive system, and is expelled as droppings (poop). The seed ends up being dropped along with manure to get it started with its life.

Nuts like cashew and almonds are made tasty, nutritious and irresistible to animals. Gray squirrels in cold countries collect a lot of nuts and hide them during summer, planning to snack on them during winter; but the squirrels usually forget where they hide the nuts, which makes new trees!

Rather than trusting the elements like wind and water, or even animals who may or may not come by, some plants have taken matters into their own hands. Their fruits burst open to explosively spill out the seeds. The fruits are in the form of pods. As the pods dry out in the sun, they become firm and hard. When water falls on them, or when the wind blows fast, the dry pods burst to release seeds. The force is like a coiled spring released suddenly. Members of the genus Impatiens (the balsams) have been named so because they are “impatient” to get dispersed!

Sandhya Sekar is a science journalist who writes about weird and wonderful creatures. You can write to her at sandysek@gmail.com or read more on her website ‘The Melting Pot’ at www.sandhyasekar.com


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Printable version | Jul 28, 2021 6:09:23 PM | https://www.thehindu.com/in-school/sh-science/when-seeds-have-wings/article6373280.ece

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