Seed_dispersal.JPG (27468 bytes)Seed Dispersal: Plants have developed a number of strategies to ensure that their seeds are spread over wide areas. Wind is often the vector used for efficient seed dispersal. Grasses and many daisies rely on the wind for scattering their progeny. Their seeds are light and easily carried with even the slightest breeze. This means of dispersal is responsible for the success, of these two major plant groups, in colonising wide areas. The image shows two capitulumís (daisy flowers). One is in full bloom and the other is past its peak with some mature seeds ready to be lifted by the wind. Each seed has a hairy tuft on one end that is known as a pappus. The pappus is caught by the wind and is often carried for long distances.
The wind plays a lesser role in the dispersal of Banksia and Hakea seeds. They both have large seeds with wings and are held in woody fruits. The woody fruits open slowly, after the plant has been burnt by a bushfire, and the seeds released. The seeds are caught by the wind and the wings act like helicopter blades and carry the seeds a short distance from the parent. Because of their weight the seeds are not carried as far as grass or daisy seeds.
Water is also a seed dispersal agent. Callistemons, Leptospermums and Melaleucas often grow beside water courses. They have small, persistent woody capsules that contain a large number of small, light seeds. If the parent is damaged or killed then the capsules open and release the seeds. The seeds drop into the water and are light enough to float. They are carried by the current some distance from the distressed parent.
Ants are useful seed dispersal agents. Grevilleas have large seeds that are surrounded by a fleshy ring. Large ants, such as Sugar Ants (Camponotus species), collect the seeds and chew off the fleshy ring. The seed is abandoned and the fleshy ring is taken into the nest. In this way Grevillea seeds are carried some distance from the parent plant.
Wattle (Acacia) seeds are also attractive to ants. Their seeds are usually circled by two fleshy structures. Firstly there is a funicle that joins the seed to the pod and this is in turn is joined to an aril that is attached to the seed. Apparently both structures, as far as ants are concerned, are edible. As the wattle pods ripen they drop their seeds and these are collected by foraging ants. They remove the fleshy attachments to take to the nest and abandon the seeds. This is the same as their treatment of Grevillea seeds and helps the dispersal of Acacia progeny.
Some native plants produce seeds with hooks that catch in the fur of animals and since European settlement in the clothing of humans. Some introduced plants that have become weeds also have hooked seeds. These hooked seeds are known as burrs. This is a very efficient means of seed dispersal and the unwitting transporters may carry plant progeny for long distances. In some cases seeds may be carried overseas.
Calotis lappulacea is one of our Yallaroo natives that have seeds that stick in socks and other clothing items. They donít worry us but graziers have trouble with these and other burrs in wool.
Some native plants have fruits that have a fleshy coat that surrounds a hard seed. Birds and sometimes animals eat these fruits, digest the edible coat and void the inner seed. Often the digestive system, of the feeder, softens the seed coat and prepares the seed for germination. Mistletoe fruits are dispersed in this way. Mistletoe Birds eat the fruits and deposit the ready-to-germinate seeds on tree branches. Using birds as a dispersal agent means that Mistletoes may be spread for long distances.