Seed Dispersal (3rd Ed, 1971)

[Music] Ever since man learned to grow his own plant crops, he has been a gatherer of seeds. Each year, millions of seeds are collected. Seeds which ensure a future harvest. Tons of fruits and vegetables are gathered not as food, but for the seeds they contain. Machines perform some tasks formerly done by hand. Seeds are first separated from the fruit. Then they are washed. Drying makes them ready for packaging and distribution throughout the world. We often aid in seed dispersal without thinking about it. One of the seeds in the apple core might eventually grow into a tree. Many food plants such as cereals, including corn, are dependent on man for dispersal. Without man, many food plants would lose the struggle for existence and become extinct. But what about the thousands of other seed producing plants? Plants that are found nearly everywhere life exists on Earth. How are their seeds scattered? One of the most common ways is by wind. These are the fruits of the Norway maple tree. One seed is in each fruit. The fruits separate and each becomes propeller-like. They spiral or glide away from the tree in late fall and then lie dormant until spring. Many pine trees also produce seeds that are wing-shaped. The seeds are enclosed in pine cones. When mature, the seeds are kept locked up inside the cone until dispersal conditions are favorable. Then, when the weather is dry, the cone opens and the seeds are exposed. Wind lifts the glider-like seeds and carries them away. Pine seeds, like many other kinds of seeds, will not germinate until they have had a resting period called dormcy. The cold period of winter is the dormant period for pine seeds. When spring comes, those that survive may begin to grow. Winged seeds travel relatively short distances, but seeds with plumes can be carried hundreds of miles. The milkweed seed is attached to a plume which acts as a parachute. The tiny dry seedlike fruits of the goats beard, which resemble the dandelion, also depend on their plumes for dispersal by wind. The plume stays open as long as the air is dry. In rain or damp air, the plume closes. When bright sunny weather returns, the plume again opens and the fruits are ready to take off. If conditions of air moisture continue to change, the parachute may open and close several times. This may push the fruit into the soil. The action here is shown greatly speeded up. Sometimes the whole plant is moved by the wind. The tumble weed, when its seeds are ripe, breaks off near the ground on windy days. Not all the seed containers are opened at once. Consequently, the plant may be blown around for many weeks before all the seeds are scattered. Some fruits, like the hickory nut, are too heavy to be moved by wind. When the tree grows on a hillside, gravity helps to disperse the nuts, which simply roll to a new location or may float away. One of the best water traveling fruits is the coconut. Fruits fall from trees near the water to begin a journey that may take them thousands of miles to a final landing place. Many seeds and fruits are carried by animals. Birds while eating may drop some of the fruit containing seeds or may later pass them through the digestive tract. Migrating water birds often carry mud on their feet which contains seeds of pond or marsh plants. As the birds travel, the seeds may get washed off in a pond many miles from the parent plant. Squirrels carry nuts such as those of the oak, the walnut, and the hickory. If hungry, a squirrel will eat the nut. If he is not hungry, he may carry it away and bury it. Because later on, squirrels do not find all the nuts they bury, some will grow into new trees. Berdo fruits are adapted for another way of traveling. Hooks are the means by which the burrs are attached to passing animals. Long-haired animals are especially liable to pick up bero burrs and carry them to new locations. Some seeds do not rely on outside means of dispersal. This is the jewel weed. It is also called the touch me nut. So another way in which plants scatter their seeds is by shooting them. The witch hazel is another plant that projects its seeds. As the witch hazel capsule opens, the smooth inner linings of the capsule pinch the shiny black seeds and shoot them as far as several yards from the parent plant. Let's watch this action again. Here is a fruit with a long corkcrew-like tail. It is called the stor's bill. When rain falls or dew forms, the corkcrew untwists. Then as it dries, it twists again. These actions repeated again and again may cause it to push into the ground. The stiff hairs on the drill-like end keep the drill from pulling out once it gets started into the ground. We are watching several cycles of wetting and drying by time-lapse photography. The wild oat is another plant well adapted to survive. Each oat grain, of which there are two or three to a spikelet, has a long bristle that twists and untwists with changes in the air's moisture. Spikelets fall to the ground where they look like insects moving about through the time-lapse camera. The grains kick themselves out of the spikelet. Then each grain goes its own way. By moving about it may enter a crack or hole into which it can burrow and begin to grow. Cultivated oats on the other hand have lost the ability for burrowing. If the cultivated oats are scattered and not covered, the roots may still penetrate the soil during rainy weather. But if they do germinate and then the soil dries out, the young plants soon die. We have seen how plants with well-developed methods of fruit and seed dispersal are distributed across continents and even the world. The plants that we regard as weeds are often those with very efficient dispersal mechanisms. Autumn is the time of year when most seeds mature. In the woodlands and fields during seed time, you can see many kinds of fruits and some of the ways in which their seeds are dispersed. Why not explore the vacant lot, the nearby parks or the countryside and discover some of these wonders of nature? [Music]