Contrary to what some people think, pineapples don't grow on trees — they grow out of the ground, from a leafy plant.
The plant consists of stocky leaves whorled around a central stem. In a healthy pineapple plant, the tapered, swordlike leaves can grow up to about 5 feet (1.5 meters) long.
The pineapple fruit grows out of the top of the central stem. The fruit is actually the result of dozens of individual fruit-producing flowers that have fused into a single fruit, which is capped with a "crown" sporting numerous short leaves.
Unlike most fruits, pineapples are not grown from seeds. Common commercial varieties of pineapples are "self-incompatible," meaning that the plants' pollen cannot fertilize members of the same variety. So unless different varieties are grown next to one another and flower simultaneously, the plant will produce a seedless fruit that develops without fertilization.
When removed, the crown of the pineapple fruit contains small roots. If it's planted into the ground (or a pot), a new fruit-producing plant will grow.
Additionally, the plant's "suckers" (side shoots that grow in between the leaves of the main stem) and slips (tiny plantlets that grow out from the base of the pineapple fruit) can produce new plants when replanted.