Seeds are the primary way that trees reproduce and their seeds vary greatly in size and shape. Some of the largest seeds come from trees, but the largest tree, Sequoiadendron giganteum, produces one of the smallest tree seeds.[67] The great diversity in tree fruits and seeds reflects the many different ways that tree species have evolved to disperse their offspring. The single extant species of Ginkgophyta (Ginkgo biloba) has fleshy seeds produced at the ends of short branches on female trees,[68] and Gnetum, a tropical and subtropical group of gymnosperms produce seeds at the tip of a shoot axis.[69] The seeds of conifers, the largest group of Gymnosperms, are enclosed in a cone and most species have seeds that are light and papery that can be blown considerable distances once free from the cone.[70] Sometimes the seed remains in the cone for years waiting for a trigger event to liberate it. Fire stimulates release and germination of seeds of the jack pine, and also enriches the forest floor with wood ash and removes competing vegetation.[71] Similarly, a number of Angiosperms including Acacia cyclops and Acacia mangium have seeds that germinate better after exposure to high temperatures.[72] Wind dispersed seed of elm (Ulmus), ash (Fraxinus) and sycamore (Acer pseudoplatanus) Angiosperm tree produce seeds in a wide variety of fruits, some of them include acorn, nut, berrie, pome, drupe, samaras, hesperdium, capsule and legume.[73] For a tree seeding to grow into an adult tree it needs light and space, if seeds only fell straight to the ground, competition among the concentrated saplings and the shade of the parent would likely pre ent it from flourishing. Many seeds such as birch are small and have papery wings to aid dispersal by the wind. Ash trees and maples have larger seeds with blade shaped wings which spiral down to the ground when released. The kapok tree has cottony threads to catch the breeze.[74] The flame tree does not rely on fire but shoots its seeds through the air when the two sides of its long pods crack apart explosively on drying.[74] The miniature cone-like catkins of Alder trees produce seeds that contain small droplets of oil that help disperse the seeds on the surface of water. Mangroves often grow in water and some species have propagules, which are buoyant fruits with seeds that start germinating before becoming detached from the parent tree.[75][76] These float on the water and may become lodged on emerging mudbanks and successfully take root.[74] Other seeds, such as apple pips and plum stones, have fleshy receptacles and smaller fruits like hawthorns have seeds enclosed in edible tissue; birds and animals[clarification needed] eat the fruits and the seeds are either discarded or are consumed and pass through the gut to be deposited in the animal's droppings well away from the parent tree. In some cases, germination is improved by being processed in this way.[77] Nuts and other large seeds are gathered by animals that hide in caches[citation needed] any not immediately consumed.[78] Many of these caches are never revisited, the nut-casing softens with rain and frost and the seed germinates in the spring.[79] Pine cones may be hoarded in a similar way by red squirrels, and grizzly bears raiding the caches may also help to disperse the seed.