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What happens when a tree seed begins to germinate?

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Tall giant sequoia trees with reddish bark and thick trunks rise closely together in a dense forest at Sequoia National Park, California. — Tree A stand of giant sequoia trees (*Sequoiadendron giganteum*) in Sequoia National Park, California.

When a tree seed begins to germinate, it undergoes a complex biological process that transitions the embryo from a state of dormancy to active growth.

The Initiation of Germination

The process typically begins with imbibition, the absorption of water by the seed. This causes the embryo's cells to rehydrate and expand. As water uptake occurs, the seed's rate of respiration increases, and metabolic processes that were suspended during dormancy resume. For many tree species, particularly those in temperate climates, germination cannot occur until the seed has undergone afterripening—a period of time or exposure to specific environmental conditions, such as chilling, that breaks dormancy.123

Seedling Emergence

Active growth follows the initial swelling:
* Root Emergence: In most species, the primary root, or radicle, emerges first and grows downward into the soil, a response known as positive geotropism.14
* Shoot Development: The young shoot, or plumule, eventually emerges and grows upward (negative geotropism) to reach light.14
* Nutritional Support: Until the seedling becomes self-supporting through photosynthesis, it relies on food reserves stored within the seed, such as in the cotyledons or endosperm.14