Seeds vary greatly in shape and size; they may be extremely small (Willow) or quite large (Horse Chestnut). However, they all contain a
I (TV^>rootiets single embryo which on germination will develop u , " into the young tree. An
I I_radicle embryo has a primary
!E root or radicle, and a primary shoot or plumule. Below and enclosing the plumule are two seed leaves or cotyledons. In non-endospermic seeds (Oak) the cotyledons are large and contain all the food reserves. The cotyledons of endospermic seeds (Ash) are usually thin and the whole embryo is surrounded by the food reserve tissue. The seed is enclosed in a seed coat or testa which in dry indehiscent fruits is very thin and may be fused to the fruit wall. On germination water is absorbed and the previously dormant embryo begins to grow, its rate of respiration greatly increases and sufficient oxygen must be available for successful germination. The radicle emerges first and grows downwards; later the plumule grows upwards. The food reserves present in the endosperm or cotyledons are utilized during germination, but once the plumule has become exposed above the soil it becomes the green seedling shoot capable of photosynthesis.
OF AN ACORN
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