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ADAPTATIONS -Offspring
Many mangrove trees have special adaptations to give their offspring the best chance in their harsh habitat. Many provide their seedlings with a good store of food and floatation devices.
In some, the fruit does not fall away when it ripens. Instead, the seed within the fruit starts to germinate while it is still on the mother tree, and the mother tree channels nutrients to the growing seedling
In some plants, the growing seed does not break through the fruit wall while the seed is on the mother plant but only after the fruit falls off. This is the case with Avicennia and the seed coat of its fruits drops away more quickly in water of the right warmth and salinity, usually in a spot best suited for an Avicennia seedling.
In others, the growing seedling breaks through the fruit wall to form a stem (called a hypocotyl), sometimes even roots (Rhizophora, Bruguiera). The whole seedling is then called a propagule (potential plant). In some trees, the seedlings only fall at high tide! When the propagule finally falls, at first it floats horizontally, and drifts with the tide. It can survive for long periods at sea. The tip is water absorbent while the top end is water repellent. After some weeks, the tip gradually absorbs water and the seedling floats vertically and starts to sprout its first leaf from the top, and roots from the bottom. When it hits land, it hauls itself upright by growing more roots, then sprouts more leaves. The long stem is a short-cut to sunlight, and oxygen as seedlings are often completely submerged at high tide. Amazingly, young seedlings can survive being completely underwater until they are big enough to grow aerial roots, at about 1-2 years. Meanwhile, they depend on stores of air in air spaces (aerenchyma) in their stems.
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