In Chapter 5 we followed the processes of reproduction through to the arrival of the seed on the ground. Here we will look at germination and early survival of the seedling, and ways of producing new trees without resorting to seed.

The seed

Seeds remind me of spaceships: they contain everything they need to colonise new worlds given favourable conditions and water once they arrive. The outside is covered by the seed coat (the testa), designed to protect the contents. At the centre of the seed is the embryo, consisting of little more than a miniature root (the radicle) and shoot (the plumule). The rest of the seed is taken up with the food supply to keep the embryo alive before it germinates and to sustain early growth before photosynthesis takes over. This food is stored usually in the cotyledons (the seed leaves, part of the plumule), although some store it outside the cotyledons in the endosperm (which can be thought of as a short-lived half-brother of the embryo; all flowering plants have endosperm but in most it is used up quickly). In ash (Fraxinus excelsior), for example (Figure 8.1), the cotyledons are small, surrounded by endosperm, but in oak (Quercus spp.) the cotyledons are bloated and fill the seed with no remaining sign of the endosperm. The best example of endosperm is in the coconut (Cocos nucifera): part liquid (the milk) and part solid (the flesh). Wherever the food is stored, it is usually in the form of starch but oils are not uncommon especially in small wind-dispersed seeds. Oils contain more calories in a given mass and volume and so allow the seeds to travel light (having said that, large seeds can also contain oil: think of walnut oil for cooking and cocoa fat in chocolate). There is a price to pay: seeds with fats cost more to produce and are fairly short-lived (they go rancid).