Lettuce Seed Germinating In Soil Cups

5 days after sowing in a cupful of damp garden soil kept at approximately 8 deg C (46 deg F), this lettuce seed shows quite clearly that the root (radicle) is first to emerge from the seed case (testa), complete with delicate root hairs to absorb water from the soil. The white hypocotyl (the part between the root hairs and the seed) has extended. Had the seed been buried, this hypocotyl would also have hooked over to drag the seed case containing the cotyledons (or 'seed leaves') out of the soil as it extended upwards.

GEOTROPISM
Note that even young roots will always grow downwards. They are said to exhibit 'positive geotropism'. Shoots exhibit 'negative geotropism' and grow upwards towards the light.

After 9 days, you can just see the cotyledons (seed leaves) emerging from this seed. Note that they have turned green in the light; the hypocotyl has also turned green. The green pigment cholorophyll is used to produce food (sugar) by 'photosynthesis'. Once this is done, the young plant is independent of the food supply in the seed. This point must be reached as soon as possible.

LIGHT AND TEMPERATURE
Lettuce seeds are sensitive to both light and temperature for germination. We have already seen how lettuce seeds react to light when germinating.

What is also not generally known, however, is that lettuce seed germination virtually 'drops off a cliff' at 25 deg C, with very slow germination even at 26 deg C. But there is also good news: Germination starts at 2 deg C (36 deg F). Peter Thompson shows this very clearly in his excellent book 'Creative Propagation'

Bibliography: Thompson, Peter(2005) 'Creative Propagation 2nd ed' (Pub:USA and UK by 'Timber Press')