What do plants need to grow?

I asked my 4 year old what he thought plants needed to grow, and he very quickly answered light and water.  I suggested we could set up an experiment to find out if he was correct, and he was really keen.  So I took three pots, some kitchen roll, cress seeds and aluminium foil and asked him to help me set it up.

I labelled one pot with 'light and water', one with 'light' and one with 'water'. Into each he folded some kitchen roll and sprinkled some cress seeds.  In those with water written on, he sprayed some water (you can also pour gently) until the tissue was soaked.  And for the ones with light on the side, we put them on a window sill uncovered.  He helped me wrap the other one in foil to keep out the light (black paper would be better but we didn't have any).  The idea here is that we had two variables we wanted to test - light, and water, and for each pot we tried to keep everything else the same so we could compare.

Cress seeds ready to grow (or not)

We left the seeds for a day and then had a look.  Both pots with water had slightly swollen seeds, but nothing was happening to the dry pot.  We then looked at them most days for just over a week to see what happened and watered the same two pots when we had a look at them.

My son was surprised to see that the seeds in the dark pot were growing.  We talked about where seeds grew outside, in the soil, and how it was dark there.  So he concluded that maybe plants didn't need light when they first started to grow.  I explained that the seeds store enough energy for the plant to start to grow and make its root and first set of leaves (cotyledons).  Then the leaves start to provide energy for the plant to grow further using photosynthesis, which he's heard about on Maddie's Do You Know.

Green leaves, pale leaves and seeds after 5 days

The leaves of the plants in the light started to turn green after about 4 days, and they stayed quite short.  The leaves of the plants in the light stayed pale (but did turn slightly green as his wrapping didn't keep all the light out) and they got taller and taller.  Had the wrapping been better, they'd have stopped growing when they exhausted the nutrients they had available from their seeds.  The dry seeds kept in the light didn't grow at all.  Also the plants in the light eventually withered when he forgot to water them for a few days.  So beyond the initial growth of the seed, he was right that plants need both light and water to grow, but the experiment showed that cress seeds only need water to germinate and start to grow!