Observations and results
Did the top stem piece grow green leaves and overall make a much better clone than the middle or bottom pieces? Did the middle piece grow at least a few small green spots, whereas the bottom had fewer green spots—if any?
When you cut the stem into three pieces, you probably saw that all three pieces had had some leaves growing from them, which you could see by the leaf stubs left from when you removed the leaves. The piece that should have had the most leaves was the top piece, the only piece that had leaves on its top side. This is relevant because plants have a type of tissue called the meristem, which is where plants can sprout and is thereby important for asexual reproduction. Meristem tissue is usually in the plant’s root tips (which had been removed from the cabbage) and the stem's tip, where it grows the stem, new leaves and buds. You should have seen that after only a day the top piece looked greener than it had after you removed its leaves—and it became greener each day, growing several leaves mostly from the top of the piece. By the time the week was over, the middle piece should have sprouted a few small green spots where the leaves meet the stem. The bottom piece may have sprouted a few green spots as well, but it probably had rotted much more than the other pieces.
Because you did not need the cabbage leaves for this activity, you could use them to make a cabbage soup or coleslaw. You can also continue trying to grow your clones on soil or compost them.
More to explore
Plant Reproduction–They'll Make More, from Biology4Kids.com
Plant Reproduction: Asexual Reproduction, from Encyclopedia Britannica
Meristem, from Kids.Net.Au
Attack of the Killer Cabbage Clones, from Science Buddies