You are probably looking at the actual flower stem of the plant which has bolted (gone to seed). That is triggered by the warm weather.
Keep watch and it may flower, and then produce seed. Collect these and plant them in late summer or early fall for a new crop. There is little hope of keeping the plant or any part of it alive thru the summer.

And propagation by SEED is the only method that works.....

Tom W27 and George5il: Thank you both. Exactly the information I was looking for.

I'll be looking for the seeds. If you can point me to tips on collecting them and starting them off I'd welcome getting them.

I didn't know anything about propagation of Cabbage but in the nice Spring we have been having, I have been planting and rooting most anything so when my wife was preparing Cabbage and I saw little roots trying to grow from the stem she cut away, I took the stem and planted it in my pot garden on the front porch. It came up with 5 little cabbage plants so I dug it up and cut them apart and so far, they all seem to be surviving in other flower pots. I may not get any cabbage to eat but it is fun watching them grow. I have already bought her a Red Cabbage so that I can try this on a red cabbage as well as the green.