The "strings" in a celery stalk are collenchyma tissue made up of thick-walled collenchyma cells that create a support structure for the plant. Collenchyma cells are filled with living protoplasm and sometimes chloroplasts.
Celery is a staple of many weight loss plans, because it is very low in calories and fat free. It also contains many key nutrients, such as vitamin A and vitamin K, and is high in fiber. Along with vitamins and minerals, celery has also been shown to contain coumarins, which help reduce damage done by free radicals and might even be beneficial in the fight against cancer, according to SFGate.