Many aquarium plants come in little plastic pots full of a white, stringy, cotton-like substance called rockwool. Fish keepers are divided over whether it's necessary to remove it from the plants or not before placing them in the aquarium.
It's possible to grow aquarium plants either by leaving them in the pots or by removing them and all the rockwool entirely. Some believe that it is a benefit to plants to have undisturbed roots and leave them in the pots, while others such as Karen Randall of "Aquatic Gardeners Magazine" believe it confines them too much. Rockwool can be potentially unsightly, and bits may break off and float around the tank.
Rockwool itself is inert and contains no chemicals that will leach into the water. Plants you purchase in rockwool tend to be more stress-resistant than bare root plants. If you keep the plant in the pot with the rockwool, it is easier and less stressful to the plant to move it later during maintenance or redecorating.
If you buy plants in plastic pots with rockwool, they may contain fertilisers that will leach out in your aquarium and cause algae. Rockwool could also potentially harbour pest snail eggs and other unwanted stowaways. Some anecdotal evidence exists that some fish may be irritated by the fibres.