If you do not have the benefit of having modern cordage with you, or you ran out or it was lost, you can use things in the natural environment to use as rope or to make it.
Unless you have miles of rope during an emergency you should try to supplement your modern cordage with natural cordage whenever appropriate.
In the photo here you see cattails, and their leaves can be used to make cordage.
Do what you must according to your circumstances. If you don't have the time or resources around you to use natural cordage then use the modern.
Things will not normally be ready to be used 'out of the box' in a way of putting it, therefore some preparation is usually required to make use of natural materials for cordage. Spruce roots however may be used as soon as you dig them out of the ground.
Here is a list of some of the things that may be used to make cordage.
1. Tree bark.
2. Tree roots like from spruce.
3. Plant stems and leaves like from cattails.
4. Animal hide and sinew.
5. Coir fibers from coconuts.
6. Animal hair.
If you can find a pliable plant with good fibers it can be used. You may need to pound the stem to open it up, and remove the pith to get to the fibers etc.
If you use stinging nettle (Urtica Dioica), the stingers contain formic acid that needs to be removed first, and that can be done by placing the stems (plants) in a running stream if there is one, and waiting a few hours. This should render the stingers less harmful. The stems need to be dried out in the sun or near a fire, pounded to get to the fibers then made into cordage.
Here are some links to help you learn more about making natural cordage.
Here are some other links that you may find interesting.
1. Wikipedia - Jute
2. Wikipedia - Sisal
Have a look at the Videos page as well for more information on the subject.