Read it through once

Before leaving the subject of tendrils, it may be interesting to notice the queer corkscrew spirals in which they roll themselves up. These spirals are formed after the end of the tendril has tied itself to the support and become woody. The free part between the end and its own stem goes on revolving; now if you tie a piece of string at both ends and make it revolve, you will see at once that it must coil itself into a double spiral, one part in one direction and the other in the opposite way, with a flat piece between them.