I start most large seeds off on paper towel, then as soon as they begin to germinate transplant them into soil. I left this cucumber seedling on the paper towel for a day too long and the root has elongated. At the tip you can see the mucilage-covered root cap, which would normally be sliding between the soil particles as the root elongates. Behind you can see the forest of root hairs that vastly increase the surface area of the root and allow it to absorb water and mineral salts efficiently. Root hairs live for about a day and new ones are developed constantly at the beginning of the root hair zone behind the root tip while the oldest, some distance further back, wither and die.