Seasonal growth and contraction. Frailea castanea, like many other diminutive cacti, such as, for instance, Turbinicarpus, Mammillaria, Rebutia, has a well-developed contractile tuberous root system. These tuberose roots have the function of storing enough sustenance so that plants can survive the periods of drought that they have to endure. Tuberous roots also have the function of anchoring the plants into the ground. The tuberose contractile roots continually pull the plants deeper into the ground as the stem elongates so the plants remain subterranean or at an appropriate level in the ground. Contractile roots are usually broad, fleshy, vertical, tapering, and very distinct of the fine absorbent roots and are capable of incredible effort. As commonly happens with tuberous-rooted cacti, in times of drought the plants lose water and shrink, the swollen root contracting and pulling the stem down, resulting in the top of the plant body lying level with, or some­times even below, ground level. In most cases, contractile roots not only produce a strong pulling force, but also push away the substratum and create a soil channel in which plant movement is made easier.