These spicules are of very varied though regular shape, but ordinarily assume a rod-like needle shape or else a stellate form.

Some plants, as many of the cactus tribe, are made up almost entirely of these needle-crystals; in some instances, every cell of the cuticle contains a stellate mass of crystals; in others, the whole interior is full of them, rendering the plant so exceedingly brittle, that the least touch will occasion a fracture;