These experiments show that the leaves do not exhibit the same polar relations that are shown by pieces of the stem and root. Vöchting points out that the results may be explained in either of two ways. The stem and the root have in general an unlimited growth with a vegetative point at the apex. The leaf has only a limited growth. Its cells form permanent tissue, hence the leaf does not produce a new plant from its outer part. The second possibility is this: the phenomenon is connected with the symmetrical relations that different structures possess. Stem and root are symmetrical in two or more directions, the leaf on the other hand is a flat structure with one plane of symmetry, and even symmetry in one plane may be absent. If the leaf could produce shoots at its apex and roots at its base, from the semilunar fibrovascular bundle of the leaf, then an individual (the leaf) with its single plane of symmetry would produce shoots and roots that are symmetrical in two planes. Such a result would be so anomalous that one may well doubt the possibility of its coming into existence.[32] Later, Vöchting attempted to see if the same relation found in the leaf would hold for other organs that have a limited growth. He found that such structures, as spines, for example, produce both shoots and roots near the base, as do leaves. These experiments of Vöchting on the regeneration of pieces of the higher plants show that a piece possesses an innate polarity, or “force,” as Vöchting sometimes calls it (although he explicitly states that he does not use the word “force” in its strict, physical sense). It does not follow, of course, that external conditions may not also influence the regeneration, but in those experiments in which the pieces were freely suspended in a moist atmosphere, the external factors are as far as possible excluded, so that the effect of the innate tendencies are most clearly seen. In another series of experiments the influence of external conditions on the regeneration was especially studied. This analysis that Vöchting has made of the problem of regeneration is in the highest degree instructive, since it shows how several factors,--some internal, others external,--take a hand in the result; and it is only possible to unravel the problem by combining different experiments carried out in such a manner that one by one the different factors at work are separated. If a piece of a young stem of _Salix viminalis_ is suspended vertically in a moist atmosphere, with the lower end in water (for ¾ of a centimetre), and the piece kept in the dark, the result is, in the main, the same as when similar pieces are suspended in moist air without coming into contact with water. Roots arise near the base, and shoots near the apex, without regard to which end is in the water.