76 RE GENERA TION 



part of the mid-vein, or of any large vein of the leaf, is cut out, leav- 

 ing a part of the lamina on each side (Fig. 35, B\ and the piece is 

 suspended vertically, roots appear on the basal end of the vein, and 

 in the same region one or more shoots arise. 



Leaves of heterocentron with the stalk attached, if kept in diffuse 

 light, produce roots along the stalk, especially near the basal end, 

 but shoots do not appear, even after five months (Fig. 35, C). 



These experiments show that the leaves do not exhibit the same 

 polar relations that are shown by pieces of the stem and root. 

 Vochting 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. 1 



Later, Vochting 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 Vochting on the regeneration of pieces of 

 the higher plants show that a piece possesses an innate polarity, or 

 "force," as Vochting 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 fac- 

 tors 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 



1 Vochting points out that a parallel case is found in certain conifers. In these there 

 arise from a vertical many-sided main stem whorls of side branches that are symmetrical in 

 one plane. These lateral branches, if cut off and planted, produce new roots and new 

 branches, but the latter are always side-branches, like the parts from which they arise. They 

 never produce a normal main axis. Nevertheless, although these branches cannot them- 

 selves produce a main shoot, a callus may be formed at the base of the piece, and from 

 this a new main stem may arise. 



