80 RE GENERA TION 



formed rudiments may be present, but also in the internodes, where 

 there are no rudiments of roots. 



Stems of heterocentron placed in a horizontal position produce a 

 circle of roots around the base, and later, in several cases, roots from 

 the under surface of the stem, both from the nodes and the inter- 

 nodes; but these roots are smaller than those at the base. Those 

 around the base are often longer on the lower side than on the 

 upper side. 



Vochting has also studied the regeneration of pieces of roots of 

 the poplar and of the elm suspended horizontally in a moist chamber. 

 A callus develops from the cambium region of the basal end, and 

 from this a thick bunch of adventitious sprouts grows out. A weak 

 callus may develop on the apical end also, from which a few roots 

 develop. In other cases adventitious shoots are produced also from 

 the apical callus, especially from the upper edge of the callus. 

 The results are variable, but show that at times leaf-shoots may 

 develop from the apical end of the root. It is also singular to find 

 that, while pieces of the root produce new leaf-shoots very readily, 

 yet they often fail to produce new roots, or produce only a few that 

 arise from the apical callus or from the sides near that region. It is 

 difficult to show that gravity has any influence on the result. 



Vochting recognizes another sort of influence that determines the 

 position of new organs on a piece. If a young, growing end of a 

 stem of Heterocentron diversifolium is suspended by two threads in 

 a horizontal position, the ends bend upward as a result of the nega- 

 tive geotropism of the piece. The new roots appear at the base of 

 the piece, and also on the convex side of the bent part of the stem, as 

 shown in Fig. 34, B. The same result can be obtained by forcibly 

 bending a twig, and then tying the ends together, so that it remains 

 in its bent position. If a piece of this sort is suspended in a moist 

 atmosphere, with the bent inner concave side turned upward, the 

 roots appear on the base and at the bend, especially on the under 

 side, both from the nodes and internodes. If now in order to see if 

 gravity takes any part in the result the next piece is suspended with 

 the outer convex side of the bent part turned upward, it is found that 

 many of the pieces produce roots only at the base, but others pro- 

 duce roots also at the bent portion of the stem, but they are fewer 

 than in the last experiment. The roots arise for the most part on the 

 under side of the arch, and only a few arise from the upper part. It 

 is clear that gravity is also one of the factors in the result. Leaf- 

 buds arise in these pieces with the concave side turned upward only 

 near the apex ; rarely one may develop on the lower part of the basal 

 end. In pieces with the concave side turned dowmvard the leaf-buds 

 arise for the most part at the apex, but sometimes they appear on the 



