744 



SCIENCE. 



[N. S. Vol. XX. No. 518. 



3. REGENERATION WHEN THE RELATIVE 

 RATE OF GROWTH DEPENDS ON PRE- 

 FORMED ELEMENTS. 



If a piece of a willow be cut off and 

 suspended iu a moist atmosphere the apical 

 buds produce new shoots (the basal buds 

 hardly developing at all). Roots develop 

 around the basal end of the piece from 

 preformed root-buds. This phenomenon is 

 so similar to what takes place in the re- 

 generation of animals that the same term, 

 'polarity, ' has been applied here also. Now 

 I believe it can be shown that the phe- 

 nomenon is not the same in the two groups 

 and is the outcome of quite different fac- 

 tors. An examination of the results in 

 plants will show that the first buds to de- 

 velop are the most vigoroTis ones, which are 

 usually the largest. In the case of the 

 willow the largest buds are those nearer, 

 although not at, the distal ends of the 

 branches. If these buds once begin to de- 

 velop they will use up the available food 

 stuffs in the piece, and thus hold in check 

 the development of the more basal buds. 

 Hence the latter fail to develop. In some 

 other plants the basal buds are the most 

 vigorous ones and these develop first. 

 Whether the same explanation will account 

 for the root development can not be stated, 

 because, so far as I can discover, no one 

 has shown whether the root-buds nearer 

 the base are more advanced than are those 

 nearer the apex. If this should prove to 

 be the case the explanation used for the 

 shoots will also account for the development 

 of the roots. These phenomena in the 

 pieces of plants led Sachs to apply his stuff 

 hypothesis to explain them. He not only 

 assumed that formative stuffs are present, 

 but also that they move, in response to 

 gravity, in definite directions. His latter 

 assumption was shown to be untenable by 

 Vochting. A number of recent investiga- 

 tors still continue, nevertheless, to make 

 use of Sachs's hypothesis in one form or 



another. Goebel, for instance, intimates 

 that the 'polarization' of the tissues them- 

 selves is the cause of some of the formative 

 (or, in some cases, nutritive) stuffs moving 

 in one direction and of others in the op- 

 posite direction. Thus, while Sachs started 

 out to explain polarity as the result of 

 fluids flowing in a given direction, Goebel, 

 nominally using Sachs's view, assumes the 

 polarity in order to make the substances 

 flow in predestined paths. 



I think we need assume neither form- 

 ative suft's nor their movements in specific 

 directions. I have stated above that we 

 can account for the results by means of a 

 simpler and, I believe, a more reasonable 

 explanation. 



4. LATERAL REGENERATION. 



If we confine our attention to the regen- 

 eration of the head and of the tail alone 

 we get a very incomplete conception of the 

 phenomena that are to be included under 

 the term polarity, for animals regenerate 

 not only in the directions of the poles of a 

 magnet, but laterally, dorsally, ventrally. 

 In short, in all three dimensions of space, 

 or combinations of them. One example 

 of many that might be given will illustrate 

 my point. 



If a planarian is split lengthwise into 

 two equal parts, each half regenerates 

 laterally its missing part. If we examine 

 carefully the method by which this takes 

 place we find that a narrow edge of new 

 material appears along the cut side and in 

 this material the new structures are laid 

 down. The chief point of interest here is 

 that the lateral organs develop long be- 

 fore the new part has reached the size of 

 the part removed. Furthermore, at the 

 outer, new edge the distal ends of the 

 branches of the digestive tract are pro- 

 duced, and along the line between the new 

 and the old material the median organs 

 are laid down. The intermediate parts 



