522 



SCIENCE 



[N. S. Vol. LIV. No. 1404 



material from the sap will cause similar ma- 

 terial to leave the cells of the stem and to dif- 

 fuse into the sap. If this purely chemical rea- 

 soning is sound it would follow that the larger 

 the mass of the stem the greater the mass of 

 chemical substances available for the growth 

 of shoots per unit of time. On this basis we 

 should expect that the mass of shoots formed 

 on the node of an isolated piece of stem would 

 be in proportion with the mass of the piece of 

 stem. That this is correct can be shown by 

 cutting a defoliated stem of Bryophyllum into 

 as many pieces as it possesses nodes. In this 

 case, each node will produce shoots but their 

 mass will be unequal in the different pieces, 

 and will be greatest where the mass of stem is 

 greatest. 



If it is true that in a long defoliated piece 

 of stem only the two shoots of the apical node 

 grow out because practically all the material 

 available in the stem flows to the apex; and 

 that the shoots in the nodes below do not grow 

 out because practically none of the material 

 reaches them, then we should expect that the 

 mass of the two shoots formed at the apex of a 

 long piece of stem should approximately equal 

 the mass of all the shoots which would have 

 been formed if the stem had been cut into as 

 many pieces as it contained nodes. A large 

 number of experiments have been made which 

 have shown that this is correct. The following 

 example may suffice : Four large stems of Bryo- 

 phyllum were defoliated and a piece contain- 

 ing 9 nodes was cut from each defoliated stem. 

 From each piece of stem the three uppermost 

 nodes were cut off and cut into three pieces 

 containing one node each. These 12 one-node 

 pieces produced 23 shoots. The 4 stems, with 

 6 nodes each, produced all together 8 shoots. 

 After 20 days the dry weight of the shoots and 

 of stems was determined. It was found that 

 the 12 small pieces of 1 node each had produced 

 23.2 mg. dry weight of shoots per gram of dry 

 weight of stems, while the 4 large pieces with 

 6 nodes each had produced 26.3 mg. dry weight 

 of shoots per gram of dry weight of stems. 



This shows that the mass of the two shoots 

 produced at the apex of a long piece of stem 

 equals approximately the mass of shoots which 



would have been produced in the same stem in 

 the same time under the same conditions if the 

 shoots could have grown out in all the nodes. 

 This leaves no doubt that the polar character 

 of the regeneration of shoots is due to the 

 fact that all the material available for growth 

 reaches the apical and none of the other nodes 

 of a long piece of stem. The average growth 

 of shoots in small pieces is slightly less than 

 in large pieces in the experiment mentioned 

 (23.2 mg. instead of 26.3 mg.), probably be- 

 cause the extreme ends of each piece die or 

 cease to participate in the supply of material 

 for growth. As a consequence the mass of a 

 stem which supplies material for growth is less 

 when the stem is cut into smaller pieces than 

 when it is left intact. 



It had been shown in previous papers that the 

 mass of shoots and roots produced by a leaf of 

 Bryophyllum is also in proportion to the mass 

 of the leaf.^ 



A fuller description of the results will be 

 given in the Journal of General Physiology. 



Jacques Loeb 

 The Eockefellee Institute 

 FOR Medical Eeseakch, 

 New Yoke 



the scattering of electrons by nickel 

 A STUDY of the electron emission from a 

 nickel target under electron bombardment has 

 revealed certain features of this emission which 

 appear to be of considerable interest on account 

 of their probable bearing on the structure of 

 the nickel atom. 



Besides the emission of slow-moving second- 

 ary electrons characteristic of all metals the 

 emission from nickel contains an appreciable 

 fraction of electrons of higher speed which 

 appear to be scattered directly from the inci- 

 dent beam of primaries by the atoms of the 

 target. The fastest of these scattered electrons 

 have speeds almost if not quite equal to the 

 speed of the primaries. It would appear that 

 the sharp deflections experienced by these scat- 

 tered electrons must result from their pene- 

 trating into the atom structure and being 

 iLoeb, J., J. Gen. Physiol, 1918-19, I., 81; 

 1919-20, II., 297, 651. 



