118 



SCIENCE 



[N. S. Vol. XL VI. No. 1179 



It seems, therefore, probable that the inhibit- 

 ing effect of the stem upon the mass of shoots 

 produced in the leaves is due to the absorption 

 of a corresponding quantity of material from 

 the leaves by the stem. 



6. Summary and Conclusions. — (1) The 

 writer had shown in a former note that the 

 mass of shoots produced in isolated sister 

 leaves of Bryophyllum calycinum is in direct 

 proportion to the masses of the leaves and that 

 this remains true if the mass of one leaf is re- 

 duced by cutting out pieces from the center of 

 the leaf, while the sister leaf remains intact. 

 In. this paper it is shown that the rate of geo- 

 tropic bending of horizontally placed stems of 

 Bryophyllum calycium, if one apical leaf is at- 

 tached to the stem, occurs at a rate increasing 

 with the mass of the leaf. When the mass of 

 the leaf is diminished by cutting away pieces 

 the rate of geotropic bending is diminished 

 also. 



(2) It had been known for a long time that 

 when a piece of stem is attached to a leaf of 

 BryophyUum calycinum the shoot production 

 in the latter is diminished or completely inhib- 

 ited. It is shown in this paper that the mass 

 of a piece of stem attached to a leaf increases 

 by approximately the same amount by which 

 the shoot production in the leaf is diminished 

 through the influence of the stem. The infer- 

 ence is drawn that the inhibiting effect of the 

 stem upon shoot production in the leaf is due 

 to the fact that the same material which would 

 have been available for shoot production in the 

 leaf, had the latter been detached from the 

 stem, is. now absorbed by the stem. 



(3) This material gives rise in the stem to 

 callus formation and to that growth of cer- 

 tain cells of the cortex which causes the geo- 

 tropit3 bending; and if the buds of the stem 

 are not removed it causes also shoot produc- 

 tion on the stem. The comparatively large 

 masses involved indicate that this material 

 must consist chiefly of the common material 

 required for growth, i. e., water, sugars, amino 

 acids, salts; but the accessory substances and 

 the hypothetical specific organ-forming sub- 

 stances of Sachs may be included in this mass; 



and this is suggested by the fact that on the 

 lower side of a horizontally placed stem, roots 

 grow out, while shoots grow out from the 

 upper side. There must, therefore, be asso- 

 ciated with the material which causes geo- 

 tropic bending also something which favors 

 the growth of roots and this may be one of 

 the hypothetical substances of Sachs. 



(4) These facts give a simple explanation 

 of the " resourcefulness " of the organism re- 

 ferred to in the beginning of this paper, 

 namely that plants may restore their lost apes 

 either by the growth of the hitherto dormant 

 buds near the wound or by a geotropic bend- 

 ing of former horizontal branches next to the 

 wound (fir trees). Our experiments suggest 

 that the cause is the same in both eases, 

 namely, a mass action of the nutritive, and 

 possibly also of some specific substances, upon 

 the cells of dormant buds or upon the cells of 

 the lower side of horizontal branches which 

 leads to a rapid synthesis and growth in these 

 cells. Without the removal of the old apex 

 this gTowth would not have taken place, for 

 the simple reason that the nutritive material 

 would have had no chance to collect near the 

 wound in masses sufS.cient for the growth. 



(5) The phenomena of geotropism thus turn 

 out to be phenomena of mass action, probably 

 of the common nutritive material circulating 

 in the sap and they are apparently of the same 

 nature as the growth of dormant buds, which 

 is also due to a mass action of the same sub- 

 stances. Gravity need play only a passive 

 role, allowing masses of liquids to " seek their 

 level." In the literature of geotropism this 

 phenomenon is treated as a case of " stimula- 

 tion," but this treatment misses the essential 

 point, namely, the chemical mass action in- 

 volved, and it substitutes a fictitious factor, 

 the " stimulus " of gravitation, which in all 

 probability does not exist. The case is similar 

 to that of heliotropism when the orientation of 

 animals to light is treated as a " reaction to a 

 stimulus " instead of as an instance of the 

 photochemical law of Bunsen and Eoscoe. 



Jacques Loeb 

 The Eockepeller Institute eok 

 Medical Eesearch 



