40S MB. F. DARWIN OS THE 



Vochting concludes that a vegetative cell, or group of cells, has 

 such a physical constitution that it requires a certain disturbing 

 force (Anstoss) to make it develop into the rudiment of a root or 

 bud. Disturbing forces or stimuli arise in the life-unit itself and 

 influence the course of cell-division ; but such stimuli may also 

 come from external forces. 



This, though not any thing like a full account of Vochting' s 

 results, may serve as an introduction to my own work. With 

 the same object, Sachs's conclusions will be briefly summarized. 

 The fundamental point of difference between Sachs's and Voch- 

 ting's theories is, that Sachs does not believe in a hereditary 

 growth-tendency. Instead of placing gravitation and light in the 

 second rank of causes, Sachs believes that the growth of organs 

 in the life-unit is entirely regulated by these external forces. 

 According to his views *, the force of gravity, acting on the de- 

 veloping cells of an organ, build up in it a predisposition to the 

 production of roots and buds at the base and apex. This after- 

 effect of gravitation he believes to produce the effects ascribed by 

 Vochting to a hereditary morphological force. 



Sachs differs entirely from Vochting in the view he takes of the 

 mode of action of the external forces. We have seen that Voch- 

 ting considers gravity as an "Anstoss " or stimulus, which tends 

 to produce certain kinds of growth because of the physical con- 

 stitution (materielle Aufbau) of the formative cells. Sachs's 

 theory rests on the belief that difference of material is a necessary 

 concomitant of difference of form. According to this view, we 

 must believe that the materials from which roots are formed are 

 chemically (in a qualified sense, loc. cit. p. 456) different from those 

 which supply branches. Sachs's theory supposes that the growth 

 of roots or buds at a given place will be determined by the dis- 

 tribution of the root- and branch-forming materials ; and, further, 

 that the distribution of these materials is regulated by the force 

 of gravity. The root-material is, in a certain sense, geotropic, 

 and flows downwards ; the branch-material is apogeotropic (nega- 

 tively geotropic) and flows upwards. The impulses which cause 

 the formative materials to flow in these directions are supposed to 

 continue as an after-effect of the force of gravity ; so that the pro- 

 duction of roots at the basal end of a cutting hung upside down is 

 an after-effect of the original tendency of the root-material to flow 

 downwards. 



* Loc. cit. p. 474. 



