POSITIVE AND NEGATIVE GEOTROPISM. 68 1 



I may remark here that the reader can only hope to understand the phenomena 

 of geotropism if he reflects most carefully on the considerations connected with 

 Fig. 383, and makes them perfectly clear to himself. 



Nevertheless the preceding simply shows how we have to figure the geotropic 

 up and down curvatures in space; though nothing has there been said as to the 

 cause of the change. We may now attempt to make this latter clearer. 



It is sufficient then, as said, to place parts capable of growth out of their 

 usual position, the erect one for instance, into another, e. g. the horizontal ; this 

 suffices as an external impulse or stimulus, which alters the processes of growth as 

 shown in Fig. 383. By thus placing the perpendicularly (upwards or downwards) 

 growing parts horizontally or obliquely, however, no more has happened than 

 a change of their position with respect to the radius of the earth; and the next 

 question is as to how far this can act as a stimulus on the growing parts. 

 There is nothing for it here but to suppose the vertical line representing the 

 radius of the earth as the direction in which a force of some kind is acting, 

 which influences the growth of the plant-organ, and that the main point is what 

 angle this force makes with the axis of growth of the organ. Every change of 

 this angle acts as a stimulus by which the growth is so influenced that the above 

 described differences between the upper and lower sides appear, until the younger 

 still growing portions again stand in the same direction to the radius of the earth 

 as before. 



Now there is but one known force which acts everywhere at the surface of the 

 earth, where plants grow, in the direction of the radius of the earth ; and that is gravita- 

 tion — the attraction of the mass of the earth. The mere reflection that the relative 

 directions of the organs of plants of like kind with respect to the earth's radius 

 at diff"erent points of the earth's surface are the same in every place, shows at 

 once that gravitation alone can be concerned here ; if at our antipodes, or in South 

 America, or in Japan, and therefore at the most different points on the globe, the stem 

 of a Fir grows vertically upwards and its primary root vertically downwards, that means 

 in other words that at each of these places the apical bud of the stem grows away 

 from the centre of gravity of the earth, while the tip of the primary root, on the 

 contrary, behaves as if it were attracted by the centre of gravity of the earth ; or, 

 both organs behave as if affected by a force supposed to be radiating in all 

 directions from the centre of gravity of the earth. There is, however, only one 

 such force, and that is gravitation — the attraction of the mass of the earth — the 

 force which causes a pendulum to hang downwards, and an air-balloon to ascend 

 vertically upwards. It is this force, therefore, which affects the growth of the 

 plant-organ, as is shown particularly clearly when the longitudinal axis of a 

 plant-organ is placed in a diff"erent direction, with reference to gravitation, than that 

 in which it had hitherto been growing, and it is well to notice that this stimulus 

 continues until every organ of the plant has resumed that direction which accords 

 with its internal nature ; or, we may also say, every plant-organ has its peculiar 

 proper angle, i. e. the property of growing at a definite angle with respect to 

 the direction of gravitation, and if this is accidentally altered, of curving until its axis 

 of growth again forms the same angle with the vertical. 



These reflections, with the necessary clearness of thought, should suffice to 



