200 LECTURE XII. 



Of the cosmic forces universally operating on the plant, I have still to mention 

 Gravitation. Plants possess an irritability — one might almost say a perception— with 

 respect to the angle which their organs make with the perpendicular where they 

 grow \ They are irritable as to the direction in which gravitation acts on each of 

 their organs, and this independently of their weight, or of any pressure. They 

 possess a sensitiveness to gravity, as we do to light or heat; while a direct 

 perception of gravitation is completely wanting to us, since we perceive it only 

 through the effect of weight and pressure. If a plant in full growth, the roots 

 of which have developed in a flower-pot, is brought from the ordinary erect position 

 into some other — e.g. laid horizontally — it is noticed after several hours, or, according 

 to circumstances, after some days, that all the growing organs, and some apparently 

 already fully grown, have been caused to move by means of this change of position : 

 the apices of the roots, the growing shoot-axes and leaves, flower stalks, etc., describe 

 the most various curvatures, until the free moveable parts of the organs have again 

 assumed those directions with respect to the horizon which they possessed before 

 the change of position of the whole plant. Those previously directed vertically 

 upwards or downwards become curved until their apices are again directed upwards 

 or downwards ; and parts previously growing obliquely or horizontally become 

 curved, after the change of position, until they can again grow forward obliquely or 

 horizontally in the same manner. We shall see later on that these movements 

 are called forth by an effect of gravitation on those organs of a plant which are 

 capable of growing. Stimulations are produced so that the organs respond to 

 every change of position with respect to the direction of gravitation, and these 

 only cease when the organs have again attained their original direction. This 

 phenomenon is termed Geotropism. The influence of gravitation makes itself felt, 

 however, in another and quite difi"erent manner, in that growing points arise at definite 

 places, the position of which is determined by the direction of gravitation. To 

 this point also we shall return subsequently. It may here be mentioned, in anticipation, 

 that the influence of gravitation on the direction of the position of equilibrium in 

 which the various organs of the plant grow onwards undisturbed can also be 

 established with certainty without special experiments ; it suffices to observe ^ that 

 the perpendicular stem of a Fir-tree or Palm stands vertical at every place on the globe 

 where it grows — i. e. has exactly that direcdon which, conUnued downwards, leads to 



^ My views on Geotropism, presented in this book, are based on the following treatises by me — 

 ' Längcnwachstlmni der Ober- und Unterseite horizontal gelegter sich aufwärtskrüninicnder Sprosse' 

 (Arbeiten des bot. Inst, in Wzbg., B. I. p. 193), and ' Über das Wachsthum der Haupt- und 

 Nebeniüurzeln'' (ibid. pp. 385 and 584), where I also first showed that organs growing obliquely 

 to the horizon are geotropic like lateral roots : this was afterwards also demonstrated by Elfving 

 for horizontal subterranean stolons. I wrote on the Geotropism of shoot-axes in 'Flora' (1873) 

 the essay, ' Über IVaehsthicm nnd Geotropism aufrechter Stengel,' a short notice on a very long 

 investigation. In addition are to be compared on this subject my treatises, ' Über Anschliessting der 

 geotropischen und helioti-opischeti I^riiinmnngen,' and further, ' Über orthotrope tind plagiotrope 

 Pflanzenthcilc,' in ' Arbeiten des TDot. Inst, in Wzbg ,' B. II. I siiecially refer to the concluding 

 remarks in the essay last mentioned. 



^ I first showed in my 'Handbuch der Experimentalphysiologie'' (iSö^:;"), p. 100, that the 

 demonstration of the influence of gravitation on plants by means of rapid rotation — undoubtedly 

 first given by Knight in 1S06 — was not needed, since the same conclusion is to be drawn with the 

 same certainty fiom every day observations, as I have pointed out in the text. 



