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LECTURE XXXIX. 



GEOTROPlSxAI AND HELIOT ROPISM '. 



It appears to naive unprejudiced mankind a self-evident fact that a tree, e.g. 

 a Fir-tree, grows with its stem upright (in this case exactly vertical) and that its apex 

 always tends upwards again in the same direction ; whereas the primary root in like 

 manner penetrates the soil vertically downwards. The main branches of the stem, 

 however, stand horizontally, and put forth horizontally lateral branches, and even the 

 needle-like leaves of the Fir lay themselves horizontally in the plane in which the 

 main branches subdivide; the lateral rootlets springing from the main root, again, 

 have their proper directions with respect to the horizon, they grow horizontally or 

 obliquely downwards, but produce in their turn lateral rootlets of second and third 

 order which are able to grow out in all directions. Exactly as in the case of the 

 Fir-tree, so with many thousands of other plants, although with some subordinate 



* The older literature on Geotropism as well as on Heliotropism, up to the year 1865, is 

 collected and criticised in my ' Handötich der Experimental- Physiologie^ pp. 38, &c., and pp. 88-1 1 2. 



Of the more recent works on Geotropism the following will be most serviceable to the 

 beginner : — 



Sachs: ' Längenzüac/isthuiii der Ober- und Unterseite horizontal gelegter, sich anficärts kriiin- 

 mender Sprosse'' (Arb. d. bot. Inst, zu Wzbg. B. I, p. 193). 



Sachs: ' Über das Wachsthtan der Haupt- tmd Nebenwurzeln'' (ibid. B. I, pp. 385 and 584). 



Sachs: ' Über IVachsthum und Geotropis/nus aufrechter Stengeln' (Flora, 1873, pp. 321, &c.). 



Sachs : ' Über Ausschliessung der geotropischen und heliotropischen Krümtnungen ivährend 

 des IFachsens' (Arb. d. bot. Inst, zu Wzbg. B. II, p. 209), where I described the Klinostat (the idea 

 of which had already been indicated in my ' PPandbuch,'' 1865), and the horizontal revolution as 

 means for eliminating heliotropic curvatures. 



Detlefsen's work referred to in the text, ' Über die von Darwin behauptete Gehirnfunktion der 

 Wurzelspitze7t^ is also found in Arb. d. bot. Inst, zu Wzbg. (B. 11, p. 627). 



I refer the reader, finally, to the detailed description of geotropic curvatures in my ' Lehrbuch ' 

 (IV. Aufl. 1874, pp. 811, &c.). 



Of recent works on Heliotropism I quote only the following : — 



Hermann Midler (Thurgau) : ' Über Helioiropismus ' (Flora, 1876, Nos. 5 and 6). I have here 

 to remark that my new theory of heliotropism was first published in the introduction to this treatise, 

 and that Müller expressly mentions this; I may indeed add that the introduction in question 

 consists of my own words. It is therefore not right that certain more recent authors should speak 

 of my theory as Miiller's. In sharp contrast to my theory of heliotropism stands that of Julius 

 Wiesner in his very extensive treatise, ' JJie heliotropischen Erscheinungen im PßanzenreicV 

 (Denkschr. der kaiserl. Akad. der Wiss. in Wien, B. 39, 1878, and B. 43, 1880). I can with 

 confidence leave it to the future to decide the matter, and entertain not the slightest doubt that my 

 theory, as soon as it is but generally understood, will be accepted on all sides. 



