INFLUENCE OF EXTERNAL STIMULI. GRAVITY 225 



of the rhizome, that it shoots out (i) if the chief shoot be taken away 

 or be restricted in growth, and (2) if this be brought into an inverted 

 position ; moreover, we can also cause it to shoot out when it is in its 

 normal position if we remove along with the chief axis all the lateral buds 

 of the rhizome. When we consider the peculiar conditions of life existing 

 here, this example in its fundamentals exhibits the same features as 

 those mentioned in the chapter upon correlation. 



The furtherance of the growth of vertical members is specially 

 apparent when we have regard to the difference between chief roots and 

 lateral roots, chief shoots and lateral shoots. That this difference 

 is not exclusively the result of the influence of gravity is quite 

 evident, inasmuch as it is seen in the formation of the chief and 

 lateral axes in Algae, upon whose relationships of configuration gravity 

 has no action 1 . The chief shoot has, when compared with the lateral 

 shoot, the advantage that it is an earlier structure than this is, and 

 therefore its relationships of nutrition and its mechanical claims are 

 points that have to be taken into consideration. On the other hand 

 we cannot deny that axial organs growing in a vertical direction appear 

 to be favoured by this direction ; shoots which are inclined at an angle 

 to it develop much more feebly, as has been long known from experi- 

 ments in fruit-culture, and they have a much greater tendency to 

 produce spur-shoots than those which have a more vertical direction. 

 The facts mentioned on preceding pages lead to the same conclusion, and 

 when we see a lateral root which has come to take the place of the 

 severed chief root 2 , or a lateral twig which has become erect, each of 

 them growing now with a much stronger construction and exhibiting 

 other peculiarities, we must admit that besides correlation the stimulus 

 of gravity must also influence them the ' shoot-forming substances ' 

 have evidently the tendency to travel upwards, the root-forming 

 substances to travel downwards. A number of cases that might be 

 mentioned here have already been brought forward when speaking of 

 relationships of symmetry and of correlation, and therefore only a few 

 additional ones need be cited. 



Sachs has observed 3 : 'If one allows a five to six-year-old silver 

 fir to grow during a year in an inverted position, that is to say with 

 its apex downwards, there arise upon the dorsal side, the side that was 

 earlier directed downwards but is now the side turned upwards, of the 

 bilateral branches new shoots which are of radial construction and 

 appear like seedling plants.' Evidently there is here a combination 

 of the effect of correlation and gravity. The growth of the chief 



1 See what has been said on p. 36. 2 See pp. 44, 214. 



'' Sachs, in Flora, 1894, p. 229. 



GOEBEL Q 



