50 GEOTROPISM AND CORRELATION [CH. 



said axis, as regards their relation to the vertical ; 

 and here again, experimental enquiry leaves us no loop- 

 hole of escape from the conclusion that gravitation is 

 at the bottom of the phenomenon, since that is the only 

 known factor of the environment with which we have 

 interfered, and that, of course, only in so far as concerns 

 its direction of action on the shoot. 



Nevertheless, this does not exhaust the subject. The 

 youngest lateral branches below the tip of the leader of 

 a Pine, Spruce, Beech, &c. shoot out into the air at an 

 angle with the vertical of somewhere near 45° : as they 

 grow older this angle widens, doubtless due to the in- 

 creasing weight of the foliage, &c. they have to support, 

 until older branches may either arch over or even form an 

 angle greater than 90° with the vertical axis above the 

 insertion, the tips curving in various ways. 



But this initial angle of divergence, of 45° or less, is at 

 once altered if we remove the leader by cutting, or if it is 

 broken off by wind, or destroyed by some parasite, &c., 

 and the previously diverging shoots are found to erect 

 themselves vertically, until one, favoured by circumstances 

 and stronger than the others, usurps the functions and 

 peculiarities of the lost leader. Here we have an ex- 

 cellent example of what, in our ignorance, we term the 

 correlation between organs. It is important to notice 

 that not only does the position of the newly constituted 

 leader change, but the whole sequence of events in its life 

 is altered : its leaves, buds, branches, &c. are all hence- 

 forward differently disposed from what would have been 

 the case had the shoot continued its life as a lateral 

 organ. 



It is impossible to believe that geotropic influences 

 alone — i.e. the directive effects of gravitation acting at 

 certain angles on the growing organ — are the only factors 



