MOVEMENTS OF IRRITATION. 



441 



then find after some time that the ends of the advancing second- 



ary roots have turned downwards in a curve. The secondary 



roots of the first 



order, therefore, in 



contrast to the main 



roots, do not grow 



vertically down- 



wards ; their posi- 



tively geotropic 



downward curvature 



ceases when they 



form a certain angle 



with the vertical, 



thegeetropic "limit- 



ing angle." It may 



further be noted 



that the lateral roots 



of the second order, 



springing from the 



lateral roots of the 



first order, are not 



at all geotropic ; 

 they do not react to 

 the influence of gravity. 1 



FIG. 142. Portion of a root of Phaseolus multiflorus 



behind a sheet of s lass - 



1 The literature concerning the geotropism of roots is collected together in 

 mjLehrbuch der Pflanzenphysiologie. As regards the method of investigation, 

 consult Sachs in Arbeiten d. bot. Inst. in Wiirzburg, Bd. 1. 



173. The Geotropic Behaviour of Shoots. 



In many stems negative geotropism is extremely well marked, 

 and we will in vestigate these in order to acquire a further insight 

 into the influence of gravitation on plants. If negatively geo- 

 tropic organs are laid horizontally, they curve upwards, as can 

 easily be demonstrated in a large variety of cases. We cover the 

 bottom of a large zinc box with moist sand, heap the sand fairly 

 high against the walls, and stick into the slopes the lower end 

 of the piece of stem whose geotropism is to be investigated, the 

 rest of the stem being free. We then close the box with a cover. 

 When, e.g., shoots of Chrysanthemum Leucanthernum bearing 



