GRAVITATION AS DETERMINING THE POSITION OF OFFSHOOTS. 



5*9 



whereas they become more or less cylindrical or prismatic when grown in the dark. 

 That the normal offshoots thus arise from the margins is an indirect consequence of 

 light ; that they arise chiefly from the upper surface of the existing segments is a 

 direct effect of gravitation, which can in its turn conquer the preceding influence of 

 the light if the segment which produces the new shoots assumes the proper position. 

 We may easily convince ourselves also by means of an experiment, that the places 

 of origin of new growing-points are actually dependent on the influence of 

 gravitation, as illustrated in Fig. 338. A plant which already consisted of the 

 segments /, //, ///, and growing in the pot c, was placed in an inverted position on 

 an iron support a, in such a manner that the plant was directed downwards and the 

 pot c upwards, the falling out of the soil being prevented by the interposition of the 

 divided cover ö. The experiment began immediately after the formation of a new 

 growing-point at the apex of the 

 plant, and this growing-point was 

 cut off. In the next few weeks 

 were then produced five new grow- 

 ing-points, from which the shoots 

 marked 1-5 in the figure developed. 

 The origin of the shoots i, 2, and 3 

 from the segments // and /// sim- 

 ply present nothing surprising, since 

 they arise at the apical portion of 

 the plant. That the segments 4 

 and 5 however have arisen also 

 from the old basal segment /, 

 proves that here, in consequence of 

 the inversion, an effect of gravitation 

 has made itself evident : the shoot- 

 forming substances which previously 

 streamed towards the apex have now 

 gone back, in part at least, upwards 

 into the segments // and /, to pro- 

 duce the shoots 4 and 5, which in fig. 3^,s.-fl/,u„ü\,ry.us, „.,■:. a. 

 the normal position of the plant 



would certainly not have happened. But the places of origin not merely of 

 the shoots but also those of the roots are determined in Opuntia by external 

 forces, and probably chiefly by gravitation. In experiments to elucidate this, how- 

 ever, difficulties are met with arising from the fact that the plant was previously 

 growing in its ordinary normal position, and a strong predisposition thus induced. 

 On cutting off vigorous Opuntia segments and then placing them with their 

 basal ends in the soil of a pot, and leaving them in the same position as in 

 Fig. 338, there appear, within a few weeks, from the basal ends (now turned up- 

 ward) of the segments, vigorous and much-branched roots which fill the soil of 

 the i)Ot. In this is expressed the tendency of the root-forming substance to be 

 driven to the basal end even when this is turned upwards ; though here the 

 moisture and darkness surrounding the basal end co-operate as favourable factors. 



