388 NATURAL SCIENCE [June 



and to a transverse geotropism which tends to give the organs a 

 horizontal direction. In the case of the roots which are turned 

 above their ' angle of geotropic limitation ' the horizontal and positive 

 geotropisnis act togetlier, whilst in the opposite case in which the 

 displacement is below the normal angle the transverse geotropism 

 acts in direct opposition to the positive geotropism. In this way 

 C^apek seeks to explain the oblique position of the rootlets and the 

 other phenomena which were noticed in his experiments. What 

 Sachs called the ' angle of geotropic limitation ' is on this view the 

 resultant of the action of two forces — positive geotropism and 

 transverse geotropism. 



In the same memoir Czapek includes his important work on the 

 directive forces acting on rhizomes and underground stems. 



Quite lately further interesting observations on the behaviour of 

 side-roots have been published by Alfred Schober (13). He ranges his 

 experiments in two series, according as the main root is compelled 

 by growth to bend on itself at an angle and thus bring the normally 

 oblique side-roots into a vertical position, or as the main root is 

 merely placed in a slanting position sufficient to bring the rootlets 

 into the vertical. The experiments were carried on in one of Sachs' 

 cases with a glass side, and the seedlings were so arranged that their 

 side-roots lay against the glass. They could be watched in every 

 phase of their growth and all changes faithfully recorded on tracing 

 paper. The plants used for these observations were Pisum sativum, 

 Phaseolus midtiflorus, Citcurhita Pepo, and Vicia Faha. It was found 

 that when the rootlets were placed in a vertical position they 

 lingered for a shorter or longer time in this direction, and then 

 usually bent aside so as to bring their apices into an oblique posi- 

 tion. In nearly every instance the curvature took place in a plane 

 passing through the side and the main root. In a few cases, as for 

 instance in one recorded for Cacurhita Pepo, the rootlet when placed 

 in the vertical line continued to grow in this direction throughout 

 the twelve days that the observation was kept up. This behaviour 

 is, however, rare, and only serves as the exception which proves the 

 rule. It was noticeable that, although the curvature of the side- 

 roots from the vertical nearly always took place, they seldom bent 

 sufficiently to attain their normal ' angle of geotropic limitation.' 

 The lingering, too, in the vertical was also sometimes very notice- 

 able ; Schober mentions the cases of Cucurhita and Vicia Fata, in 

 which the rootlets continued to grow for 2 cm. in a straight line 

 before they manifested the characteristic curvatures. 



These experiments bring out another interesting point, viz., that 

 the direction of the curvature is partly fixed and partly irregular. 

 It may be said, in general terms, that the rootlets springing from 

 the hypocotyl (the part of the stem below the seed-leaves) and from 



