1898] RECENT PROGRESS IN ROOT-PHYSIOLOGY 887 



normal, the vigour of the reaction became less and less, but still 

 remained higher than at 90\ When the tip of the root is exactly 

 upside down, that is to say when it points accurately vertically 

 iipwards, it is theoretically once more in a position of equilibrium, 

 and the geotropic reaction should equal 0. As a matter of fact the 

 root curves round until the tip is once again directed vertically 

 downwards. Sachs had already explained this as an effect of nuta- 

 tion, which carried the root out of the exactly upright position and 

 so brought it under the influence of gravity again, and the experi- 

 ments of Czapek tend to bear out this view. 



The latter part of Czapek's work is occupied with an interesting 

 account of ' autotropism,' and the elimination of geotropic curvatures 

 by virtue of autotropism. By autotropism is implied the inherent 

 tendency of vegetable organs to grow in a straight line unless they 

 be compelled by outward influences (gravity, light, etc.) to bend aside 

 from that direction. Moreover, having bent aside to the action of a 

 transient external stimulus they subsequently straighten themselves 

 out again through the agency of the same inherent quality. The 

 reader, however, must refer to the original for these matters. 



Up to this point we have only referred to phenomena presented 

 by the main root. Both in the paper already mentioned and in other 

 special ones Czapek (2 & 3) has studied the geotropism of side-roots. 

 These apparently are governed by somewhat different laws ; they 

 do not grow vertically downwards like the tap-root, but occupy a 

 position in which they form a greater or less angle with the vertical. 

 Dealing with side-roots of the first order, Sachs was inclined to attri- 

 bute their oblique position to a weaker geotropism residing in them. 

 The angle they form witli the vertical he called ' the angle of geo- 

 tropic limitation.' In turning his attention to this subject Dr 

 Czapek first satisfied himself that the only external force inducing 

 the oblique position was gravity. Having settled this point by 

 clinostat experiments, he next enquired whether the oblique position 

 of the rootlets could be explained in the manner indicated by Sachs, 

 viz., by a limited action of gravity, or whether a different explana- 

 tion was needed. A number of plantlets of Vicia Faha were taken 

 and divided into two groups. In the first group the side-roots were 

 turned 60° above their normal position, whilst in the other set the 

 rootlets were placed 60° below their usual direction. The experi- 

 ment, which was carried on in darkness, showed that the rootlets of 

 the first group returned to their normal position more rapidly than 

 those of the second batch. The intensity with which side-roots, 

 placed above their usual position, bend is therefore greater than that 

 manifested by rootlets displaced below the normal direction. Czapek 

 believes that these facts point to a two-fold geotropic action : to a 

 positive geotropism common to the side and the main roots alike. 



