386 NATURAL SCIENCE [June 



perature was kept very low. Roots left for twenty-four hours in a 

 vacuum at a temperature of 0° to 1° C. were quite healthy at the 

 end of that time. On being placed once more in air at the ordinary 

 temperature of the room, they resumed growth, and showed all the 

 properties which are normal to these organs. In the actual experi- 

 ments on geotropisra, roots of L^qnnus were placed horizontally for 

 twenty-four hours in an atmosphere free from oxygen, and at a tem- 

 perature of 0^ to 2° C. During the whole of this time growth was 

 entirely arrested in the organs, and no reaction could therefore be 

 manifested by them. The roots were then placed on the clinostat, 

 at the temperature of the room and in ordinary air. In a few hours 

 geotropic curvatures were clearly shown. 



From these results of Czapek and of the older workers it will 

 be seen that, whilst the absence of oxygen inhibits the growth of 

 the roots and prevents the display of a reaction, it does not arrest 

 the perception of the stimulus by the root-apex. The previous work 

 of Wortmann, Correns, and others had already shown that want of 

 oxygen stopped the growth, even at ordinary temperatures, whilst 

 the experiments of Czapek have now indicated that the perceptive 

 faculty of the root is not prevented by the absence of this gas. As 

 in the case of low temperatures, so also here the perception of and 

 reaction to the stimulus are governed by different laws. 



The effect of mechanical prevention of growth in geotropic 

 organs was also examined by Czapek, but, beyond mentioning that 

 in these cases the perception of the stimulus was not arrested at the 

 same time, we cannot ofter any comments in the present paper. 



When a root which normally grows straight downwards is dis- 

 placed from that position, its growing part bends round until the 

 apex is once more in the vertical. A question which naturally 

 rises to the mind in connection with this is, in what position is the 

 reaction at its maximum ? Sachs, partly on experimental and partly 

 on theoretical grounds, believed that the horizontal position was the 

 one in which geotropic reactions were most marked. As the root 

 was displaced more and more from the vertical, the reaction became 

 more and more vigorous, until when the horizontal direction was 

 attained the highest point was reached ; as the root was displaced 

 above the horizontal the reaction gradually became less, until it 

 completely died away when 180° had been passed through. 



Czapek investigated this question both by studying the magni- 

 tude of the after-effect following inductions of equal length when the 

 roots were placed at different angles from the vertical, and also by 

 measuring the time-relations of the reaction. He found that the 

 horizontal was not the position of maximum reaction, but that this 

 was some 45° above the horizontal. From this point onwards, until 

 the roots were once more vertical but in the reverse direction to the 



