HYDROTROPISM. 7l5 



to it. After the older ve<;etablc physiologists, especially Dutrochec, had suggested 

 that roots may be caused to curve by the moisture of their environment, but 

 owing to unsuitable experiments had arrived at no results, I found in 1872 that this 

 supposition is, as a matter of fact, correct. Fig. 404 will make the facts easily intel- 

 ligible, a a represents the vertical transverse section of a very shallow cylinder or 

 ring of zinc, which by means of three threads c c is suspended obliquely at d. The 

 zinc frame a a had been previously covered with large meshed netting and then filled 

 with damp saw-dust, the upper surface of which is shown at// Peas or any other 

 seeds saturated with water were then laid in this saw-dust g g. In virtue of their 

 strong geotropism, the seedling roots at first grow vertically downwards and at 

 length come forth into the air between the meshes of the nelling at the oblique lower 



Fig. 404.— Apparatus for observing the liytlrotropisin of the primary 



surface of the apparatus. If the air is completely or nearly saturated with vapour the 

 roots grow down vertically into it ; if this is not the case, however, and the air is 

 only to a certain extent moist, but not saturated, the root-apices projecting from the 

 meshes curve laterally until they again come into contact with the lower side of the 

 saw-dust, as at // and i. Very often they grow obliquely downwards closely applied 

 to this oblique surface ; occasionally the tip of the root again penetrates through the 

 meshes into the damp saw- dust, at once bending downwards again geotropically, 

 however, to repeat the same process. In this way a root (w /;/) may fairly sew itself 

 like a needle and thread passing up and down between the meshes of the netting. 



It is certainly, as numerous further experiments showed, only the difference 

 in the moisture of the surrounding air which produces these phenomena : aqueous 



