356 THE EFFECT OF WATER UPON GROWTH [Ch. XII 



which the whole plant is rolled into a ball capable of being 

 transported by the wind, perchance to a moister region. 

 These hygroscopic movements occurring in a homogeneous 

 medium are to be distinguished from true hydrotropism, for 

 they are not properly growth phenomena. 



True hydrotropism occurs in growing elongated organs, such 

 as roots or stolons, which grow from or toward a region of 

 greater or less moisture. The observations on this phenom- 

 enon have not been numerous, and are difficult to bring under 

 one point of view ; consequently, we shall do best to classify 

 the cases studied on the basis of the organs considered. 



1. Roots. — The first studies upon hydrotropism in roots 

 were made in the middle of the eighteenth century ; but they 

 were crude and uncritical. The first adequate experiments 

 were made by Knight ('11). He half -buried some beans in a 

 flower-pot filled with earth, inverted the pot (in which the 

 earth and seeds were retained by a grating), and kept the 

 earth moist by adding water through the hole in the bottom of 

 the pot. The radicles, instead of growing vertically down- 

 wards as radicles normally do, ran horizontally along the sur- 

 face of the moist earth. The same results were got bj- John- 

 son ('29), who found in addition that if the mouth of the 

 inverted flower-pot, or other seed receptacle, be placed in a 

 moist atmosphere, the roots grow vertically downwards ; they 

 are no longer turned aside by dry air. Then Dxjchaetre 

 ('56) discovered that when a seedling was grown in relatively 

 dry earth, with its aerial part in a close, moist chamber, the 

 roots did not penetrate vertically into the soil, but grew out 

 horizontally, and even upwards. Sachs ('72) varied this 

 experiment by planting his seeds in a basket made of netting, 

 fixed to a metallic frame, and hung with its sides inclined at 

 an angle of 45° with the horizontal plane. When the appa- 

 ratus was placed in a damp chamber the radicles grew verti- 

 cally downwards ; but in dry air they turned back towards the 

 bottom of the sieve containing the damp earth, and ran along 

 its under surface. Sachs called especial attention to the fact 

 that it is the damper side which becomes concave ; and this 

 shows that the turning is not due to direct physical causes. 



The second epoch in the study of hydrotropism now began. 



