476 PHYSIOLOGY OF GROWTH. 



consists of the netting. We fill the sieve with moist sawdust, 

 and lay in it soaked seeds. I experimented with Phaseolus seeds ; 

 we may, however, equally well employ other seeds (Pisum, Zea, 

 etc.). The apparatus is then hung obliquely, by means of three 

 threads, in a cupboard or under a large cardboard box, the base of 

 the sieve being inclined at an angle of about 45 with the horizon- 

 tal. The main roots of the seedlings, which develop in complete 

 darkness, soon pass out through the meshes of the netting ; they 

 do not, however, grow straight downwards, but their tips at once 



FIG. 160. Apparatus for observations on hydrotropic 

 root curvatures. (After Sachs.) 



apply themselves obliquely to the under surface of the netting, 

 and now continue their downward growth closely pressed to it. 

 The roots on emergence turn, as our illustration also shows, to- 

 wards the side on which the seed-bed makes the smallest angle 

 "with the vertical. The curvatures are due to a difference in the 

 distribution of moisture on the sides of the roots turned towards 

 and away from the seed-bed, though it seems specially noteworthy 

 that just that side of the root becomes convex, and therefore 

 grows most vigorously, which is not directed towards the moist 

 base of the sieve. The root curvatures referred to completely 



