CH. VIIl] HYDROTROPISM. 213 



to plant an iron rod in the centre of a Phycomyces culture 

 (from which light is carefully excluded), and leave it for 

 12 or 18 hours, when the hyphse are seen bending from all 

 sides towards the rod. The cause of this remarkable 

 phenomenon is still obscure \ 



(247) Hydroti^opism. 



The curvature of roots towards a moist surface can be 

 demonstrated by the well-known method of Sachs I A 

 sieve is constructed by fastening netting to a bottomless 

 box or stretching and tying it over the mouth of a short, 

 wide^ cylinder open at both ends and made of galvanised 

 iron or tin-plate. A thin layer of moist sawdust or finely 

 divided cocoa-fibre is spread on the netting, seeds are 

 placed on the layer and covered with 2 inches of the 

 same material. The sieve is now hung up so that the 

 bottom makes an angle of about 50° wdth the horizon. 

 As the roots emerge they leave the vertical and grow 

 along the moist surface of the sieve. We find that 

 cereals such as rye or barley answer w^ell. The only 

 difficulty is to provide a suitable atmosphere in which to 

 hang up the sieve. If the air is too dry the roots wither 

 before they have time to bend ; if too damp the surface of 

 the sieve does not supply a sufficiently strong contrast to 

 the surrounding air. A greenhouse atmosphere answers 

 fairly well, or, as Sachs recommends, a large cupboard or 



1 See however Elfving's interesting paper in Nature, March 15, 1894, 

 where references to his original paper and to Errera's work on the subject 

 are given. 



' Sachs' Arheiten, i. p. 212, Fig. 3. 



^ 5 cm. deep, 20 cm. in diameter. 



