CH. VIIl] HYDROTROPISM. 197 



another as the filaments are made to contract on the 

 corresponding sides. 



To see the extrusion of pollen another flower must be 

 used: the best plan is to remove the pollen at the free 

 end of the anther-tube with a camel-hair brush. If this 

 is done with a rapid touch, the filaments are irritated and 

 a ribbon of pollen emerges immediately after the blow. 



(246) Phycomyces: curvature towards iron. 



Elfving has shown that the sporangiferous hyphse of 

 Phycomyces nitens curve towards iron. It is only necessary 

 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) Hydrotropism. 



The curvature of roots towards a moist surface can be 

 demonstrated by the well-known method of Sachs". 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 two inches of the 



' 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. 



2 Sachs' Arbeiten, i. p. 212, Fig. 3. 



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



