MOVEMENTS OF IRRITATION. 477 



cease when the apparatus is suspended horizontally, or even 

 obliquely in an atmosphere completely saturated with aqueous 

 vapour, e.g. under a large bell-glass wet on the inside. In this 

 case the main roots of the seedlings grow straight downwards. 

 Growth curvatures due to non-uniform distribution of moisture 

 cannot here take place ; the roots growing in absence of light, in a 

 space saturated with moisture, only answer to the directive influ- 

 ence of gravity. 



See Sachs, Arbeiten d. botan. Inst. in Wiirzburg, Bd. 1, p. 209. 



181. The Hydrotropism of Mucor Mucedo. 



We have already made ourselves acquainted with Mucor Mucedo 

 in 36. The sporangiophores of Mucor are distinguished, as Wort- 

 mann 1 determined, by being negatively hydrotropic. Hence, if 

 they are growing in the neighbourhood of a moist body, they 

 curve away from it. I have satisfied myself that this can easily 

 be determined as follows. 



Some cow-dung or horse-dung is left under a bell-glass fora few 

 days. There develops a luxuriant growth of Mucor Mucedo, the 

 fungus which we require for our experiment. We lay a small 

 cube of bread, saturated with water, in a flat glass dish, and cover 

 the dish with a sheet of glass, which fits closely to its edge, and is 

 provided in the middle with a hole a few mm. wide. Before 

 covering with the sheet of glass, we transfer to the bread, by 

 means of a needle which has been sterilised by heating, some ripe 

 sporangia from the Mucor growth on the dung, and distribute the 

 spores on it. The spores quickly germinate ; after one or two 

 days sporangiophores are already growing out through the hole in 

 the sheet of glass, and it is now our object to prove that these 

 organs are negatively hydrotropic. For this purpose a strip of 

 thick cardboard is fastened on a cork by means of shellac. We 

 then saturate the cardboard with water, and place it in the imme- 

 diate neighbourhood of the sporangiophores growing through the 

 hole in the glass. The whole is now covered with a cardboard box 

 in order to exclude the light, as also earlier in the course of the 

 experiment, viz. after the sowing of the spores. After about 

 twenty-four hours, the sporangiophores will have increased con- 

 siderably in length, and it is easy to see that they have not grown 



