460 PHYSIOLOGY OF GROWTH. 



parallel with the window panes. The cube is held securely at the 

 middle of the shaft by means of tightly fitting corks slipped on to 

 the shaft to the right and leffc of it. Below the shaft of the 

 clinostat is a zinc dish, 50 cm. long in the side, containing water. 

 A glass case traversed by the shaft, and standing in the water, 

 serves to keep the air about the bread moist. The case has a 

 zinc framework ; this, 011 the two sides where the shaft passes 

 through it, possesses slits, which when the apparatus is com- 

 pletely put together can be sufficiently closed. The front and 

 back, and also the roof of the case, is formed by panes of glass. 



To make an experiment, some sporangia of Mucor or Phyco- 

 myces nitens are distributed in sterilised water. By means of 

 a flat needle, sterilised by heating-, we sow all six faces of the 

 cube of bread with spores, and after covering with the glass box, 

 at once set the clinostat in rotation. After a few days the sporan- 

 giophores rise from the substratum, and quickly attain a con- 

 siderable length. The sporangiophores on the flanks of the cube, 

 which we shall not farther consider, are certainly somewhat 

 curved, since from time to time, at each revolution, they come 

 into the shade of the shaft, whereby heliotropic curvatures are 

 occasioned. On the other faces of the cube, however, the sporan- 

 giophores are straight ; they are directed at right angles to the 

 substratum.* This relation of the organs to the substratum, which 

 is certainly not, as supposed by some physiologists, caused by the 

 mass of the bread, comes about in rather a complicated manner. 5 



It cannot be due to geotropisrn, since, of course, the one-sided 

 influence of gravitation is eliminated by the rotation of the object 

 on the clinostat. f Bat negative hydro tropism, which will be more 

 closely considered below, is of importance, and the capacity of 

 the sporophores to re -act by irritation curvatures to the stimulus 

 of contact, 6 and of one-sided illumination, also demands attention. 



We can easily convince ourselves of the heliotropic irritability 



* The sporophores on the edges of the cube are disposed in a direction which 

 bisects the angle made by the corresponding faces. 



f That does not say, of course, that the sporophores of Mucor are notgeotropic- 

 ally irritable. They are highly irritable. If, e.g., a cube of bread, sown with 

 spores of Mucor, is stuck on a long pin and suspended by means of it in a large 

 glass cylinder, whose base is covered with water, then in a few days, the cylinder 

 being kept in the dark, erect sporangiophores spring from the upper face of the 

 cube. The sporangiophores which develop on the flanks of the cube arch 

 upwards, exhibiting negative geotropic irritability. From the base of the cube 

 grow branched mycelial threads, which exhibit positive geotropism. 



