MOVEMENTS OF GROWTH. 381 



divided into 100 parts. The microscope is levelled by means of the 

 levelling screws. /, and the spirit level, Z. 



It remains to be remarked that the focal distance of the micro- 

 scope for 20-fold magnification is 80 mm. The ocular micrometer 

 is divided into 120 parts. Each division, with 20-fold magnification, 

 has a value of 0'07 mm. ; one revolution of the screw as we have 

 said corresponds with 0*792 mm. The screw permits the measure- 

 ment of distances up to 3 cm., the micrometer distances up to 8 mm. 

 Both ways of measuring may be employed. 



In using the microscope here described, it is exceedingly im- 

 portant that it should be fixed so as not to vibrate, e.g. on a suit- 

 able bracket, or on a substantial table, supported by a strong 

 wooden tripod, and movable in a vertical direction. The support also 

 carries a clinostat (see Section V.), whose axis is vertical, so that 

 its disc rotates slowly in a horizontal plane (say once every hour). 

 As research material we may first work with sporophores of 

 Phycomyces or Mucor. 



A piece of bread is moderately moistened with O'l per cent, grape- 

 sugar solution, then sterilised by exposure for some time to a tem- 

 perature of 100 C. in the dry ing chamber, and finally sown by means 

 of a sterilised needle with a few spores of Phycomyces or Mucor.* 

 Mucor spores may readily be obtained as described in 36. The 

 bread is left under a bell-glass, in the dark, till the sporophores 

 have attained a suitable size ; it is then placed in a glass dish 

 on the disc of the clinostat, and again covered with a bell-glass, 

 which, however, must be tubulated for the accommodation of a 

 thermometer. The sporophores now grow straight upwards, even 

 in the light, when the clinostat is set in motion. Under the con- 

 ditions indicated, heliotropic curvatures cannot take place. Before 

 starting the clockwork of the clinostat, we focus the microscope on 

 a sporophore in such a way that the top of the sporangium appears 

 to touch one of the divisions of the ocular-micrometer. If we now 

 start the experiment, the disc of the clinostat making one complete 

 rotation per hour, it will be found that the sporangium is not again 

 distinctly in the field of view for an hour. The amount of growth 

 made during the hour may readily be determined by rotating the 

 screw, m (Fig. 125). Not infrequently our sporophores grow 1-2 



* If a few sporangia are transferred to sterilised water, they burst and 

 liberate their spores. It is best to infect the bread with a few drops of the 

 spore-containing fluid so obtained. 



