MOVEMENTS OF GROWTH. 413 



investigation, so that the plants are already rooted. Several days 

 before the beginning of the experiments the soil in the flower-pots 

 is thoroughly moistened, and then in order to prevent it from 

 drying up, which would interfere considerably with the course 

 of the observations, the pots are put into a zinc receiver, the 

 cover of which is in halves. We employ an auxanometer (see 

 153). The thread may be fixed say below the leaves of the third, 

 fourth, or fifth internode. These leaves, and also those below them, 

 it is then advisable to cut off close to the base, and always deter- 

 mine the total growth of the parts of the stem lying below the 

 point of attachment of the thread, that is, e.g., of the second and 

 third or fourth and fifth internode. The plants are to be screened 

 from direct sunlight ; they must therefore be placed e.g. in a room 

 with a south aspect, at a sufficient distance from the window. 

 Mirrors serve to prevent heliotropic curvatures (see 153). The 

 temperature and hygrometric state of the air are to be deter- 

 mined by means of wet and dry bulb thermometers hanging free 

 in the neighbourhood of the research plants (see 76). In order 

 to keep the air in the laboratory nearly uniformly moist, the floor 

 must be sprinkled several times a day with water. Attention 

 must also be paid to the effect of overclouding. 



The interpretation of the numerical data afforded by the tem- 

 perature readings and the measurements of the hourly growth is 

 not quite easy. The peculiar relationship between amount of 

 growth and conditions of illumination stands out fairly clearly, 

 if we simply calculate from the hourly values the increments of 

 growth and mean temperatures for periods of three hours. The 

 relationship between growth and conditions of illumination comes 

 out most clearly of all if the results of the observations are repre- 

 sented graphically. The curves are plotted in the usual manner. 

 Sachs (Arbeiten d. lot. Inst. in Wtirzburg, Bd. 1, pp. 126, 185, and 

 192) has drawn attention to some points deserving attention, which 

 for want of room we shall here pass over. The curves teach that 

 from morning till evening the growth becomes less, while from 

 evening till morning it increases. This stands out clearly even 

 when the temperature in the night is somewhat lower than in the 

 daytime. 



Lastly, it is very instructive to observe the growth of the sporo- 

 phores of Mucor in light and darkness. We proceed exactly as 

 described in 153. The bell-glasses, under which the Mucor plants 

 grow, are alternately covered with a cardboard case, and left un- 



