THE MEASUREMENT OF GROWTH 23 



heat. The latter error vanishes if the experiments are performed in a room at 

 constant temperature. Threads of mixed hemp and silk may be constructed whose 

 length is unaffected by moisture owing to the opposed hygroscopic changes of 

 shape of the two substances. 



Photographic registration will probably be largely employed in the future, for 

 series of pictures may be obtained which when placed in a kinematograph show 

 the phases of several days' or weeks' growth in a minute or so l . 



1 Pfeffer, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 738. Cf. also Langendorff, 1. c., p. 89 ; Marey, 

 Die Chromophotographie, 1893. 



