GROWTH 205 



for projection upon a screen are described by Kohl, Berichte der deutschen 

 botanischen Gesellschaft, 20, 1902, 208, and by Pfeffer in a paper earliei 

 cited (page 23). 



The preceding experiment will yield an accurate record, 

 which should be transferred to a graph, of the increase of a stem 

 in length. The same apparatus may also be applied to grow- 

 ing leaves and roots; and if the student has time and desires to 

 follow this subject, he will be aided by the following suggestions : 



Suggested Experiments, (<z) Prepare a flower-pot moist-chamber, 

 as described later under Geotropism, and fasten to a cork near its top a seed 

 (e.g., Horse Bean) with a strong-growing root. Place over the root a small 

 sealed glass tube 5 mm. deep, which is connected by threads, through the 

 hole in the pot, with an auxograph. Or the arrangement described by Stone 

 in Botanical Gazette (22, 1896, 258) may be used. (6) Arrange a growing 

 leaf so that it is supported in a horizontal position, but is free to expand; 

 attach threads, by shellac, on its tip and one margin, and carry these out 

 horizontally to auxograph wheels. 



A striking feature of the auxographic records is the great 

 fluctuations they show in the rate of growth, and it is natural 

 ' to infer that these fluctuations are probably due to changes in 

 the variable external physical conditions. Obviously this is 

 a matter of consequence, requiring a definite experimental study, 

 and we consider first the most important of the external condi- 

 tions, and ask: 



Whit effect is produced upon growth by temperature? 



This may be tested by either of three methods: first, by a com- 

 parison of the growth and temperature graphs of the preceding 

 experiment at times when the other conditions are fairly constant, or, 

 second, by conducting an experiment similar to the preceding in a 

 meteorostat (page 37), where temperature may be varied while the 

 other conditions are kept constant, or, third, by exposing a series of 

 similar plants to various degrees of temperature, the other conditions 

 being kept the same for all, an end which can be accomplished by 

 use of thermostats. 



Experiment. Prepare 10 two-inch pots with seedling mixture and 

 sow, exactly alike in each, 10 similar Oats. Place the pots in the cham- 

 bers of a differential thermostat kept heated from 5°-6o°. Supply 

 uniform light, daily aeration, and sufficient water to each. When the 

 tallest seedlings reach the top of their chamber, close the experiment, 

 measure the average height of the plants in each set, and plot a graph 

 of the results. 



