108 PLANT PHYSIOLOGY 
hours to determine in what segment so marked the most 
rapid growth occurs. It must be remembered that this zone 
of most rapid growth is rapidly passing down the root all 
the time, keeping about the same distance back from the root 
tip, so that the marked root must not be left too long before 
examination or the conclusions will be faulty. 
(c) Attach the thread of an auxanometer (instrument for 
measuring growth) to the tip of a leaf just growing out of an 
onion or hyacinth bulb or to the tip of the flower scape of such 
a plant, or just below the cotyledons of a sunflower seedling. 
If possible have the plant in a situation where 
it is almost equally lighted from all directions. 
If the instrument is not self-recording readings 
should be made every one or two hours during 
the day and night. If the records are automat- 
ically made the readings need not be taken during 
the course of the experiment but the records can 
Fic. 48.— be studied afterward. So far as possible keep the 
ecredhs int temperature constant. Interesting results may 
’ be obtained by varying the temperature while 
keeping the intensity of the light the same or by varying 
the light with constant temperature. The effect of keeping 
the soil very wet and very dry may also be compared. 
(d) Observe a potato that has started to grow in a dark 
corner of a cellar and compare its growth with that from a tuber 
that has been grown in full light. 
(e) Place potted plants under bell jars as follows: (1) clear 
white glass, (2) double bell jar with space filled with saturated 
K.Cr.0; solution, (3) double jar with space filled with saturated 
cuprammonia solution. Compare the growth. Note also the 
differences in the color and development of the leaves. The 
cuprammonia solution is prepared by carefully adding to a 
copper sulphate solution sufficient ammonia to precipitate all of 
the copper as copper hydroxide but not adding enough ammonia 
to redissolve the precipitate. Filter and wash the precipitate 
and then dissolve it in strong ammonia using only enough of 
the latter to completely dissolve it. This must not be done 
_ on the filter paper as the solution thus formed dissolves cellulose. 
(f) The rate of growth under normal conditions can be meas- 
ured by an auxanometer or with a horizontal microscope or in 
the case of rapidly growing plants, such as Indian corn, morn- 
Oe 
