22 STUDIES IN PHYSIOLOGY. 



iu the sawdust, near the glass side, so that the tip of each radicle 

 will stait down about one fourth of an inch from it. If the glass 

 is proporl}- inolincd, the radicle will quickly press itself against 

 it and thus be the more roadil3' seen and studied in its subse- 

 quent growth. Wlien the radicles are about two inches in 

 length, withdraw them, and by tlie aid of a fine carael's-hair 

 brush and India ink mark them off with precision at regular 

 intervals of one or two millimeters, then place each in the same 

 place and position from which it was taken. It will be found 

 that only tlftir tips grow ; the marks above the tips remaining 

 the same distance apart. 



Put a thermometer in the sawdust in order to observe the tem- 

 perature, upon which it will be found the rate of growth depends. 

 Place the seedlings near the stoA'c or over a register where the 

 temperature of the sawdust can be gradually raised to from 28° 

 to 30° C. Having previously measured and noted the exact 

 length of the radicle of each plant, observe its increase, while 

 the temperature remains constant, for a given period of saj- from 

 five to ten hours. Next place the case containing tlie seedlings 

 in an improvised ice-chest (any box which can be well closed will 

 answer), and when the temperature has been reduced to 10° C, 

 or nearlj- that, measure the roots carefully again. Hold this 

 degree of cold as nearlj- constant as possible for five or ten hours, 

 whichever may have been the period of time in the first ease. 

 Compare the growth in the two periods and note the difference. 



SECOND SERIES. — SPECIAL EXPERIMENTS. 



I. Diffusion. 



Place a tumbler containing an inch or two of pure water upon 

 a firm shelf where it will not be subject to any jarring, and put 

 in it a vial filled to the brim with some colored liquid, for instance 

 blue or purple ink. Then by means of a tube or " thistle-funnel" 

 resting on the bottom of the tumbler pour into the tumbler water 

 enough to come up to the mouth of the vial, and verjj^ cautiously 

 add more until the mouth is covered to a depth of about an incli. 

 If the pouring has been skilfullj' done, there will be scarcely any 

 of the inJc mixed with the surrounding water. Let the apparatus 

 stand undisturbed for a week or so, and note anj- changes in the 

 color which maj- be observed from day to day. 



Try the same experiment with a saturated solution of common 

 salt in place of the ink, and at intervals of three daj's cautiously 



