22 STUDIES IN PHYSIOLOGY. 



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

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

 is properly inclined, the radicle will quickly press itself against 

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

 quent growth. When the radicles are about two inches in 

 length, withdraw them, and by the aid of a fine camel'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 their 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 stove 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 say from 

 five to ten hours. Next place the case containing the 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 nearly that, measure the roots care full}- again. Hold this 

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

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

 Compare the growtli 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 very cautiously 

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

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

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

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

 color which may 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 days cautiously 



